-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. There are
separate text files for the pcregrep and pcretest commands.
-----------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions


INTRODUCTION

       The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a  few differences. (Certain features that appeared in Python and
       PCRE before they appeared in Perl are also available using  the  Python
       syntax.)

       The  current  implementation of PCRE (release 7.x) corresponds approxi-
       mately with Perl 5.10, including support for UTF-8 encoded strings  and
       Unicode general category properties. However, UTF-8 and Unicode support
       has to be explicitly enabled; it is not the default. The Unicode tables
       correspond to Unicode release 5.0.0.

       In  addition to the Perl-compatible matching function, PCRE contains an
       alternative matching function that matches the same  compiled  patterns
       in  a different way. In certain circumstances, the alternative function
       has some advantages. For a discussion of the two  matching  algorithms,
       see the pcrematching page.

       PCRE  is  written  in C and released as a C library. A number of people
       have written wrappers and interfaces of various kinds.  In  particular,
       Google  Inc.   have  provided  a comprehensive C++ wrapper. This is now
       included as part of the PCRE distribution. The pcrecpp page has details
       of  this  interface.  Other  people's contributions can be found in the
       Contrib directory at the primary FTP site, which is:

       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

       Details of exactly which Perl regular expression features are  and  are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern and pcrecompat pages. There is a syntax summary in the  pcresyntax
       page.

       Some  features  of  PCRE can be included, excluded, or changed when the
       library is built. The pcre_config() function makes it  possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcrebuild page. Documentation about  build-
       ing  PCRE for various operating systems can be found in the README file
       in the source distribution.

       The library contains a number of undocumented  internal  functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre_", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external  symbols  are  exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.


USER DOCUMENTATION

       The user documentation for PCRE comprises a number  of  different  sec-
       tions.  In the "man" format, each of these is a separate "man page". In
       the HTML format, each is a separate page, linked from the  index  page.
       In  the  plain text format, all the sections are concatenated, for ease
       of searching. The sections are as follows:

         pcre              this document
         pcre-config       show PCRE installation configuration information
         pcreapi           details of PCRE's native C API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper
         pcregrep          description of the pcregrep command
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcresyntax        quick syntax reference
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the sample program
         pcrestack         discussion of stack usage
         pcretest          description of the pcretest testing command

       In  addition,  in the "man" and HTML formats, there is a short page for
       each C library function, listing its arguments and results.


LIMITATIONS

       There are some size limitations in PCRE but it is hoped that they  will
       never in practice be relevant.

       The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
       is compiled with the default internal linkage size of 2. If you want to
       process  regular  expressions  that are truly enormous, you can compile
       PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
       the  source  distribution and the pcrebuild documentation for details).
       In these cases the limit is substantially larger.  However,  the  speed
       of execution is slower.

       All values in repeating quantifiers must be less than 65536.

       There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns.

       The maximum length of name for a named subpattern is 32 characters, and
       the maximum number of named subpatterns is 10000.

       The  maximum  length of a subject string is the largest positive number
       that an integer variable can hold. However, when using the  traditional
       matching function, PCRE uses recursion to handle subpatterns and indef-
       inite repetition.  This means that the available stack space may  limit
       the size of a subject string that can be processed by certain patterns.
       For a discussion of stack issues, see the pcrestack documentation.


UTF-8 AND UNICODE PROPERTY SUPPORT

       From release 3.3, PCRE has  had  some  support  for  character  strings
       encoded  in the UTF-8 format. For release 4.0 this was greatly extended
       to cover most common requirements, and in release 5.0  additional  sup-
       port for Unicode general category properties was added.

       In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
       support in the code, and, in addition,  you  must  call  pcre_compile()
       with  the PCRE_UTF8 option flag. When you do this, both the pattern and
       any subject strings that are matched against it are  treated  as  UTF-8
       strings instead of just strings of bytes.

       If  you compile PCRE with UTF-8 support, but do not use it at run time,
       the library will be a bit bigger, but the additional run time  overhead
       is limited to testing the PCRE_UTF8 flag occasionally, so should not be
       very big.

       If PCRE is built with Unicode character property support (which implies
       UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
       ported.  The available properties that can be tested are limited to the
       general  category  properties such as Lu for an upper case letter or Nd
       for a decimal number, the Unicode script names such as Arabic  or  Han,
       and  the  derived  properties  Any  and L&. A full list is given in the
       pcrepattern documentation. Only the short names for properties are sup-
       ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
       ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may
       optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
       does not support this.

   Validity of UTF-8 strings

       When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and
       subjects are (by default) checked for validity on entry to the relevant
       functions. From release 7.3 of PCRE, the check is according  the  rules
       of  RFC  3629, which are themselves derived from the Unicode specifica-
       tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which
       allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current
       check allows only values in the range U+0 to U+10FFFF, excluding U+D800
       to U+DFFF.

       The  excluded  code  points are the "Low Surrogate Area" of Unicode, of
       which the Unicode Standard says this: "The Low Surrogate Area does  not
       contain  any  character  assignments,  consequently  no  character code
       charts or namelists are provided for this area. Surrogates are reserved
       for  use  with  UTF-16 and then must be used in pairs." The code points
       that are encoded by UTF-16 pairs  are  available  as  independent  code
       points  in  the  UTF-8  encoding.  (In other words, the whole surrogate
       thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)

       If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return
       (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
       that your strings are valid, and therefore want to skip these checks in
       order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
       compile time or at run time, PCRE assumes that the pattern  or  subject
       it  is  given  (respectively)  contains only valid UTF-8 codes. In this
       case, it does not diagnose an invalid UTF-8 string.

       If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,
       what  happens  depends on why the string is invalid. If the string con-
       forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
       string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,
       apart from the initial validity test, PCRE (when in UTF-8 mode) handles
       strings  according  to  the more liberal rules of RFC 2279. However, if
       the string does not even conform to RFC 2279, the result is  undefined.
       Your program may crash.

       If  you  want  to  process  strings  of  values  in the full range 0 to
       0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
       set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
       this situation, you will have to apply your own validity check.

   General comments about UTF-8 mode

       1. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
       two-byte UTF-8 character if the value is greater than 127.

       2.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
       characters for values greater than \177.

       3. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
       vidual bytes, for example: \x{100}{3}.

       4.  The dot metacharacter matches one UTF-8 character instead of a sin-
       gle byte.

       5. The escape sequence \C can be used to match a single byte  in  UTF-8
       mode,  but  its  use can lead to some strange effects. This facility is
       not available in the alternative matching function, pcre_dfa_exec().

       6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
       test  characters of any code value, but the characters that PCRE recog-
       nizes as digits, spaces, or word characters  remain  the  same  set  as
       before, all with values less than 256. This remains true even when PCRE
       includes Unicode property support, because to do otherwise  would  slow
       down  PCRE in many common cases. If you really want to test for a wider
       sense of, say, "digit", you must use Unicode  property  tests  such  as
       \p{Nd}.

       7.  Similarly,  characters that match the POSIX named character classes
       are all low-valued characters.

       8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
       escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
       acters.

       9. Case-insensitive matching applies only to  characters  whose  values
       are  less than 128, unless PCRE is built with Unicode property support.
       Even when Unicode property support is available, PCRE  still  uses  its
       own  character  tables when checking the case of low-valued characters,
       so as not to degrade performance.  The Unicode property information  is
       used only for characters with higher values. Even when Unicode property
       support is available, PCRE supports case-insensitive matching only when
       there  is  a  one-to-one  mapping between a letter's cases. There are a
       small number of many-to-one mappings in Unicode;  these  are  not  sup-
       ported by PCRE.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

       Putting  an actual email address here seems to have been a spam magnet,
       so I've taken it away. If you want to email me, use  my  two  initials,
       followed by the two digits 10, at the domain cam.ac.uk.


REVISION

       Last updated: 09 August 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREBUILD(3)                                                      PCREBUILD(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE BUILD-TIME OPTIONS

       This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. It assumes use of the  configure
       script,  where the optional features are selected or deselected by pro-
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of  CMakeSetup  if  you  are  using
       CMake instead of configure to build PCRE.

       The complete list of options for configure (which includes the standard
       ones such as the  selection  of  the  installation  directory)  can  be
       obtained by running

         ./configure --help

       The  following  sections  include  descriptions  of options whose names
       begin with --enable or --disable. These settings specify changes to the
       defaults  for  the configure command. Because of the way that configure
       works, --enable and --disable always come in pairs, so  the  complemen-
       tary  option always exists as well, but as it specifies the default, it
       is not described.


C++ SUPPORT

       By default, the configure script will search for a C++ compiler and C++
       header files. If it finds them, it automatically builds the C++ wrapper
       library for PCRE. You can disable this by adding

         --disable-cpp

       to the configure command.


UTF-8 SUPPORT

       To build PCRE with support for UTF-8 character strings, add

         --enable-utf8

       to the configure command. Of itself, this  does  not  make  PCRE  treat
       strings  as UTF-8. As well as compiling PCRE with this option, you also
       have have to set the PCRE_UTF8 option when you call the  pcre_compile()
       function.


UNICODE CHARACTER PROPERTY SUPPORT

       UTF-8  support allows PCRE to process character values greater than 255
       in the strings that it handles. On its own, however, it does  not  pro-
       vide any facilities for accessing the properties of such characters. If
       you want to be able to use the pattern escapes \P, \p,  and  \X,  which
       refer to Unicode character properties, you must add

         --enable-unicode-properties

       to  the configure command. This implies UTF-8 support, even if you have
       not explicitly requested it.

       Including Unicode property support adds around 30K  of  tables  to  the
       PCRE  library.  Only  the general category properties such as Lu and Nd
       are supported. Details are given in the pcrepattern documentation.


CODE VALUE OF NEWLINE

       By default, PCRE interprets character 10 (linefeed, LF)  as  indicating
       the  end  of  a line. This is the normal newline character on Unix-like
       systems. You can compile PCRE to use character 13 (carriage return, CR)
       instead, by adding

         --enable-newline-is-cr

       to  the  configure  command.  There  is  also  a --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which  causes  PCRE  to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

         --enable-newline-is-any

       causes PCRE to recognize any Unicode newline sequence.

       Whatever  line  ending convention is selected when PCRE is built can be
       overridden when the library functions are called. At build time  it  is
       conventional to use the standard for your operating system.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line  ending  sequence.  If
       you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.


BUILDING SHARED AND STATIC LIBRARIES

       The  PCRE building process uses libtool to build both shared and static
       Unix libraries by default. You can suppress one of these by adding  one
       of

         --disable-shared
         --disable-static

       to the configure command, as required.


POSIX MALLOC USAGE

       When PCRE is called through the POSIX interface (see the pcreposix doc-
       umentation), additional working storage is  required  for  holding  the
       pointers  to capturing substrings, because PCRE requires three integers
       per substring, whereas the POSIX interface provides only  two.  If  the
       number of expected substrings is small, the wrapper function uses space
       on the stack, because this is faster than using malloc() for each call.
       The default threshold above which the stack is no longer used is 10; it
       can be changed by adding a setting such as

         --with-posix-malloc-threshold=20

       to the configure command.


HANDLING VERY LARGE PATTERNS

       Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, two-byte values are used  for  these
       offsets,  leading  to  a  maximum size for a compiled pattern of around
       64K. This is sufficient to handle all but the most  gigantic  patterns.
       Nevertheless,  some  people do want to process enormous patterns, so it
       is possible to compile PCRE to use three-byte or four-byte  offsets  by
       adding a setting such as

         --with-link-size=3

       to  the  configure  command.  The value given must be 2, 3, or 4. Using
       longer offsets slows down the operation of PCRE because it has to  load
       additional bytes when handling them.


AVOIDING EXCESSIVE STACK USAGE

       When matching with the pcre_exec() function, PCRE implements backtrack-
       ing by making recursive calls to an internal function  called  match().
       In  environments  where  the size of the stack is limited, this can se-
       verely limit PCRE's operation. (The Unix environment does  not  usually
       suffer from this problem, but it may sometimes be necessary to increase
       the maximum stack size.  There is a discussion in the  pcrestack  docu-
       mentation.)  An alternative approach to recursion that uses memory from
       the heap to remember data, instead of using recursive  function  calls,
       has  been  implemented to work round the problem of limited stack size.
       If you want to build a version of PCRE that works this way, add

         --disable-stack-for-recursion

       to the configure command. With this configuration, PCRE  will  use  the
       pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
       ment functions. By default these point to malloc() and free(), but  you
       can replace the pointers so that your own functions are used.

       Separate  functions  are  provided  rather  than  using pcre_malloc and
       pcre_free because the  usage  is  very  predictable:  the  block  sizes
       requested  are  always  the  same,  and  the blocks are always freed in
       reverse order. A calling program might be able to  implement  optimized
       functions  that  perform  better  than  malloc()  and free(). PCRE runs
       noticeably more slowly when built in this way. This option affects only
       the   pcre_exec()   function;   it   is   not   relevant  for  the  the
       pcre_dfa_exec() function.


LIMITING PCRE RESOURCE USAGE

       Internally, PCRE has a function called match(), which it calls  repeat-
       edly   (sometimes   recursively)  when  matching  a  pattern  with  the
       pcre_exec() function. By controlling the maximum number of  times  this
       function  may be called during a single matching operation, a limit can
       be placed on the resources used by a single call  to  pcre_exec().  The
       limit  can be changed at run time, as described in the pcreapi documen-
       tation. The default is 10 million, but this can be changed by adding  a
       setting such as

         --with-match-limit=500000

       to   the   configure  command.  This  setting  has  no  effect  on  the
       pcre_dfa_exec() matching function.

       In some environments it is desirable to limit the  depth  of  recursive
       calls of match() more strictly than the total number of calls, in order
       to restrict the maximum amount of stack (or heap,  if  --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it defaults to the value that  is  set  for  --with-match-limit,  which
       imposes  no  additional constraints. However, you can set a lower limit
       by adding, for example,

         --with-match-limit-recursion=10000

       to the configure command. This value can  also  be  overridden  at  run
       time.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE  uses fixed tables for processing characters whose code values are
       less than 256. By default, PCRE is built with a set of tables that  are
       distributed  in  the  file pcre_chartables.c.dist. These tables are for
       ASCII codes only. If you add

         --enable-rebuild-chartables

       to the configure command, the distributed tables are  no  longer  used.
       Instead,  a  program  called dftables is compiled and run. This outputs
       the source for new set of tables, created in the default locale of your
       C runtime system. (This method of replacing the tables does not work if
       you are cross compiling, because dftables is run on the local host.  If
       you  need  to  create alternative tables when cross compiling, you will
       have to do so "by hand".)


USING EBCDIC CODE

       PCRE assumes by default that it will run in an  environment  where  the
       character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
       This is the case for most computer operating systems.  PCRE  can,  how-
       ever, be compiled to run in an EBCDIC environment by adding

         --enable-ebcdic

       to the configure command. This setting implies --enable-rebuild-charta-
       bles. You should only use it if you know that  you  are  in  an  EBCDIC
       environment (for example, an IBM mainframe operating system).


PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of

         --enable-pcregrep-libz
         --enable-pcregrep-libbz2

       to the configure command. These options naturally require that the rel-
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.


PCRETEST OPTION FOR LIBREADLINE SUPPORT

       If you add

         --enable-pcretest-libreadline

       to  the  configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licenced, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.


SEE ALSO

       pcreapi(3), pcre_config(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 18 December 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREMATCHING(3)                                                PCREMATCHING(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE for matching a compiled regular expression against a given sub-
       ject  string.  The  "standard"  algorithm  is  the  one provided by the
       pcre_exec() function.  This works in the same was  as  Perl's  matching
       function, and provides a Perl-compatible matching operation.

       An  alternative  algorithm is provided by the pcre_dfa_exec() function;
       this operates in a different way, and is not  Perl-compatible.  It  has
       advantages  and disadvantages compared with the standard algorithm, and
       these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  In PCRE, there is an option to stop the algorithm after the first
       match (which is necessarily the shortest) has been found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)

       is  matched  against the string "the caterpillar catchment", the result
       will be the three strings "cat", "cater", and "caterpillar" that  start
       at the fourth character of the subject. The algorithm does not automat-
       ically move on to find matches that start at later positions.

       There are a number of features of PCRE regular expressions that are not
       supported by the alternative matching algorithm. They are as follows:

       1.  Because  the  algorithm  finds  all possible matches, the greedy or
       ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
       ungreedy quantifiers are treated in exactly the same way. However, pos-
       sessive quantifiers can make a difference when what follows could  also
       match what is quantified, for example in a pattern like this:

         ^a++\w!

       This  pattern matches "aaab!" but not "aaa!", which would be matched by
       a non-possessive quantifier. Similarly, if an atomic group is  present,
       it  is matched as if it were a standalone pattern at the current point,
       and the longest match is then "locked in" for the rest of  the  overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is not straightforward to keep track of  captured  substrings  for  the
       different  matching  possibilities,  and  PCRE's implementation of this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3.  Because no substrings are captured, back references within the pat-
       tern are not supported, and cause errors if encountered.

       4. For the same reason, conditional expressions that use  a  backrefer-
       ence  as  the  condition or test for a specific group recursion are not
       supported.

       5. Because many paths through the tree may be  active,  the  \K  escape
       sequence, which resets the start of the match when encountered (but may
       be on some paths and not on others), is not  supported.  It  causes  an
       error if encountered.

       6.  Callouts  are  supported, but the value of the capture_top field is
       always 1, and the value of the capture_last field is always -1.

       7. The \C escape sequence, which (in the standard algorithm) matches  a
       single  byte, even in UTF-8 mode, is not supported because the alterna-
       tive algorithm moves through the subject  string  one  character  at  a
       time, for all active paths through the tree.

       8.  None  of  the  backtracking control verbs such as (*PRUNE) are sup-
       ported.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using the alternative matching algorithm provides the following  advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically found, and in particular, the longest match is  found.  To  find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2. There is much better support for partial matching. The  restrictions
       on  the content of the pattern that apply when using the standard algo-
       rithm for partial matching do not apply to the  alternative  algorithm.
       For  non-anchored patterns, the starting position of a partial match is
       available.

       3. Because the alternative algorithm  scans  the  subject  string  just
       once,  and  never  needs to backtrack, it is possible to pass very long
       subject strings to the matching function in  several  pieces,  checking
       for partial matching each time.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1.  It  is  substantially  slower  than the standard algorithm. This is
       partly because it has to search for all possible matches, but  is  also
       because it is less susceptible to optimization.

       2. Capturing parentheses and back references are not supported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 August 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREAPI(3)                                                          PCREAPI(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE NATIVE API

       #include <pcre.h>

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre_extra *pcre_study(const pcre *code, int options,
            const char **errptr);

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       void pcre_free_substring(const char *stringptr);

       void pcre_free_substring_list(const char **stringptr);

       const unsigned char *pcre_maketables(void);

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

       int pcre_refcount(pcre *code, int adjust);

       int pcre_config(int what, void *where);

       char *pcre_version(void);

       void *(*pcre_malloc)(size_t);

       void (*pcre_free)(void *);

       void *(*pcre_stack_malloc)(size_t);

       void (*pcre_stack_free)(void *);

       int (*pcre_callout)(pcre_callout_block *);


PCRE API OVERVIEW

       PCRE has its own native API, which is described in this document. There
       are also some wrapper functions that correspond to  the  POSIX  regular
       expression  API.  These  are  described in the pcreposix documentation.
       Both of these APIs define a set of C function calls. A C++  wrapper  is
       distributed with PCRE. It is documented in the pcrecpp page.

       The  native  API  C  function prototypes are defined in the header file
       pcre.h, and on Unix systems the library itself is called  libpcre.   It
       can normally be accessed by adding -lpcre to the command for linking an
       application  that  uses  PCRE.  The  header  file  defines  the  macros
       PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
       bers for the library.  Applications can use these  to  include  support
       for different releases of PCRE.

       The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
       pcre_exec() are used for compiling and matching regular expressions  in
       a  Perl-compatible  manner. A sample program that demonstrates the sim-
       plest way of using them is provided in the file  called  pcredemo.c  in
       the  source distribution. The pcresample documentation describes how to
       compile and run it.

       A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble,  is  also provided. This uses a different algorithm for the match-
       ing. The alternative algorithm finds all possible matches (at  a  given
       point  in  the subject), and scans the subject just once. However, this
       algorithm does not return captured substrings. A description of the two
       matching  algorithms and their advantages and disadvantages is given in
       the pcrematching documentation.

       In addition to the main compiling and  matching  functions,  there  are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

         pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

       pcre_free_substring() and pcre_free_substring_list() are also provided,
       to free the memory used for extracted strings.

       The  function  pcre_maketables()  is  used  to build a set of character
       tables  in  the  current  locale   for   passing   to   pcre_compile(),
       pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
       provided for specialist use.  Most  commonly,  no  special  tables  are
       passed,  in  which case internal tables that are generated when PCRE is
       built are used.

       The function pcre_fullinfo() is used to find out  information  about  a
       compiled  pattern; pcre_info() is an obsolete version that returns only
       some of the available information, but is retained for  backwards  com-
       patibility.   The function pcre_version() returns a pointer to a string
       containing the version of PCRE and its date of release.

       The function pcre_refcount() maintains a  reference  count  in  a  data
       block  containing  a compiled pattern. This is provided for the benefit
       of object-oriented applications.

       The global variables pcre_malloc and pcre_free  initially  contain  the
       entry  points  of  the  standard malloc() and free() functions, respec-
       tively. PCRE calls the memory management functions via these variables,
       so  a  calling  program  can replace them if it wishes to intercept the
       calls. This should be done before calling any PCRE functions.

       The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
       indirections  to  memory  management functions. These special functions
       are used only when PCRE is compiled to use  the  heap  for  remembering
       data, instead of recursive function calls, when running the pcre_exec()
       function. See the pcrebuild documentation for  details  of  how  to  do
       this.  It  is  a non-standard way of building PCRE, for use in environ-
       ments that have limited stacks. Because of the greater  use  of  memory
       management,  it  runs  more  slowly. Separate functions are provided so
       that special-purpose external code can be  used  for  this  case.  When
       used,  these  functions  are always called in a stack-like manner (last
       obtained, first freed), and always for memory blocks of the same  size.
       There  is  a discussion about PCRE's stack usage in the pcrestack docu-
       mentation.

       The global variable pcre_callout initially contains NULL. It can be set
       by  the  caller  to  a "callout" function, which PCRE will then call at
       specified points during a matching operation. Details are given in  the
       pcrecallout documentation.


NEWLINES

       PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each  of  the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE is built, a  default
       can  be  specified.  The default default is LF, which is the Unix stan-
       dard. When PCRE is run, the default can be overridden,  either  when  a
       pattern is compiled, or when it is matched.

       At compile time, the newline convention can be specified by the options
       argument of pcre_compile(), or it can be specified by special  text  at
       the start of the pattern itself; this overrides any other settings. See
       the pcrepattern page for details of the special character sequences.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter  or pair of characters that indicate a line break". The choice of
       newline convention affects the handling of  the  dot,  circumflex,  and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre_exec() options below.

       The choice of newline convention does not affect the interpretation  of
       the  \n  or  \r  escape  sequences, nor does it affect what \R matches,
       which is controlled in a similar way, but by separate options.


MULTITHREADING

       The PCRE functions can be used in  multi-threading  applications,  with
       the  proviso  that  the  memory  management  functions  pointed  to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, are shared by all threads.

       The compiled form of a regular expression is not altered during  match-
       ing, so the same compiled pattern can safely be used by several threads
       at once.


SAVING PRECOMPILED PATTERNS FOR LATER USE

       The compiled form of a regular expression can be saved and re-used at a
       later  time,  possibly by a different program, and even on a host other
       than the one on which  it  was  compiled.  Details  are  given  in  the
       pcreprecompile  documentation.  However, compiling a regular expression
       with one version of PCRE for use with a different version is not  guar-
       anteed to work and may cause crashes.


CHECKING BUILD-TIME OPTIONS

       int pcre_config(int what, void *where);

       The  function pcre_config() makes it possible for a PCRE client to dis-
       cover which optional features have been compiled into the PCRE library.
       The  pcrebuild documentation has more details about these optional fea-
       tures.

       The first argument for pcre_config() is an  integer,  specifying  which
       information is required; the second argument is a pointer to a variable
       into which the information is  placed.  The  following  information  is
       available:

         PCRE_CONFIG_UTF8

       The  output is an integer that is set to one if UTF-8 support is avail-
       able; otherwise it is set to zero.

         PCRE_CONFIG_UNICODE_PROPERTIES

       The output is an integer that is set to  one  if  support  for  Unicode
       character properties is available; otherwise it is set to zero.

         PCRE_CONFIG_NEWLINE

       The  output  is  an integer whose value specifies the default character
       sequence that is recognized as meaning "newline". The four values  that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and -1 for ANY. The default should normally be  the  standard  sequence
       for your operating system.

         PCRE_CONFIG_BSR

       The output is an integer whose value indicates what character sequences
       the \R escape sequence matches by default. A value of 0 means  that  \R
       matches  any  Unicode  line ending sequence; a value of 1 means that \R
       matches only CR, LF, or CRLF. The default can be overridden when a pat-
       tern is compiled or matched.

         PCRE_CONFIG_LINK_SIZE

       The  output  is  an  integer that contains the number of bytes used for
       internal linkage in compiled regular expressions. The value is 2, 3, or
       4.  Larger  values  allow larger regular expressions to be compiled, at
       the expense of slower matching. The default value of  2  is  sufficient
       for  all  but  the  most massive patterns, since it allows the compiled
       pattern to be up to 64K in size.

         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

       The output is an integer that contains the threshold  above  which  the
       POSIX  interface  uses malloc() for output vectors. Further details are
       given in the pcreposix documentation.

         PCRE_CONFIG_MATCH_LIMIT

       The output is an integer that gives the default limit for the number of
       internal  matching  function  calls in a pcre_exec() execution. Further
       details are given with pcre_exec() below.

         PCRE_CONFIG_MATCH_LIMIT_RECURSION

       The output is an integer that gives the default limit for the depth  of
       recursion  when calling the internal matching function in a pcre_exec()
       execution. Further details are given with pcre_exec() below.

         PCRE_CONFIG_STACKRECURSE

       The output is an integer that is set to one if internal recursion  when
       running pcre_exec() is implemented by recursive function calls that use
       the stack to remember their state. This is the usual way that  PCRE  is
       compiled. The output is zero if PCRE was compiled to use blocks of data
       on the  heap  instead  of  recursive  function  calls.  In  this  case,
       pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
       blocks on the heap, thus avoiding the use of the stack.


COMPILING A PATTERN

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the two interfaces is that pcre_compile2() has an additional  argument,
       errorcodeptr, via which a numerical error code can be returned.

       The pattern is a C string terminated by a binary zero, and is passed in
       the pattern argument. A pointer to a single block  of  memory  that  is
       obtained  via  pcre_malloc is returned. This contains the compiled code
       and related data. The pcre type is defined for the returned block; this
       is a typedef for a structure whose contents are not externally defined.
       It is up to the caller to free the memory (via pcre_free) when it is no
       longer required.

       Although  the compiled code of a PCRE regex is relocatable, that is, it
       does not depend on memory location, the complete pcre data block is not
       fully  relocatable, because it may contain a copy of the tableptr argu-
       ment, which is an address (see below).

       The options argument contains various bit settings that affect the com-
       pilation.  It  should be zero if no options are required. The available
       options are described below. Some of them, in  particular,  those  that
       are  compatible  with  Perl,  can also be set and unset from within the
       pattern (see the detailed description  in  the  pcrepattern  documenta-
       tion).  For  these options, the contents of the options argument speci-
       fies their initial settings at the start of compilation and  execution.
       The  PCRE_ANCHORED  and PCRE_NEWLINE_xxx options can be set at the time
       of matching as well as at compile time.

       If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
       sets the variable pointed to by errptr to point to a textual error mes-
       sage. This is a static string that is part of the library. You must not
       try to free it. The offset from the start of the pattern to the charac-
       ter where the error was discovered is placed in the variable pointed to
       by erroffset, which must not be NULL. If it is, an immediate  error  is
       given.

       If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
       codeptr argument is not NULL, a non-zero error code number is  returned
       via  this argument in the event of an error. This is in addition to the
       textual error message. Error codes and messages are listed below.

       If the final argument, tableptr, is NULL, PCRE uses a  default  set  of
       character  tables  that  are  built  when  PCRE  is compiled, using the
       default C locale. Otherwise, tableptr must be an address  that  is  the
       result  of  a  call to pcre_maketables(). This value is stored with the
       compiled pattern, and used again by pcre_exec(), unless  another  table
       pointer is passed to it. For more discussion, see the section on locale
       support below.

       This code fragment shows a typical straightforward  call  to  pcre_com-
       pile():

         pcre *re;
         const char *error;
         int erroffset;
         re = pcre_compile(
           "^A.*Z",          /* the pattern */
           0,                /* default options */
           &error,           /* for error message */
           &erroffset,       /* for error offset */
           NULL);            /* use default character tables */

       The  following  names  for option bits are defined in the pcre.h header
       file:

         PCRE_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

         PCRE_AUTO_CALLOUT

       If this bit is set, pcre_compile() automatically inserts callout items,
       all  with  number  255, before each pattern item. For discussion of the
       callout facility, see the pcrecallout documentation.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. The default is specified when
       PCRE is built. It can be overridden from within the pattern, or by set-
       ting an option when a compiled pattern is matched.

         PCRE_CASELESS

       If this bit is set, letters in the pattern match both upper  and  lower
       case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
       changed within a pattern by a (?i) option setting. In UTF-8 mode,  PCRE
       always  understands the concept of case for characters whose values are
       less than 128, so caseless matching is always possible. For  characters
       with  higher  values,  the concept of case is supported if PCRE is com-
       piled with Unicode property support, but not otherwise. If you want  to
       use  caseless  matching  for  characters 128 and above, you must ensure
       that PCRE is compiled with Unicode property support  as  well  as  with
       UTF-8 support.

         PCRE_DOLLAR_ENDONLY

       If  this bit is set, a dollar metacharacter in the pattern matches only
       at the end of the subject string. Without this option,  a  dollar  also
       matches  immediately before a newline at the end of the string (but not
       before any other newlines). The PCRE_DOLLAR_ENDONLY option  is  ignored
       if  PCRE_MULTILINE  is  set.   There is no equivalent to this option in
       Perl, and no way to set it within a pattern.

         PCRE_DOTALL

       If this bit is set, a dot metacharater in the pattern matches all char-
       acters,  including  those that indicate newline. Without it, a dot does
       not match when the current position is at a  newline.  This  option  is
       equivalent  to Perl's /s option, and it can be changed within a pattern
       by a (?s) option setting. A negative class such as [^a] always  matches
       newline characters, independent of the setting of this option.

         PCRE_DUPNAMES

       If  this  bit is set, names used to identify capturing subpatterns need
       not be unique. This can be helpful for certain types of pattern when it
       is  known  that  only  one instance of the named subpattern can ever be
       matched. There are more details of named subpatterns  below;  see  also
       the pcrepattern documentation.

         PCRE_EXTENDED

       If  this  bit  is  set,  whitespace  data characters in the pattern are
       totally ignored except when escaped or inside a character class. White-
       space does not include the VT character (code 11). In addition, charac-
       ters between an unescaped # outside a character class and the next new-
       line,  inclusive,  are  also  ignored.  This is equivalent to Perl's /x
       option, and it can be changed within a pattern by a  (?x)  option  set-
       ting.

       This  option  makes  it possible to include comments inside complicated
       patterns.  Note, however, that this applies only  to  data  characters.
       Whitespace   characters  may  never  appear  within  special  character
       sequences in a pattern, for  example  within  the  sequence  (?(  which
       introduces a conditional subpattern.

         PCRE_EXTRA

       This  option  was invented in order to turn on additional functionality
       of PCRE that is incompatible with Perl, but it  is  currently  of  very
       little  use. When set, any backslash in a pattern that is followed by a
       letter that has no special meaning  causes  an  error,  thus  reserving
       these  combinations  for  future  expansion.  By default, as in Perl, a
       backslash followed by a letter with no special meaning is treated as  a
       literal.  (Perl can, however, be persuaded to give a warning for this.)
       There are at present no other features controlled by  this  option.  It
       can also be set by a (?X) option setting within a pattern.

         PCRE_FIRSTLINE

       If  this  option  is  set,  an  unanchored pattern is required to match
       before or at the first  newline  in  the  subject  string,  though  the
       matched text may continue over the newline.

         PCRE_MULTILINE

       By  default,  PCRE  treats the subject string as consisting of a single
       line of characters (even if it actually contains newlines). The  "start
       of  line"  metacharacter  (^)  matches only at the start of the string,
       while the "end of line" metacharacter ($) matches only at  the  end  of
       the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
       is set). This is the same as Perl.

       When PCRE_MULTILINE it is set, the "start of line" and  "end  of  line"
       constructs  match  immediately following or immediately before internal
       newlines in the subject string, respectively, as well as  at  the  very
       start  and  end.  This is equivalent to Perl's /m option, and it can be
       changed within a pattern by a (?m) option setting. If there are no new-
       lines  in  a  subject string, or no occurrences of ^ or $ in a pattern,
       setting PCRE_MULTILINE has no effect.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These options override the default newline definition that  was  chosen
       when  PCRE  was built. Setting the first or the second specifies that a
       newline is indicated by a single character (CR  or  LF,  respectively).
       Setting  PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
       two-character CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF  specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should  be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus the single characters VT (vertical  tab,  U+000B),  FF  (formfeed,
       U+000C),  NEL  (next line, U+0085), LS (line separator, U+2028), and PS
       (paragraph separator, U+2029). The last  two  are  recognized  only  in
       UTF-8 mode.

       The  newline  setting  in  the  options  word  uses three bits that are
       treated as a number, giving eight possibilities. Currently only six are
       used  (default  plus the five values above). This means that if you set
       more than one newline option, the combination may or may not be  sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers  and
       cause an error.

       The  only time that a line break is specially recognized when compiling
       a pattern is if PCRE_EXTENDED is set, and  an  unescaped  #  outside  a
       character  class  is  encountered.  This indicates a comment that lasts
       until after the next line break sequence. In other circumstances,  line
       break   sequences   are   treated  as  literal  data,  except  that  in
       PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
       and are therefore ignored.

       The newline option that is set at compile time becomes the default that
       is used for pcre_exec() and pcre_dfa_exec(), but it can be  overridden.

         PCRE_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses in the pattern. Any opening parenthesis that is not followed  by
       ?  behaves as if it were followed by ?: but named parentheses can still
       be used for capturing (and they acquire  numbers  in  the  usual  way).
       There is no equivalent of this option in Perl.

         PCRE_UNGREEDY

       This  option  inverts  the "greediness" of the quantifiers so that they
       are not greedy by default, but become greedy if followed by "?". It  is
       not  compatible  with Perl. It can also be set by a (?U) option setting
       within the pattern.

         PCRE_UTF8

       This option causes PCRE to regard both the pattern and the  subject  as
       strings  of  UTF-8 characters instead of single-byte character strings.
       However, it is available only when PCRE is built to include UTF-8  sup-
       port.  If not, the use of this option provokes an error. Details of how
       this option changes the behaviour of PCRE are given in the  section  on
       UTF-8 support in the main pcre page.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically checked. There is a  discussion  about  the  validity  of
       UTF-8  strings  in  the main pcre page. If an invalid UTF-8 sequence of
       bytes is found, pcre_compile() returns an error. If  you  already  know
       that your pattern is valid, and you want to skip this check for perfor-
       mance reasons, you can set the PCRE_NO_UTF8_CHECK option.  When  it  is
       set,  the  effect  of  passing  an invalid UTF-8 string as a pattern is
       undefined. It may cause your program to crash. Note  that  this  option
       can  also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
       UTF-8 validity checking of subject strings.


COMPILATION ERROR CODES

       The following table lists the error  codes  than  may  be  returned  by
       pcre_compile2(),  along with the error messages that may be returned by
       both compiling functions. As PCRE has developed, some error codes  have
       fallen out of use. To avoid confusion, they have not been re-used.

          0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (? or (?-
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  [this code is not in use]
         20  regular expression is too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?[+-]digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is not compiled with PCRE_UTF8 support
         33  [this code is not in use]
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error in subpattern name (missing terminator)
         43  two named subpatterns have the same name
         44  invalid UTF-8 string
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10000)
         50  [this code is not in use]
         51  octal value is greater than \377 (not in UTF-8 mode)
         52  internal error: overran compiling workspace
         53   internal  error:  previously-checked  referenced  subpattern not
       found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options
         57  \g is not followed by a braced name or an optionally braced
               non-zero number
         58  (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
         59  (*VERB) with an argument is not supported
         60  (*VERB) not recognized
         61  number is too big
         62  subpattern name expected
         63  digit expected after (?+

       The numbers 32 and 10000 in errors 48 and 49  are  defaults;  different
       values may be used if the limits were changed when PCRE was built.


STUDYING A PATTERN

       pcre_extra *pcre_study(const pcre *code, int options
            const char **errptr);

       If  a  compiled  pattern is going to be used several times, it is worth
       spending more time analyzing it in order to speed up the time taken for
       matching.  The function pcre_study() takes a pointer to a compiled pat-
       tern as its first argument. If studying the pattern produces additional
       information  that  will  help speed up matching, pcre_study() returns a
       pointer to a pcre_extra block, in which the study_data field points  to
       the results of the study.

       The  returned  value  from  pcre_study()  can  be  passed  directly  to
       pcre_exec(). However, a pcre_extra block  also  contains  other  fields
       that  can  be  set  by the caller before the block is passed; these are
       described below in the section on matching a pattern.

       If studying the pattern does not  produce  any  additional  information
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants to pass any of the other fields to pcre_exec(), it  must  set  up
       its own pcre_extra block.

       The  second  argument of pcre_study() contains option bits. At present,
       no options are defined, and this argument should always be zero.

       The third argument for pcre_study() is a pointer for an error  message.
       If  studying  succeeds  (even  if no data is returned), the variable it
       points to is set to NULL. Otherwise it is set to  point  to  a  textual
       error message. This is a static string that is part of the library. You
       must not try to free it. You should test the  error  pointer  for  NULL
       after calling pcre_study(), to be sure that it has run successfully.

       This is a typical call to pcre_study():

         pcre_extra *pe;
         pe = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options exist */
           &error);        /* set to NULL or points to a message */

       At present, studying a pattern is useful only for non-anchored patterns
       that do not have a single fixed starting character. A bitmap of  possi-
       ble starting bytes is created.


LOCALE SUPPORT

       PCRE  handles  caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables,  indexed
       by  character  value.  When running in UTF-8 mode, this applies only to
       characters with codes less than 128. Higher-valued  codes  never  match
       escapes  such  as  \w or \d, but can be tested with \p if PCRE is built
       with Unicode character property support. The use of locales  with  Uni-
       code  is discouraged. If you are handling characters with codes greater
       than 128, you should either use UTF-8 and Unicode, or use locales,  but
       not try to mix the two.

       PCRE  contains  an  internal set of tables that are used when the final
       argument of pcre_compile() is  NULL.  These  are  sufficient  for  many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

       The internal tables can always be overridden by tables supplied by  the
       application that calls PCRE. These may be created in a different locale
       from the default. As more and more applications change  to  using  Uni-
       code, the need for this locale support is expected to die away.

       External  tables  are  built by calling the pcre_maketables() function,
       which has no arguments, in the relevant locale. The result can then  be
       passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
       example, to build and use tables that are appropriate  for  the  French
       locale  (where  accented  characters  with  values greater than 128 are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre_maketables();
         re = pcre_compile(..., tables);

       The locale name "fr_FR" is used on Linux and other  Unix-like  systems;
       if you are using Windows, the name for the French locale is "french".

       When  pcre_maketables()  runs,  the  tables are built in memory that is
       obtained via pcre_malloc. It is the caller's responsibility  to  ensure
       that  the memory containing the tables remains available for as long as
       it is needed.

       The pointer that is passed to pcre_compile() is saved with the compiled
       pattern,  and the same tables are used via this pointer by pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

       It is possible to pass a table pointer or NULL (indicating the  use  of
       the  internal  tables)  to  pcre_exec(). Although not intended for this
       purpose, this facility could be used to match a pattern in a  different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.


INFORMATION ABOUT A PATTERN

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       The pcre_fullinfo() function returns information about a compiled  pat-
       tern. It replaces the obsolete pcre_info() function, which is neverthe-
       less retained for backwards compability (and is documented below).

       The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
       pattern.  The second argument is the result of pcre_study(), or NULL if
       the pattern was not studied. The third argument specifies  which  piece
       of  information  is required, and the fourth argument is a pointer to a
       variable to receive the data. The yield of the  function  is  zero  for
       success, or one of the following negative numbers:

         PCRE_ERROR_NULL       the argument code was NULL
                               the argument where was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found
         PCRE_ERROR_BADOPTION  the value of what was invalid

       The  "magic  number" is placed at the start of each compiled pattern as
       an simple check against passing an arbitrary memory pointer. Here is  a
       typical  call  of pcre_fullinfo(), to obtain the length of the compiled
       pattern:

         int rc;
         size_t length;
         rc = pcre_fullinfo(
           re,               /* result of pcre_compile() */
           pe,               /* result of pcre_study(), or NULL */
           PCRE_INFO_SIZE,   /* what is required */
           &length);         /* where to put the data */

       The possible values for the third argument are defined in  pcre.h,  and
       are as follows:

         PCRE_INFO_BACKREFMAX

       Return  the  number  of  the highest back reference in the pattern. The
       fourth argument should point to an int variable. Zero  is  returned  if
       there are no back references.

         PCRE_INFO_CAPTURECOUNT

       Return  the  number of capturing subpatterns in the pattern. The fourth
       argument should point to an int variable.

         PCRE_INFO_DEFAULT_TABLES

       Return a pointer to the internal default character tables within  PCRE.
       The  fourth  argument should point to an unsigned char * variable. This
       information call is provided for internal use by the pcre_study() func-
       tion.  External  callers  can  cause PCRE to use its internal tables by
       passing a NULL table pointer.

         PCRE_INFO_FIRSTBYTE

       Return information about the first byte of any matched  string,  for  a
       non-anchored  pattern. The fourth argument should point to an int vari-
       able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old  name
       is still recognized for backwards compatibility.)

       If  there  is  a  fixed first byte, for example, from a pattern such as
       (cat|cow|coyote), its value is returned. Otherwise, if either

       (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
       branch starts with "^", or

       (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
       set (if it were set, the pattern would be anchored),

       -1 is returned, indicating that the pattern matches only at  the  start
       of  a  subject string or after any newline within the string. Otherwise
       -2 is returned. For anchored patterns, -2 is returned.

         PCRE_INFO_FIRSTTABLE

       If the pattern was studied, and this resulted in the construction of  a
       256-bit table indicating a fixed set of bytes for the first byte in any
       matching string, a pointer to the table is returned. Otherwise NULL  is
       returned.  The fourth argument should point to an unsigned char * vari-
       able.

         PCRE_INFO_HASCRORLF

       Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
       characters,  otherwise  0.  The  fourth argument should point to an int
       variable. An explicit match is either a literal CR or LF character,  or
       \r or \n.

         PCRE_INFO_JCHANGED

       Return  1  if  the (?J) or (?-J) option setting is used in the pattern,
       otherwise 0. The fourth argument should point to an int variable.  (?J)
       and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

         PCRE_INFO_LASTLITERAL

       Return  the  value of the rightmost literal byte that must exist in any
       matched string, other than at its  start,  if  such  a  byte  has  been
       recorded. The fourth argument should point to an int variable. If there
       is no such byte, -1 is returned. For anchored patterns, a last  literal
       byte  is  recorded only if it follows something of variable length. For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.

         PCRE_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

       PCRE  supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre_get_named_substring() are provided for  extracting  captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers in the output vector (described with pcre_exec() below). To do
       the conversion, you need  to  use  the  name-to-number  map,  which  is
       described by these three values.

       The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of  each  entry;  both  of  these  return  an int value. The entry size
       depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
       a  pointer  to  the  first  entry of the table (a pointer to char). The
       first two bytes of each entry are the number of the capturing parenthe-
       sis,  most  significant byte first. The rest of the entry is the corre-
       sponding name, zero terminated. The names are  in  alphabetical  order.
       When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
       theses numbers. For example, consider  the  following  pattern  (assume
       PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
       ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named subpatterns, so the table has  four  entries,  and
       each  entry  in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When  writing  code  to  extract  data from named subpatterns using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE_INFO_OKPARTIAL

       Return  1 if the pattern can be used for partial matching, otherwise 0.
       The fourth argument should point to an int  variable.  The  pcrepartial
       documentation  lists  the restrictions that apply to patterns when par-
       tial matching is used.

         PCRE_INFO_OPTIONS

       Return a copy of the options with which the pattern was  compiled.  The
       fourth  argument  should  point to an unsigned long int variable. These
       option bits are those specified in the call to pcre_compile(), modified
       by any top-level option settings at the start of the pattern itself. In
       other words, they are the options that will be in force  when  matching
       starts.  For  example, if the pattern /(?im)abc(?-i)d/ is compiled with
       the PCRE_EXTENDED option, the result is PCRE_CASELESS,  PCRE_MULTILINE,
       and PCRE_EXTENDED.

       A  pattern  is  automatically  anchored by PCRE if all of its top-level
       alternatives begin with one of the following:

         ^     unless PCRE_MULTILINE is set
         \A    always
         \G    always
         .*    if PCRE_DOTALL is set and there are no back
                 references to the subpattern in which .* appears

       For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().

         PCRE_INFO_SIZE

       Return  the  size  of the compiled pattern, that is, the value that was
       passed as the argument to pcre_malloc() when PCRE was getting memory in
       which to place the compiled data. The fourth argument should point to a
       size_t variable.

         PCRE_INFO_STUDYSIZE

       Return the size of the data block pointed to by the study_data field in
       a  pcre_extra  block.  That  is,  it  is  the  value that was passed to
       pcre_malloc() when PCRE was getting memory into which to place the data
       created  by  pcre_study(). The fourth argument should point to a size_t
       variable.


OBSOLETE INFO FUNCTION

       int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

       The pcre_info() function is now obsolete because its interface  is  too
       restrictive  to return all the available data about a compiled pattern.
       New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of
       pcre_info()  is the number of capturing subpatterns, or one of the fol-
       lowing negative numbers:

         PCRE_ERROR_NULL       the argument code was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found

       If the optptr argument is not NULL, a copy of the  options  with  which
       the  pattern  was  compiled  is placed in the integer it points to (see
       PCRE_INFO_OPTIONS above).

       If the pattern is not anchored and the  firstcharptr  argument  is  not
       NULL,  it is used to pass back information about the first character of
       any matched string (see PCRE_INFO_FIRSTBYTE above).


REFERENCE COUNTS

       int pcre_refcount(pcre *code, int adjust);

       The pcre_refcount() function is used to maintain a reference  count  in
       the data block that contains a compiled pattern. It is provided for the
       benefit of applications that  operate  in  an  object-oriented  manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

       When a pattern is compiled, the reference count field is initialized to
       zero.   It is changed only by calling this function, whose action is to
       add the adjust value (which may be positive or  negative)  to  it.  The
       yield of the function is the new value. However, the value of the count
       is constrained to lie between 0 and 65535, inclusive. If the new  value
       is outside these limits, it is forced to the appropriate limit value.

       Except  when it is zero, the reference count is not correctly preserved
       if a pattern is compiled on one host and then  transferred  to  a  host
       whose byte-order is different. (This seems a highly unlikely scenario.)


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       The function pcre_exec() is called to match a subject string against  a
       compiled  pattern, which is passed in the code argument. If the pattern
       has been studied, the result of the study should be passed in the extra
       argument.  This  function is the main matching facility of the library,
       and it operates in a Perl-like manner. For specialist use there is also
       an  alternative matching function, which is described below in the sec-
       tion about the pcre_dfa_exec() function.

       In most applications, the pattern will have been compiled (and  option-
       ally  studied)  in the same process that calls pcre_exec(). However, it
       is possible to save compiled patterns and study data, and then use them
       later  in  different processes, possibly even on different hosts. For a
       discussion about this, see the pcreprecompile documentation.

       Here is an example of a simple call to pcre_exec():

         int rc;
         int ovector[30];
         rc = pcre_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

   Extra data for pcre_exec()

       If the extra argument is not NULL, it must point to a  pcre_extra  data
       block.  The pcre_study() function returns such a block (when it doesn't
       return NULL), but you can also create one for yourself, and pass  addi-
       tional  information  in it. The pcre_extra block contains the following
       fields (not necessarily in this order):

         unsigned long int flags;
         void *study_data;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;

       The flags field is a bitmap that specifies which of  the  other  fields
       are set. The flag bits are:

         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_TABLES

       Other  flag  bits should be set to zero. The study_data field is set in
       the pcre_extra block that is returned by  pcre_study(),  together  with
       the appropriate flag bit. You should not set this yourself, but you may
       add to the block by setting the other fields  and  their  corresponding
       flag bits.

       The match_limit field provides a means of preventing PCRE from using up
       a vast amount of resources when running patterns that are not going  to
       match,  but  which  have  a very large number of possibilities in their
       search trees. The classic  example  is  the  use  of  nested  unlimited
       repeats.

       Internally,  PCRE uses a function called match() which it calls repeat-
       edly (sometimes recursively). The limit set by match_limit  is  imposed
       on  the  number  of times this function is called during a match, which
       has the effect of limiting the amount of  backtracking  that  can  take
       place. For patterns that are not anchored, the count restarts from zero
       for each position in the subject string.

       The default value for the limit can be set  when  PCRE  is  built;  the
       default  default  is 10 million, which handles all but the most extreme
       cases. You can override the default  by  suppling  pcre_exec()  with  a
       pcre_extra     block    in    which    match_limit    is    set,    and
       PCRE_EXTRA_MATCH_LIMIT is set in the  flags  field.  If  the  limit  is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

       The  match_limit_recursion field is similar to match_limit, but instead
       of limiting the total number of times that match() is called, it limits
       the  depth  of  recursion. The recursion depth is a smaller number than
       the total number of calls, because not all calls to match() are  recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting the recursion depth limits the amount of  stack  that  can  be
       used, or, when PCRE has been compiled to use memory on the heap instead
       of the stack, the amount of heap memory that can be used.

       The default value for match_limit_recursion can be  set  when  PCRE  is
       built;  the  default  default  is  the  same  value  as the default for
       match_limit. You can override the default by suppling pcre_exec()  with
       a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

       The  pcre_callout  field is used in conjunction with the "callout" fea-
       ture, which is described in the pcrecallout documentation.

       The tables field  is  used  to  pass  a  character  tables  pointer  to
       pcre_exec();  this overrides the value that is stored with the compiled
       pattern. A non-NULL value is stored with the compiled pattern  only  if
       custom  tables  were  supplied to pcre_compile() via its tableptr argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's  internal  tables  to be used. This facility is helpful when re-
       using patterns that have been saved after compiling  with  an  external
       set  of  tables,  because  the  external tables might be at a different
       address when pcre_exec() is called. See the  pcreprecompile  documenta-
       tion for a discussion of saving compiled patterns for later use.

   Option bits for pcre_exec()

       The  unused  bits of the options argument for pcre_exec() must be zero.
       The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
       PCRE_PARTIAL.

         PCRE_ANCHORED

       The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
       matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
       turned out to be anchored by virtue of its contents, it cannot be  made
       unachored at matching time.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence matches. The choice is either to match only CR, LF,  or  CRLF,
       or  to  match  any Unicode newline sequence. These options override the
       choice that was made or defaulted when the pattern was compiled.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These options override  the  newline  definition  that  was  chosen  or
       defaulted  when the pattern was compiled. For details, see the descrip-
       tion of pcre_compile()  above.  During  matching,  the  newline  choice
       affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
       ters. It may also alter the way the match position is advanced after  a
       match failure for an unanchored pattern.

       When  PCRE_NEWLINE_CRLF,  PCRE_NEWLINE_ANYCRLF,  or PCRE_NEWLINE_ANY is
       set, and a match attempt for an unanchored pattern fails when the  cur-
       rent  position  is  at  a  CRLF  sequence,  and the pattern contains no
       explicit matches for  CR  or  LF  characters,  the  match  position  is
       advanced by two characters instead of one, in other words, to after the
       CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected.  For  example,  if  the  pattern  is .+A (and the PCRE_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing  at the start, it skips both the CR and the LF before retrying.
       However, the pattern [\r\n]A does match that string,  because  it  con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those  characters,  or  one  of the \r or \n escape sequences. Implicit
       matches such as [^X] do not count, nor does \s (which includes  CR  and
       LF in the characters that it matches).

       Notwithstanding  the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.

         PCRE_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before it. Setting this without PCRE_MULTILINE (at compile time)
       causes circumflex never to match. This option affects only  the  behav-
       iour of the circumflex metacharacter. It does not affect \A.

         PCRE_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option  affects only the behaviour of the dollar metacharacter. It does
       not affect \Z or \z.

         PCRE_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a string not beginning with "a" or "b", it matches the
       empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
       match is not valid, so PCRE searches further into the string for occur-
       rences of "a" or "b".

       Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
       cial  case  of  a  pattern match of the empty string within its split()
       function, and when using the /g modifier. It  is  possible  to  emulate
       Perl's behaviour after matching a null string by first trying the match
       again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
       if  that  fails by advancing the starting offset (see below) and trying
       an ordinary match again. There is some code that demonstrates how to do
       this in the pcredemo.c sample program.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8 string is automatically checked when pcre_exec() is  subsequently
       called.   The  value  of  startoffset is also checked to ensure that it
       points to the start of a UTF-8 character. There is a  discussion  about
       the  validity  of  UTF-8 strings in the section on UTF-8 support in the
       main pcre page. If  an  invalid  UTF-8  sequence  of  bytes  is  found,
       pcre_exec()  returns  the error PCRE_ERROR_BADUTF8. If startoffset con-
       tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.

       If you already know that your subject is valid, and you  want  to  skip
       these    checks    for   performance   reasons,   you   can   set   the
       PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
       do  this  for the second and subsequent calls to pcre_exec() if you are
       making repeated calls to find all  the  matches  in  a  single  subject
       string.  However,  you  should  be  sure  that the value of startoffset
       points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
       set,  the  effect of passing an invalid UTF-8 string as a subject, or a
       value of startoffset that does not point to the start of a UTF-8  char-
       acter, is undefined. Your program may crash.

         PCRE_PARTIAL

       This  option  turns  on  the  partial  matching feature. If the subject
       string fails to match the pattern, but at some point during the  match-
       ing  process  the  end of the subject was reached (that is, the subject
       partially matches the pattern and the failure to  match  occurred  only
       because  there were not enough subject characters), pcre_exec() returns
       PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
       used,  there  are restrictions on what may appear in the pattern. These
       are discussed in the pcrepartial documentation.

   The string to be matched by pcre_exec()

       The subject string is passed to pcre_exec() as a pointer in subject,  a
       length  in  length, and a starting byte offset in startoffset. In UTF-8
       mode, the byte offset must point to the start  of  a  UTF-8  character.
       Unlike  the  pattern string, the subject may contain binary zero bytes.
       When the starting offset is zero, the search for a match starts at  the
       beginning of the subject, and this is by far the most common case.

       A  non-zero  starting offset is useful when searching for another match
       in the same subject by calling pcre_exec() again after a previous  suc-
       cess.   Setting  startoffset differs from just passing over a shortened
       string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which  finds  occurrences  of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not  a  word  boundary.)
       When  applied  to the string "Mississipi" the first call to pcre_exec()
       finds the first occurrence. If pcre_exec() is called  again  with  just
       the  remainder  of  the  subject,  namely  "issipi", it does not match,
       because \B is always false at the start of the subject, which is deemed
       to  be  a  word  boundary. However, if pcre_exec() is passed the entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence  of "iss" because it is able to look behind the starting point to
       discover that it is preceded by a letter.

       If a non-zero starting offset is passed when the pattern  is  anchored,
       one attempt to match at the given offset is made. This can only succeed
       if the pattern does not require the match to be at  the  start  of  the
       subject.

   How pcre_exec() returns captured substrings

       In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
       this is called "capturing" in what follows, and the  phrase  "capturing
       subpattern"  is  used for a fragment of a pattern that picks out a sub-
       string. PCRE supports several other kinds of  parenthesized  subpattern
       that do not cause substrings to be captured.

       Captured  substrings are returned to the caller via a vector of integer
       offsets whose address is passed in ovector. The number of  elements  in
       the  vector is passed in ovecsize, which must be a non-negative number.
       Note: this argument is NOT the size of ovector in bytes.

       The first two-thirds of the vector is used to pass back  captured  sub-
       strings,  each  substring using a pair of integers. The remaining third
       of the vector is used as workspace by pcre_exec() while  matching  cap-
       turing  subpatterns, and is not available for passing back information.
       The length passed in ovecsize should always be a multiple of three.  If
       it is not, it is rounded down.

       When  a  match  is successful, information about captured substrings is
       returned in pairs of integers, starting at the  beginning  of  ovector,
       and  continuing  up  to two-thirds of its length at the most. The first
       element of a pair is set to the offset of the first character in a sub-
       string,  and  the  second  is  set to the offset of the first character
       after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
       tor[1],  identify  the  portion  of  the  subject string matched by the
       entire pattern. The next pair is used for the first  capturing  subpat-
       tern, and so on. The value returned by pcre_exec() is one more than the
       highest numbered pair that has been set. For example, if two substrings
       have  been captured, the returned value is 3. If there are no capturing
       subpatterns, the return value from a successful match is 1,  indicating
       that just the first pair of offsets has been set.

       If a capturing subpattern is matched repeatedly, it is the last portion
       of the string that it matched that is returned.

       If the vector is too small to hold all the captured substring  offsets,
       it is used as far as possible (up to two-thirds of its length), and the
       function returns a value of zero. In particular, if the substring  off-
       sets are not of interest, pcre_exec() may be called with ovector passed
       as NULL and ovecsize as zero. However, if  the  pattern  contains  back
       references  and  the  ovector is not big enough to remember the related
       substrings, PCRE has to get additional memory for use during  matching.
       Thus it is usually advisable to supply an ovector.

       The  pcre_info()  function  can  be used to find out how many capturing
       subpatterns there are in a compiled  pattern.  The  smallest  size  for
       ovector  that  will allow for n captured substrings, in addition to the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

       It is possible for capturing subpattern number n+1 to match  some  part
       of the subject when subpattern n has not been used at all. For example,
       if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2 is not. When this happens, both values in  the  offset  pairs  corre-
       sponding to unused subpatterns are set to -1.

       Offset  values  that correspond to unused subpatterns at the end of the
       expression are also set to -1. For example,  if  the  string  "abc"  is
       matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
       matched. The return from the function is 2, because  the  highest  used
       capturing subpattern number is 1. However, you can refer to the offsets
       for the second and third capturing subpatterns if  you  wish  (assuming
       the vector is large enough, of course).

       Some  convenience  functions  are  provided for extracting the captured
       substrings as separate strings. These are described below.

   Error return values from pcre_exec()

       If pcre_exec() fails, it returns a negative number. The  following  are
       defined in the header file:

         PCRE_ERROR_NOMATCH        (-1)

       The subject string did not match the pattern.

         PCRE_ERROR_NULL           (-2)

       Either  code  or  subject  was  passed as NULL, or ovector was NULL and
       ovecsize was not zero.

         PCRE_ERROR_BADOPTION      (-3)

       An unrecognized bit was set in the options argument.

         PCRE_ERROR_BADMAGIC       (-4)

       PCRE stores a 4-byte "magic number" at the start of the compiled  code,
       to catch the case when it is passed a junk pointer and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an  environment  with the other endianness. This is the error that PCRE
       gives when the magic number is not present.

         PCRE_ERROR_UNKNOWN_OPCODE (-5)

       While running the pattern match, an unknown item was encountered in the
       compiled  pattern.  This  error  could be caused by a bug in PCRE or by
       overwriting of the compiled pattern.

         PCRE_ERROR_NOMEMORY       (-6)

       If a pattern contains back references, but the ovector that  is  passed
       to pcre_exec() is not big enough to remember the referenced substrings,
       PCRE gets a block of memory at the start of matching to  use  for  this
       purpose.  If the call via pcre_malloc() fails, this error is given. The
       memory is automatically freed at the end of matching.

         PCRE_ERROR_NOSUBSTRING    (-7)

       This error is used by the pcre_copy_substring(),  pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see  below).  It  is  never
       returned by pcre_exec().

         PCRE_ERROR_MATCHLIMIT     (-8)

       The backtracking limit, as specified by  the  match_limit  field  in  a
       pcre_extra  structure  (or  defaulted) was reached. See the description
       above.

         PCRE_ERROR_CALLOUT        (-9)

       This error is never generated by pcre_exec() itself. It is provided for
       use  by  callout functions that want to yield a distinctive error code.
       See the pcrecallout documentation for details.

         PCRE_ERROR_BADUTF8        (-10)

       A string that contains an invalid UTF-8 byte sequence was passed  as  a
       subject.

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       The UTF-8 byte sequence that was passed as a subject was valid, but the
       value of startoffset did not point to the beginning of a UTF-8  charac-
       ter.

         PCRE_ERROR_PARTIAL        (-12)

       The  subject  string did not match, but it did match partially. See the
       pcrepartial documentation for details of partial matching.

         PCRE_ERROR_BADPARTIAL     (-13)

       The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
       items  that are not supported for partial matching. See the pcrepartial
       documentation for details of partial matching.

         PCRE_ERROR_INTERNAL       (-14)

       An unexpected internal error has occurred. This error could  be  caused
       by a bug in PCRE or by overwriting of the compiled pattern.

         PCRE_ERROR_BADCOUNT       (-15)

       This  error is given if the value of the ovecsize argument is negative.

         PCRE_ERROR_RECURSIONLIMIT (-21)

       The internal recursion limit, as specified by the match_limit_recursion
       field  in  a  pcre_extra  structure (or defaulted) was reached. See the
       description above.

         PCRE_ERROR_BADNEWLINE     (-23)

       An invalid combination of PCRE_NEWLINE_xxx options was given.

       Error numbers -16 to -20 and -22 are not used by pcre_exec().


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       Captured substrings can be  accessed  directly  by  using  the  offsets
       returned  by  pcre_exec()  in  ovector.  For convenience, the functions
       pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
       string_list()  are  provided for extracting captured substrings as new,
       separate, zero-terminated strings. These functions identify  substrings
       by  number.  The  next section describes functions for extracting named
       substrings.

       A substring that contains a binary zero is correctly extracted and  has
       a  further zero added on the end, but the result is not, of course, a C
       string.  However, you can process such a string  by  referring  to  the
       length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not  adequate for handling strings containing binary zeros, because the
       end of the final string is not independently indicated.

       The first three arguments are the same for all  three  of  these  func-
       tions:  subject  is  the subject string that has just been successfully
       matched, ovector is a pointer to the vector of integer offsets that was
       passed to pcre_exec(), and stringcount is the number of substrings that
       were captured by the match, including the substring  that  matched  the
       entire regular expression. This is the value returned by pcre_exec() if
       it is greater than zero. If pcre_exec() returned zero, indicating  that
       it  ran out of space in ovector, the value passed as stringcount should
       be the number of elements in the vector divided by three.

       The functions pcre_copy_substring() and pcre_get_substring() extract  a
       single  substring,  whose  number  is given as stringnumber. A value of
       zero extracts the substring that matched the  entire  pattern,  whereas
       higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
       string(), the string is placed in buffer,  whose  length  is  given  by
       buffersize,  while  for  pcre_get_substring()  a new block of memory is
       obtained via pcre_malloc, and its address is  returned  via  stringptr.
       The  yield  of  the function is the length of the string, not including
       the terminating zero, or one of these error codes:

         PCRE_ERROR_NOMEMORY       (-6)

       The buffer was too small for pcre_copy_substring(), or the  attempt  to
       get memory failed for pcre_get_substring().

         PCRE_ERROR_NOSUBSTRING    (-7)

       There is no substring whose number is stringnumber.

       The  pcre_get_substring_list()  function  extracts  all  available sub-
       strings and builds a list of pointers to them. All this is  done  in  a
       single block of memory that is obtained via pcre_malloc. The address of
       the memory block is returned via listptr, which is also  the  start  of
       the  list  of  string pointers. The end of the list is marked by a NULL
       pointer. The yield of the function is zero if all  went  well,  or  the
       error code

         PCRE_ERROR_NOMEMORY       (-6)

       if the attempt to get the memory block failed.

       When  any of these functions encounter a substring that is unset, which
       can happen when capturing subpattern number n+1 matches  some  part  of
       the  subject, but subpattern n has not been used at all, they return an
       empty string. This can be distinguished from a genuine zero-length sub-
       string  by inspecting the appropriate offset in ovector, which is nega-
       tive for unset substrings.

       The two convenience functions pcre_free_substring() and  pcre_free_sub-
       string_list()  can  be  used  to free the memory returned by a previous
       call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
       tively.  They  do  nothing  more  than  call the function pointed to by
       pcre_free, which of course could be called directly from a  C  program.
       However,  PCRE is used in some situations where it is linked via a spe-
       cial  interface  to  another  programming  language  that  cannot   use
       pcre_free  directly;  it is for these cases that the functions are pro-
       vided.


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       To extract a substring by name, you first have to find associated  num-
       ber.  For example, for this pattern

         (a+)b(?<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
       subpattern of that name.

       Given the number, you can extract the substring directly, or use one of
       the functions described in the previous section. For convenience, there
       are also two functions that do the whole job.

       Most   of   the   arguments    of    pcre_copy_named_substring()    and
       pcre_get_named_substring()  are  the  same  as  those for the similarly
       named functions that extract by number. As these are described  in  the
       previous  section,  they  are not re-described here. There are just two
       differences:

       First, instead of a substring number, a substring name is  given.  Sec-
       ond, there is an extra argument, given at the start, which is a pointer
       to the compiled pattern. This is needed in order to gain access to  the
       name-to-number translation table.

       These  functions call pcre_get_stringnumber(), and if it succeeds, they
       then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
       ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
       behaviour may not be what you want (see the next section).


DUPLICATE SUBPATTERN NAMES

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
       subpatterns  are  not  required  to  be unique. Normally, patterns with
       duplicate names are such that in any one match, only one of  the  named
       subpatterns  participates. An example is shown in the pcrepattern docu-
       mentation.

       When   duplicates   are   present,   pcre_copy_named_substring()    and
       pcre_get_named_substring()  return the first substring corresponding to
       the given name that is set. If  none  are  set,  PCRE_ERROR_NOSUBSTRING
       (-7)  is  returned;  no  data  is returned. The pcre_get_stringnumber()
       function returns one of the numbers that are associated with the  name,
       but it is not defined which it is.

       If  you want to get full details of all captured substrings for a given
       name, you must use  the  pcre_get_stringtable_entries()  function.  The
       first argument is the compiled pattern, and the second is the name. The
       third and fourth are pointers to variables which  are  updated  by  the
       function. After it has run, they point to the first and last entries in
       the name-to-number table  for  the  given  name.  The  function  itself
       returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
       there are none. The format of the table is described above in the  sec-
       tion  entitled  Information  about  a  pattern.  Given all the relevant
       entries for the name, you can extract each of their numbers, and  hence
       the captured data, if any.


FINDING ALL POSSIBLE MATCHES

       The  traditional  matching  function  uses a similar algorithm to Perl,
       which stops when it finds the first match, starting at a given point in
       the  subject.  If you want to find all possible matches, or the longest
       possible match, consider using the alternative matching  function  (see
       below)  instead.  If you cannot use the alternative function, but still
       need to find all possible matches, you can kludge it up by  making  use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.   When your callout function is called, extract and save the cur-
       rent matched substring. Then return  1,  which  forces  pcre_exec()  to
       backtrack  and  try other alternatives. Ultimately, when it runs out of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       The function pcre_dfa_exec()  is  called  to  match  a  subject  string
       against  a  compiled pattern, using a matching algorithm that scans the
       subject string just once, and does not backtrack.  This  has  different
       characteristics  to  the  normal  algorithm, and is not compatible with
       Perl. Some of the features of PCRE patterns are not  supported.  Never-
       theless,  there are times when this kind of matching can be useful. For
       a discussion of the two matching algorithms, see the pcrematching docu-
       mentation.

       The  arguments  for  the  pcre_dfa_exec()  function are the same as for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent  way,  and  this is described below. The other common arguments are
       used in the same way as for pcre_exec(), so their  description  is  not
       repeated here.

       The  two  additional  arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It  is  used  for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace will be needed for patterns and subjects where  there  are  a
       lot of potential matches.

       Here is an example of a simple call to pcre_dfa_exec():

         int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_exec()

       The  unused  bits  of  the options argument for pcre_dfa_exec() must be
       zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
       LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
       PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
       three of these are the same as for pcre_exec(), so their description is
       not repeated here.

         PCRE_PARTIAL

       This has the same general effect as it does for  pcre_exec(),  but  the
       details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
       pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
       PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
       been no complete matches, but there is still at least one matching pos-
       sibility.  The portion of the string that provided the partial match is
       set as the first matching string.

         PCRE_DFA_SHORTEST

       Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
       stop as soon as it has found one match. Because of the way the alterna-
       tive algorithm works, this is necessarily the shortest  possible  match
       at the first possible matching point in the subject string.

         PCRE_DFA_RESTART

       When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
       returns a partial match, it is possible to call it  again,  with  addi-
       tional  subject  characters,  and have it continue with the same match.
       The PCRE_DFA_RESTART option requests this action; when it is  set,  the
       workspace  and wscount options must reference the same vector as before
       because data about the match so far is left in  them  after  a  partial
       match.  There  is  more  discussion of this facility in the pcrepartial
       documentation.

   Successful returns from pcre_dfa_exec()

       When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
       string in the subject. Note, however, that all the matches from one run
       of the function start at the same point in  the  subject.  The  shorter
       matches  are all initial substrings of the longer matches. For example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something>
         <something> <something else>
         <something> <something else> <something further>

       On success, the yield of the function is a number  greater  than  zero,
       which  is  the  number of matched substrings. The substrings themselves
       are returned in ovector. Each string uses two elements;  the  first  is
       the  offset  to  the start, and the second is the offset to the end. In
       fact, all the strings have the same start  offset.  (Space  could  have
       been  saved by giving this only once, but it was decided to retain some
       compatibility with the way pcre_exec() returns data,  even  though  the
       meaning of the strings is different.)

       The strings are returned in reverse order of length; that is, the long-
       est matching string is given first. If there were too many  matches  to
       fit  into ovector, the yield of the function is zero, and the vector is
       filled with the longest matches.

   Error returns from pcre_dfa_exec()

       The pcre_dfa_exec() function returns a negative number when  it  fails.
       Many  of  the  errors  are  the  same as for pcre_exec(), and these are
       described above.  There are in addition the following errors  that  are
       specific to pcre_dfa_exec():

         PCRE_ERROR_DFA_UITEM      (-16)

       This  return is given if pcre_dfa_exec() encounters an item in the pat-
       tern that it does not support, for instance, the use of \C  or  a  back
       reference.

         PCRE_ERROR_DFA_UCOND      (-17)

       This  return  is  given  if pcre_dfa_exec() encounters a condition item
       that uses a back reference for the condition, or a test  for  recursion
       in a specific group. These are not supported.

         PCRE_ERROR_DFA_UMLIMIT    (-18)

       This  return  is given if pcre_dfa_exec() is called with an extra block
       that contains a setting of the match_limit field. This is not supported
       (it is meaningless).

         PCRE_ERROR_DFA_WSSIZE     (-19)

       This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
       workspace vector.

         PCRE_ERROR_DFA_RECURSE    (-20)

       When a recursive subpattern is processed, the matching  function  calls
       itself  recursively,  using  private vectors for ovector and workspace.
       This error is given if the output vector  is  not  large  enough.  This
       should be extremely rare, as a vector of size 1000 is used.


SEE ALSO

       pcrebuild(3),  pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
       tial(3), pcreposix(3), pcreprecompile(3), pcresample(3),  pcrestack(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 23 January 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------


PCRECALLOUT(3)                                                  PCRECALLOUT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout. By  default,  this
       variable contains NULL, which disables all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called.  Different  callout  points  can  be
       identified  by  putting  a number less than 256 after the letter C. The
       default value is zero.  For  example,  this  pattern  has  two  callout
       points:

         (?C1)abc(?C2)def

       If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
       called, PCRE automatically  inserts  callouts,  all  with  number  255,
       before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
       used with the pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice that there is a callout before and after  each  parenthesis  and
       alternation  bar.  Automatic  callouts  can  be  used  for tracking the
       progress of pattern matching. The pcretest command has an  option  that
       sets  automatic callouts; when it is used, the output indicates how the
       pattern is matched. This is useful information when you are  trying  to
       optimize the performance of a particular pattern.


MISSING CALLOUTS

       You  should  be  aware  that,  because of optimizations in the way PCRE
       matches patterns, callouts sometimes do not happen. For example, if the
       pattern is

         ab(?C4)cd

       PCRE knows that any matching string must contain the letter "d". If the
       subject string is "abyz", the lack of "d" means that  matching  doesn't
       ever  start,  and  the  callout is never reached. However, with "abyd",
       though the result is still no match, the callout is obeyed.


THE CALLOUT INTERFACE

       During matching, when PCRE reaches a callout point, the external  func-
       tion  defined by pcre_callout is called (if it is set). This applies to
       both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
       only  argument  to  the callout function is a pointer to a pcre_callout
       block. This structure contains the following fields:

         int          version;
         int          callout_number;
         int         *offset_vector;
         const char  *subject;
         int          subject_length;
         int          start_match;
         int          current_position;
         int          capture_top;
         int          capture_last;
         void        *callout_data;
         int          pattern_position;
         int          next_item_length;

       The version field is an integer containing the version  number  of  the
       block  format. The initial version was 0; the current version is 1. The
       version number will change again in future  if  additional  fields  are
       added, but the intention is never to remove any of the existing fields.

       The callout_number field contains the number of the  callout,  as  com-
       piled  into  the pattern (that is, the number after ?C for manual call-
       outs, and 255 for automatically generated callouts).

       The offset_vector field is a pointer to the vector of offsets that  was
       passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
       pcre_exec() is used, the contents can be inspected in order to  extract
       substrings  that  have  been  matched  so  far,  in the same way as for
       extracting substrings after a match has completed. For  pcre_dfa_exec()
       this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to pcre_exec().

       The start_match field normally contains the offset within  the  subject
       at  which  the  current  match  attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect  the
       modified  starting  point.  If the pattern is not anchored, the callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       When the pcre_exec() function is used, the capture_top  field  contains
       one  more than the number of the highest numbered captured substring so
       far. If no substrings have been captured, the value of  capture_top  is
       one.  This  is always the case when pcre_dfa_exec() is used, because it
       does not support captured substrings.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring. If no substrings have been captured, its value is -1.
       This is always the case when pcre_dfa_exec() is used.

       The callout_data field contains a value that is passed  to  pcre_exec()
       or  pcre_dfa_exec() specifically so that it can be passed back in call-
       outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
       structure.  If  no such data was passed, the value of callout_data in a
       pcre_callout block is NULL. There is a description  of  the  pcre_extra
       structure in the pcreapi documentation.

       The  pattern_position field is present from version 1 of the pcre_call-
       out structure. It contains the offset to the next item to be matched in
       the pattern string.

       The  next_item_length field is present from version 1 of the pcre_call-
       out structure. It contains the length of the next item to be matched in
       the  pattern  string. When the callout immediately precedes an alterna-
       tion bar, a closing parenthesis, or the end of the pattern, the  length
       is  zero.  When the callout precedes an opening parenthesis, the length
       is that of the entire subpattern.

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts.


RETURN VALUES

       The external callout function returns an integer to PCRE. If the  value
       is  zero,  matching  proceeds  as  normal. If the value is greater than
       zero, matching fails at the current point, but  the  testing  of  other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  and
       pcre_exec() (or pcre_dfa_exec()) returns the negative value.

       Negative   values   should   normally   be   chosen  from  the  set  of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be  used  by  PCRE
       itself.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 29 May 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCRECOMPAT(3)                                                    PCRECOMPAT(3)


NAME
       PCRE - Perl-compatible regular expressions


DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences described here  are  mainly
       with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
       some features that are expected to be in the forthcoming Perl 5.10.

       1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
       of  what  it does have are given in the section on UTF-8 support in the
       main pcre page.

       2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
       permits  them,  but they do not mean what you might think. For example,
       (?!a){3} does not assert that the next three characters are not "a". It
       just asserts that the next character is not "a" three times.

       3.  Capturing  subpatterns  that occur inside negative lookahead asser-
       tions are counted, but their entries in the offsets  vector  are  never
       set.  Perl sets its numerical variables from any such patterns that are
       matched before the assertion fails to match something (thereby succeed-
       ing),  but  only  if the negative lookahead assertion contains just one
       branch.

       4. Though binary zero characters are supported in the  subject  string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5.  The  following Perl escape sequences are not supported: \l, \u, \L,
       \U, and \N. In fact these are implemented by Perl's general string-han-
       dling  and are not part of its pattern matching engine. If any of these
       are encountered by PCRE, an error is generated.

       6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
       is  built  with Unicode character property support. The properties that
       can be tested with \p and \P are limited to the general category  prop-
       erties  such  as  Lu and Nd, script names such as Greek or Han, and the
       derived properties Any and L&.

       7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters  in  between  are  treated as literals. This is slightly different
       from Perl in that $ and @ are  also  handled  as  literals  inside  the
       quotes.  In Perl, they cause variable interpolation (but of course PCRE
       does not have variables). Note the following examples:

           Pattern            PCRE matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

       8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
       constructions. However, there is support for recursive  patterns.  This
       is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
       "callout" feature allows an external function to be called during  pat-
       tern matching. See the pcrecallout documentation for details.

       9.  Subpatterns  that  are  called  recursively or as "subroutines" are
       always treated as atomic groups in  PCRE.  This  is  like  Python,  but
       unlike Perl.

       10.  There are some differences that are concerned with the settings of
       captured strings when part of  a  pattern  is  repeated.  For  example,
       matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
       unset, but in PCRE it is set to "b".

       11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
       (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
       the forms without an  argument.  PCRE  does  not  support  (*MARK).  If
       (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-
       ture group; this is different to Perl.

       12. PCRE provides some extensions to the Perl regular expression facil-
       ities.   Perl  5.10  will  include new features that are not in earlier
       versions, some of which (such as named parentheses) have been  in  PCRE
       for some time. This list is with respect to Perl 5.10:

       (a)  Although  lookbehind  assertions  must match fixed length strings,
       each alternative branch of a lookbehind assertion can match a different
       length of string. Perl requires them all to have the same length.

       (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
       meta-character matches only at the very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
       cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
       ignored.  (Perl can be made to issue a warning.)

       (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

       (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
       TURE options for pcre_exec() have no Perl equivalents.

       (g) The \R escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.

       (k) The alternative matching function (pcre_dfa_exec())  matches  in  a
       different way and is not Perl-compatible.

       (l)  PCRE  recognizes some special sequences such as (*CR) at the start
       of a pattern that set overall options that cannot be changed within the
       pattern.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 11 September 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREPATTERN(3)                                                  PCREPATTERN(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE are described in detail below. There is a quick-reference  syn-
       tax  summary  in  the  pcresyntax  page. Perl's regular expressions are
       described in its own documentation, and regular expressions in  general
       are  covered in a number of books, some of which have copious examples.
       Jeffrey  Friedl's  "Mastering  Regular   Expressions",   published   by
       O'Reilly,  covers regular expressions in great detail. This description
       of PCRE's regular expressions is intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, you must build PCRE to  include  UTF-8  support,  and  then  call
       pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
       matching is mentioned in several places below. There is also a  summary
       of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
       page.

       The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE when its main matching function, pcre_exec(), is used.
       From  release  6.0,   PCRE   offers   a   second   matching   function,
       pcre_dfa_exec(),  which matches using a different algorithm that is not
       Perl-compatible. Some of the features discussed below are not available
       when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
       alternative function, and how it differs from the normal function,  are
       discussed in the pcrematching page.


NEWLINE CONVENTIONS

       PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The pcreapi page  has  further
       discussion  about newlines, and shows how to set the newline convention
       in the options arguments for the compiling and matching functions.

       It is also possible to specify a newline convention by starting a  pat-
       tern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to pcre_compile(). For
       example, on a Unix system where LF is the default newline sequence, the
       pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. Note that these special settings,  which  are  not
       Perl-compatible,  are  recognized  only at the very start of a pattern,
       and that they must be in upper case.  If  more  than  one  of  them  is
       present, the last one is used.

       The  newline  convention  does  not  affect what the \R escape sequence
       matches. By default, this is any Unicode  newline  sequence,  for  Perl
       compatibility.  However, this can be changed; see the description of \R
       in the section entitled "Newline sequences" below. A change of \R  set-
       ting can be combined with a change of newline convention.


CHARACTERS AND METACHARACTERS

       A  regular  expression  is  a pattern that is matched against a subject
       string from left to right. Most characters stand for  themselves  in  a
       pattern,  and  match  the corresponding characters in the subject. As a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless  matching is specified (the PCRE_CASELESS option), letters are
       matched independently of case. In UTF-8 mode, PCRE  always  understands
       the  concept  of case for characters whose values are less than 128, so
       caseless matching is always possible. For characters with  higher  val-
       ues,  the concept of case is supported if PCRE is compiled with Unicode
       property support, but not otherwise.   If  you  want  to  use  caseless
       matching  for  characters  128  and above, you must ensure that PCRE is
       compiled with Unicode property support as well as with UTF-8 support.

       The power of regular expressions comes  from  the  ability  to  include
       alternatives  and  repetitions in the pattern. These are encoded in the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There  are  two different sets of metacharacters: those that are recog-
       nized anywhere in the pattern except within square brackets, and  those
       that  are  recognized  within square brackets. Outside square brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part of a pattern that is in square brackets  is  called  a  "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The  following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a  non-alphanumeric  character,  it takes away any special meaning that
       character may have. This  use  of  backslash  as  an  escape  character
       applies both inside and outside character classes.

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

       If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap-
       ing  backslash  can  be  used to include a whitespace or # character as
       part of the pattern.

       If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters, apart from the binary zero that
       terminates a pattern, but when a pattern  is  being  prepared  by  text
       editing,  it  is  usually  easier  to  use  one of the following escape
       sequences than the binary character it represents:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or backreference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh..

       The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;
       becomes hex 7B.

       After  \x, from zero to two hexadecimal digits are read (letters can be
       in upper or lower case). Any number of hexadecimal  digits  may  appear
       between  \x{  and  },  but the value of the character code must be less
       than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
       the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
       than the largest Unicode code point, which is 10FFFF.

       If characters other than hexadecimal digits appear between \x{  and  },
       or if there is no terminating }, this form of escape is not recognized.
       Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal
       escape,  with  no  following  digits, giving a character whose value is
       zero.

       Characters whose value is less than 256 can be defined by either of the
       two  syntaxes  for  \x. There is no difference in the way they are han-
       dled. For example, \xdc is exactly the same as \x{dc}.

       After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those  that  are  present  are used. Thus the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code  value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its as a decimal number. If the number is less than  10,  or  if  there
       have been at least that many previous capturing left parentheses in the
       expression, the entire  sequence  is  taken  as  a  back  reference.  A
       description  of how this works is given later, following the discussion
       of parenthesized subpatterns.

       Inside a character class, or if the decimal number is  greater  than  9
       and  there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the backslash, and uses them to gen-
       erate  a data character. Any subsequent digits stand for themselves. In
       non-UTF-8 mode, the value of a character specified  in  octal  must  be
       less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
       example:

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

       Note that octal values of 100 or greater must not be  introduced  by  a
       leading zero, because no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class,  the  sequence \b is interpreted as the backspace character (hex
       08), and the sequences \R and \X are interpreted as the characters  "R"
       and  "X", respectively. Outside a character class, these sequences have
       different meanings (see below).

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Generic character types

       Another use of backslash is for specifying generic character types. The
       following are always recognized:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal whitespace character
         \H     any character that is not a horizontal whitespace character
         \s     any whitespace character
         \S     any character that is not a whitespace character
         \v     any vertical whitespace character
         \V     any character that is not a vertical whitespace character
         \w     any "word" character
         \W     any "non-word" character

       Each pair of escape sequences partitions the complete set of characters
       into  two disjoint sets. Any given character matches one, and only one,
       of each pair.

       These character type sequences can appear both inside and outside char-
       acter  classes.  They each match one character of the appropriate type.
       If the current matching point is at the end of the subject string,  all
       of them fail, since there is no character to match.

       For  compatibility  with Perl, \s does not match the VT character (code
       11).  This makes it different from the the POSIX "space" class. The  \s
       characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If
       "use locale;" is included in a Perl script, \s may match the VT charac-
       ter. In PCRE, it never does.

       In  UTF-8 mode, characters with values greater than 128 never match \d,
       \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
       code  character  property  support is available. These sequences retain
       their original meanings from before UTF-8 support was available, mainly
       for efficiency reasons.

       The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
       the other sequences, these do match certain high-valued  codepoints  in
       UTF-8 mode.  The horizontal space characters are:

         U+0009     Horizontal tab
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed
         U+000B     Vertical tab
         U+000C     Formfeed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

       A "word" character is an underscore or any character less than 256 that
       is a letter or digit. The definition of  letters  and  digits  is  con-
       trolled  by PCRE's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the  pcreapi
       page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
       systems, or "french" in Windows, some character codes greater than  128
       are  used for accented letters, and these are matched by \w. The use of
       locales with Unicode is discouraged.

   Newline sequences

       Outside a character class, by default, the escape sequence  \R  matches
       any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
       mode \R is equivalent to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details  of  which  are  given
       below.  This particular group matches either the two-character sequence
       CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

       In  UTF-8  mode, two additional characters whose codepoints are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator,  U+2029).   Unicode character property support is not needed for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode  line  endings)  by  setting the option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when PCRE is built; if this is the case, the  other  behaviour  can  be
       requested  via  the  PCRE_BSR_UNICODE  option.   It is also possible to
       specify these settings by starting a pattern string  with  one  of  the
       following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to pcre_compile(), but
       they can be overridden by options given to pcre_exec(). Note that these
       special settings, which are not Perl-compatible, are recognized only at
       the very start of a pattern, and that they must be in  upper  case.  If
       more  than  one  of  them is present, the last one is used. They can be
       combined with a change of newline convention, for  example,  a  pattern
       can start with:

         (*ANY)(*BSR_ANYCRLF)

       Inside a character class, \R matches the letter "R".

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional escape sequences that match characters with specific  properties
       are  available.   When not in UTF-8 mode, these sequences are of course
       limited to testing characters whose codepoints are less than  256,  but
       they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

       The  property  names represented by xx above are limited to the Unicode
       script names, the general category properties, and "Any", which matches
       any character (including newline). Other properties such as "InMusical-
       Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does
       not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A character from one of these sets can be matched using a script  name.
       For example:

         \p{Greek}
         \P{Han}

       Those  that are not part of an identified script are lumped together as
       "Common". The current list of scripts is:

       Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
       Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
       Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
       gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,
       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,
       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
       Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.

       Each character has exactly one general category property, specified  by
       a two-letter abbreviation. For compatibility with Perl, negation can be
       specified by including a circumflex between the opening brace  and  the
       property name. For example, \p{^Lu} is the same as \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral category properties that start with that letter. In this case,  in
       the  absence of negation, the curly brackets in the escape sequence are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The special property L& is also supported: it matches a character  that
       has  the  Lu,  Ll, or Lt property, in other words, a letter that is not
       classified as a modifier or "other".

       The Cs (Surrogate) property applies only to  characters  in  the  range
       U+D800  to  U+DFFF. Such characters are not valid in UTF-8 strings (see
       RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
       ing  has  been  turned off (see the discussion of PCRE_NO_UTF8_CHECK in
       the pcreapi page).

       The long synonyms for these properties  that  Perl  supports  (such  as
       \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying caseless matching does not affect  these  escape  sequences.
       For example, \p{Lu} always matches only upper case letters.

       The  \X  escape  matches  any number of Unicode characters that form an
       extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

       That is, it matches a character without the "mark"  property,  followed
       by  zero  or  more  characters with the "mark" property, and treats the
       sequence as an atomic group (see below).  Characters  with  the  "mark"
       property  are  typically  accents  that affect the preceding character.
       None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
       matches any one character.

       Matching  characters  by Unicode property is not fast, because PCRE has
       to search a structure that contains  data  for  over  fifteen  thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE.

   Resetting the match start

       The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
       ously  matched  characters  not  to  be  included  in the final matched
       sequence. For example, the pattern:

         foo\Kbar

       matches "foobar", but reports that it has matched "bar".  This  feature
       is  similar  to  a lookbehind assertion (described below).  However, in
       this case, the part of the subject before the real match does not  have
       to  be of fixed length, as lookbehind assertions do. The use of \K does
       not interfere with the setting of captured  substrings.   For  example,
       when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

   Simple assertions

       The  final use of backslash is for certain simple assertions. An asser-
       tion specifies a condition that has to be met at a particular point  in
       a  match, without consuming any characters from the subject string. The
       use of subpatterns for more complicated assertions is described  below.
       The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       These  assertions may not appear in character classes (but note that \b
       has a different meaning, namely the backspace character, inside a char-
       acter class).

       A  word  boundary is a position in the subject string where the current
       character and the previous character do not both match \w or  \W  (i.e.
       one  matches  \w  and the other matches \W), or the start or end of the
       string if the first or last character matches \w, respectively.

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in the next section) in that they only ever match
       at the very start and end of the subject string, whatever  options  are
       set.  Thus,  they are independent of multiline mode. These three asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect  only the behaviour of the circumflex and dollar metacharacters.
       However, if the startoffset argument of pcre_exec() is non-zero,  indi-
       cating that matching is to start at a point other than the beginning of
       the subject, \A can never match. The difference between \Z  and  \z  is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

       The \G assertion is true only when the current matching position is  at
       the  start point of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times with appropriate argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note,  however,  that  PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end  of  the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one  match
       at a time, it cannot reproduce this behaviour.

       If  all  the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
       PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
       has an entirely different meaning (see below).

       Circumflex need not be the first character of the pattern if  a  number
       of  alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever  to  match  that
       branch.  If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start  of  the  sub-
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       A dollar character is an assertion that is true  only  if  the  current
       matching  point  is  at  the  end of the subject string, or immediately
       before a newline at the end of the string (by default). Dollar need not
       be  the  last  character of the pattern if a number of alternatives are
       involved, but it should be the last item in  any  branch  in  which  it
       appears. Dollar has no special meaning in a character class.

       The  meaning  of  dollar  can be changed so that it matches only at the
       very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex
       matches  immediately after internal newlines as well as at the start of
       the subject string. It does not match after a  newline  that  ends  the
       string.  A dollar matches before any newlines in the string, as well as
       at the very end, when PCRE_MULTILINE is set. When newline is  specified
       as  the  two-character  sequence CRLF, isolated CR and LF characters do
       not indicate newlines.

       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
       (where  \n  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
       PCRE_MULTILINE is set.

       Note that the sequences \A, \Z, and \z can be used to match  the  start
       and  end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not  PCRE_MULTILINE  is
       set.


FULL STOP (PERIOD, DOT)

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies  the  end  of  a line. In UTF-8 mode, the matched character may be
       more than one byte long.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE_DOTALL option is set, a dot matches  any  one  character,  without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the  only relationship being that they both involve
       newlines. Dot has no special meaning in a character class.


MATCHING A SINGLE BYTE

       Outside a character class, the escape sequence \C matches any one byte,
       both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any
       line-ending characters. The feature is provided in  Perl  in  order  to
       match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
       acters into individual bytes, what remains in the string may be a  mal-
       formed  UTF-8  string.  For this reason, the \C escape sequence is best
       avoided.

       PCRE does not allow \C to appear in  lookbehind  assertions  (described
       below),  because  in UTF-8 mode this would make it impossible to calcu-
       late the length of the lookbehind.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial. If a closing square bracket is required as a member of the class,
       it  should  be  the first data character in the class (after an initial
       circumflex, if present) or escaped with a backslash.

       A character class matches a single character in the subject.  In  UTF-8
       mode,  the character may occupy more than one byte. A matched character
       must be in the set of characters defined by the class, unless the first
       character  in  the  class definition is a circumflex, in which case the
       subject character must not be in the set defined by  the  class.  If  a
       circumflex  is actually required as a member of the class, ensure it is
       not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case  vowel,
       while  [^aeiou]  matches  any character that is not a lower case vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters  that  are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion: it still  con-
       sumes  a  character  from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

       In UTF-8 mode, characters with values greater than 255 can be  included
       in  a  class as a literal string of bytes, or by using the \x{ escaping
       mechanism.

       When caseless matching is set, any letters in a  class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported  if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching for characters 128 and above, you  must  ensure  that
       PCRE  is  compiled  with Unicode property support as well as with UTF-8
       support.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when  matching  character  classes,  whatever line-ending
       sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character  class.  For  example,  [d-m] matches any letter
       between d and m, inclusive. If a  minus  character  is  required  in  a
       class,  it  must  be  escaped  with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as  the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges operate in the collating sequence of character values. They  can
       also   be  used  for  characters  specified  numerically,  for  example
       [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
       are greater than 255, for example [\x{100}-\x{2ff}].

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
       character tables for a French locale are in  use,  [\xc8-\xcb]  matches
       accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
       concept of case for characters with values greater than 128  only  when
       it is compiled with Unicode property support.

       The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
       in a character class, and add the characters that  they  match  to  the
       class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
       flex can conveniently be used with the upper case  character  types  to
       specify  a  more  restricted  set of characters than the matching lower
       case type. For example, the class [^\W_] matches any letter  or  digit,
       but not underscore.

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see  the
       next  section),  and  the  terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by  [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT  character  (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.


VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns.  For
       example, the pattern

         gilbert|sullivan

       matches  either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty  alternative  is  permitted  (matching  the  empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the  alternatives
       are  within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the  subpattern.


INTERNAL OPTION SETTING

       The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
       PCRE_EXTENDED options (which are Perl-compatible) can be  changed  from
       within  the  pattern  by  a  sequence  of  Perl option letters enclosed
       between "(?" and ")".  The option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
       LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
       is also permitted. If a  letter  appears  both  before  and  after  the
       hyphen, the option is unset.

       The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
       can be changed in the same way as the Perl-compatible options by  using
       the characters J, U and X respectively.

       When  an option change occurs at top level (that is, not inside subpat-
       tern parentheses), the change applies to the remainder of  the  pattern
       that follows.  If the change is placed right at the start of a pattern,
       PCRE extracts it into the global options (and it will therefore show up
       in data extracted by the pcre_fullinfo() function).

       An  option  change  within a subpattern (see below for a description of
       subpatterns) affects only that part of the current pattern that follows
       it, so

         (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).  By this means, options can be made to have  different  settings
       in  different parts of the pattern. Any changes made in one alternative
       do carry on into subsequent branches within the  same  subpattern.  For
       example,

         (a(?i)b|c)

       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
       first branch is abandoned before the option setting.  This  is  because
       the  effects  of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       Note: There are other PCRE-specific options that  can  be  set  by  the
       application  when  the  compile  or match functions are called. In some
       cases the pattern can contain special  leading  sequences  to  override
       what  the  application  has set or what has been defaulted. Details are
       given in the section entitled "Newline sequences" above.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches  one  of the words "cat", "cataract", or "caterpillar". Without
       the parentheses, it would match  "cataract",  "erpillar"  or  an  empty
       string.

       2.  It  sets  up  the  subpattern as a capturing subpattern. This means
       that, when the whole pattern  matches,  that  portion  of  the  subject
       string that matched the subpattern is passed back to the caller via the
       ovector argument of pcre_exec(). Opening parentheses are  counted  from
       left  to  right  (starting  from 1) to obtain numbers for the capturing
       subpatterns.

       For example, if the string "the red king" is matched against  the  pat-
       tern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil  two  functions  is  not  always
       helpful.   There are often times when a grouping subpattern is required
       without a capturing requirement. If an opening parenthesis is  followed
       by  a question mark and a colon, the subpattern does not do any captur-
       ing, and is not counted when computing the  number  of  any  subsequent
       capturing  subpatterns. For example, if the string "the white queen" is
       matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

       As  a  convenient shorthand, if any option settings are required at the
       start of a non-capturing subpattern,  the  option  letters  may  appear
       between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until  the  end  of
       the  subpattern is reached, an option setting in one branch does affect
       subsequent branches, so the above patterns match "SUNDAY"  as  well  as
       "Saturday".


DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses the same numbers for its capturing parentheses. Such a  subpattern
       starts  with (?| and is itself a non-capturing subpattern. For example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of  cap-
       turing  parentheses  are  numbered one. Thus, when the pattern matches,
       you can look at captured substring number  one,  whichever  alternative
       matched.  This  construct  is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses  are  numbered as usual, but the number is reset at the start of
       each branch. The numbers of any capturing buffers that follow the  sub-
       pattern  start after the highest number used in any branch. The follow-
       ing example is taken from the Perl documentation.  The  numbers  under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A  backreference  or  a  recursive call to a numbered subpattern always
       refers to the first one in the pattern with the given number.

       An alternative approach to using this "branch reset" feature is to  use
       duplicate named subpatterns, as described in the next section.


NAMED SUBPATTERNS

       Identifying  capturing  parentheses  by number is simple, but it can be
       very hard to keep track of the numbers in complicated  regular  expres-
       sions.  Furthermore,  if  an  expression  is  modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of  sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had the feature earlier, and PCRE introduced it at release  4.0,  using
       the  Python syntax. PCRE now supports both the Perl and the Python syn-
       tax.

       In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)
       or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References
       to capturing parentheses from other parts of the pattern, such as back-
       references,  recursion,  and conditions, can be made by name as well as
       by number.

       Names consist of up to  32  alphanumeric  characters  and  underscores.
       Named  capturing  parentheses  are  still  allocated numbers as well as
       names, exactly as if the names were not present. The PCRE API  provides
       function calls for extracting the name-to-number translation table from
       a compiled pattern. There is also a convenience function for extracting
       a captured substring by name.

       By  default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time.  This  can  be useful for patterns where only one instance of the
       named parentheses can match. Suppose you want to match the  name  of  a
       weekday,  either as a 3-letter abbreviation or as the full name, and in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There  are  five capturing substrings, but only one is ever set after a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The  convenience  function  for extracting the data by name returns the
       substring for the first (and in this example, the only)  subpattern  of
       that  name  that  matched.  This saves searching to find which numbered
       subpattern it was. If you make a reference to a non-unique  named  sub-
       pattern  from elsewhere in the pattern, the one that corresponds to the
       lowest number is used. For further details of the interfaces  for  han-
       dling named subpatterns, see the pcreapi documentation.


REPETITION

       Repetition  is  specified  by  quantifiers, which can follow any of the
       following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         the \R escape sequence
         an escape such as \d that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (unless it is an assertion)

       The general repetition quantifier specifies a minimum and maximum  num-
       ber  of  permitted matches, by giving the two numbers in curly brackets
       (braces), separated by a comma. The numbers must be  less  than  65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but  the  comma  is
       present,  there  is  no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of  required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches  exactly  8  digits. An opening curly bracket that appears in a
       position where a quantifier is not allowed, or one that does not  match
       the  syntax of a quantifier, is taken as a literal character. For exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended  sequences,  each of which may be several bytes long (and they
       may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present.

       For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the .*  item.

       However,  if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE_UNGREEDY option is set (an option that is not available  in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
       the pattern is implicitly anchored, because whatever  follows  will  be
       tried  against every character position in the subject string, so there
       is no point in retrying the overall match at  any  position  after  the
       first.  PCRE  normally treats such a pattern as though it were preceded
       by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However, there is one situation where the optimization cannot be  used.
       When  .*   is  inside  capturing  parentheses that are the subject of a
       backreference elsewhere in the pattern, a match at the start  may  fail
       where a later one succeeds. Consider, for example:

         (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are  nested  capturing  subpatterns,
       the  corresponding captured values may have been set in previous itera-
       tions. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
       repetition,  failure  of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats  allows  the
       rest  of  the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it  fail  earlier
       than  it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to  the  subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  gives
       up  immediately  on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       This  kind  of  parenthesis "locks up" the  part of the pattern it con-
       tains once it has matched, and a failure further into  the  pattern  is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a subpattern of  this  type  matches
       the  string  of  characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
       pared  to  adjust  the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists  of  an  additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive   quantifiers   are   always  greedy;  the  setting  of  the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler  forms  of atomic group. However, there is no difference in the
       meaning of a possessive quantifier and  the  equivalent  atomic  group,
       though  there  may  be a performance difference; possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax is an extension to the Perl  5.8  syn-
       tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built  Sun's Java package, and PCRE copied it from there. It ultimately
       found its way into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain sim-
       ple  pattern  constructs.  For  example, the sequence A+B is treated as
       A++B because there is no point in backtracking into a sequence  of  A's
       when B must follow.

       When  a  pattern  contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use  of  an
       atomic  group  is  the  only way to avoid some failing matches taking a
       very long time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE and Perl have an optimization that allows  for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens  quickly.


BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern  earlier  (that is, to its left) in the pattern, provided there
       have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it  is  always  taken  as a back reference, and causes an error only if
       there are not that many capturing left parentheses in the  entire  pat-
       tern.  In  other words, the parentheses that are referenced need not be
       to the left of the reference for numbers less than 10. A "forward  back
       reference"  of  this  type can make sense when a repetition is involved
       and the subpattern to the right has participated in an  earlier  itera-
       tion.

       It  is  not  possible to have a numerical "forward back reference" to a
       subpattern whose number is 10 or  more  using  this  syntax  because  a
       sequence  such  as  \50 is interpreted as a character defined in octal.
       See the subsection entitled "Non-printing characters" above for further
       details  of  the  handling of digits following a backslash. There is no
       such problem when named parentheses are used. A back reference  to  any
       subpattern is possible using named parentheses (see below).

       Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g escape sequence, which is a fea-
       ture  introduced  in  Perl  5.10.  This  escape  must be followed by an
       unsigned number or a negative number, optionally  enclosed  in  braces.
       These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An  unsigned number specifies an absolute reference without the ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started captur-
       ing  subpattern  before \g, that is, is it equivalent to \2. Similarly,
       \g{-2} would be equivalent to \1. The use of relative references can be
       helpful  in  long  patterns,  and  also in patterns that are created by
       joining together fragments that contain references within themselves.

       A back reference matches whatever actually matched the  capturing  sub-
       pattern  in  the  current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of the back reference, the case of letters is relevant. For exam-
       ple,

         ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
       original capturing subpattern is matched caselessly.

       There  are  several  different ways of writing back references to named
       subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or
       \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and  named  references,  is  also supported. We could rewrite the above
       example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A subpattern that is referenced by  name  may  appear  in  the  pattern
       before or after the reference.

       There  may be more than one back reference to the same subpattern. If a
       subpattern has not actually been used in a particular match,  any  back
       references to it always fail. For example, the pattern

         (a|(bc))\2

       always  fails if it starts to match "a" rather than "bc". Because there
       may be many capturing parentheses in a pattern,  all  digits  following
       the  backslash  are taken as part of a potential back reference number.
       If the pattern continues with a digit character, some delimiter must be
       used  to  terminate  the back reference. If the PCRE_EXTENDED option is
       set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-
       ments" below) can be used.

       A  back reference that occurs inside the parentheses to which it refers
       fails when the subpattern is first used, so, for example,  (a\1)  never
       matches.   However,  such references can be useful inside repeated sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation  of  the  subpattern,  the  back  reference matches the character
       string corresponding to the previous iteration. In order  for  this  to
       work,  the  pattern must be such that the first iteration does not need
       to match the back reference. This can be done using alternation, as  in
       the example above, or by a quantifier with a minimum of zero.


ASSERTIONS

       An  assertion  is  a  test on the characters following or preceding the
       current matching point that does not actually consume  any  characters.
       The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are
       described above.

       More complicated assertions are coded as  subpatterns.  There  are  two
       kinds:  those  that  look  ahead of the current position in the subject
       string, and those that look  behind  it.  An  assertion  subpattern  is
       matched  in  the  normal way, except that it does not cause the current
       matching position to be changed.

       Assertion subpatterns are not capturing subpatterns,  and  may  not  be
       repeated,  because  it  makes no sense to assert the same thing several
       times. If any kind of assertion contains capturing  subpatterns  within
       it,  these are counted for the purposes of numbering the capturing sub-
       patterns in the whole pattern.  However, substring capturing is carried
       out  only  for  positive assertions, because it does not make sense for
       negative assertions.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

         foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

         (?!foo)bar

       does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.

   Lookbehind assertions

       Lookbehind  assertions start with (?<= for positive assertions and (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does find an occurrence of "bar" that is not  preceded  by  "foo".  The
       contents  of  a  lookbehind  assertion are restricted such that all the
       strings it matches must have a fixed length. However, if there are sev-
       eral  top-level  alternatives,  they  do  not all have to have the same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes an error at compile time. Branches that match  different  length
       strings  are permitted only at the top level of a lookbehind assertion.
       This is an extension compared with  Perl  (at  least  for  5.8),  which
       requires  all branches to match the same length of string. An assertion
       such as

         (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different  lengths,  but  it is acceptable if rewritten to use two top-
       level branches:

         (?<=abc|abde)

       In some cases, the Perl 5.10 escape sequence \K (see above) can be used
       instead  of  a lookbehind assertion; this is not restricted to a fixed-
       length.

       The implementation of lookbehind assertions is, for  each  alternative,
       to  temporarily  move the current position back by the fixed length and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode) to appear in lookbehind assertions, because it makes it  impossi-
       ble  to  calculate the length of the lookbehind. The \X and \R escapes,
       which can match different numbers of bytes, are also not permitted.

       Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
       assertions  to  specify  efficient  matching  at the end of the subject
       string. Consider a simple pattern such as

         abcd$

       when applied to a long string that does  not  match.  Because  matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and then see if what follows matches the rest of the  pattern.  If  the
       pattern is specified as

         ^.*abcd$

       the  initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last  character,  then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to  left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there  can  be  no backtracking for the .*+ item; it can match only the
       entire string. The subsequent lookbehind assertion does a  single  test
       on  the last four characters. If it fails, the match fails immediately.
       For long strings, this approach makes a significant difference  to  the
       processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches  "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the previous three
       characters are all digits, and then there is  a  check  that  the  same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the  preceding  six  characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in  turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is  another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It is possible to cause the matching process to obey a subpattern  con-
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a previous capturing  subpat-
       tern  matched  or not. The two possible forms of conditional subpattern
       are

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern  (if  present)  is used. If there are more than two alterna-
       tives in the subpattern, a compile-time error occurs.

       There are four kinds of condition: references  to  subpatterns,  refer-
       ences to recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for a used subpattern by number

       If  the  text between the parentheses consists of a sequence of digits,
       the condition is true if the capturing subpattern of  that  number  has
       previously  matched.  An  alternative notation is to precede the digits
       with a plus or minus sign. In this case, the subpattern number is rela-
       tive rather than absolute.  The most recently opened parentheses can be
       referenced by (?(-1), the next most recent by (?(-2),  and  so  on.  In
       looping constructs it can also make sense to refer to subsequent groups
       with constructs such as (?(+2).

       Consider the following pattern, which  contains  non-significant  white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The first part matches an optional opening  parenthesis,  and  if  that
       character is present, sets it as the first captured substring. The sec-
       ond part matches one or more characters that are not  parentheses.  The
       third part is a conditional subpattern that tests whether the first set
       of parentheses matched or not. If they did, that is, if subject started
       with an opening parenthesis, the condition is true, and so the yes-pat-
       tern is executed and a  closing  parenthesis  is  required.  Otherwise,
       since  no-pattern  is  not  present, the subpattern matches nothing. In
       other words,  this  pattern  matches  a  sequence  of  non-parentheses,
       optionally enclosed in parentheses.

       If  you  were  embedding  this pattern in a larger one, you could use a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This makes the fragment independent of the parentheses  in  the  larger
       pattern.

   Checking for a used subpattern by name

       Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
       used subpattern by name. For compatibility  with  earlier  versions  of
       PCRE,  which  had this facility before Perl, the syntax (?(name)...) is
       also recognized. However, there is a possible ambiguity with this  syn-
       tax,  because  subpattern  names  may  consist entirely of digits. PCRE
       looks first for a named subpattern; if it cannot find one and the  name
       consists  entirely  of digits, PCRE looks for a subpattern of that num-
       ber, which must be greater than zero. Using subpattern names that  con-
       sist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )


   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name R, the condition is true if a recursive call to the whole  pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into the  subpat-
       tern  whose  number or name is given. This condition does not check the
       entire recursion stack.

       At "top level", all these recursion test conditions are  false.  Recur-
       sive patterns are described below.

   Defining subpatterns for use by reference only

       If  the  condition  is  the string (DEFINE), and there is no subpattern
       with the name DEFINE, the condition is  always  false.  In  this  case,
       there  may  be  only  one  alternative  in the subpattern. It is always
       skipped if control reaches this point  in  the  pattern;  the  idea  of
       DEFINE  is that it can be used to define "subroutines" that can be ref-
       erenced from elsewhere. (The use of "subroutines" is described  below.)
       For  example,  a pattern to match an IPv4 address could be written like
       this (ignore whitespace and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition.

       The rest of the pattern uses references to the named group to match the
       four  dot-separated  components of an IPv4 address, insisting on a word
       boundary at each end.

   Assertion conditions

       If the condition is not in any of the above  formats,  it  must  be  an
       assertion.   This may be a positive or negative lookahead or lookbehind
       assertion. Consider  this  pattern,  again  containing  non-significant
       white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The  condition  is  a  positive  lookahead  assertion  that  matches an
       optional sequence of non-letters followed by a letter. In other  words,
       it  tests  for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first  alternative;
       otherwise  it  is  matched  against  the  second.  This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
       letters and dd are digits.


COMMENTS

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses  are  not  permitted.  The
       characters  that make up a comment play no part in the pattern matching
       at all.

       If the PCRE_EXTENDED option is set, an unescaped # character outside  a
       character  class  introduces  a  comment  that continues to immediately
       after the next newline in the pattern.


RECURSIVE PATTERNS

       Consider the problem of matching a string in parentheses, allowing  for
       unlimited  nested  parentheses.  Without the use of recursion, the best
       that can be done is to use a pattern that  matches  up  to  some  fixed
       depth  of  nesting.  It  is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions  to recurse (amongst other things). It does this by interpolating
       Perl code in the expression at run time, and the code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE cannot support the interpolation of Perl code. Instead,
       it supports special syntax for recursion of  the  entire  pattern,  and
       also  for  individual  subpattern  recursion. After its introduction in
       PCRE and Python, this kind of recursion was  introduced  into  Perl  at
       release 5.10.

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive call of the subpattern of
       the  given  number, provided that it occurs inside that subpattern. (If
       not, it is a "subroutine" call, which is described  in  the  next  sec-
       tion.)  The special item (?R) or (?0) is a recursive call of the entire
       regular expression.

       In PCRE (like Python, but unlike Perl), a recursive subpattern call  is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.

       This  PCRE  pattern  solves  the nested parentheses problem (assume the
       PCRE_EXTENDED option is set so that white space is ignored):

         \( ( (?>[^()]+) | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a  correctly  parenthe-
       sized substring).  Finally there is a closing parenthesis.

       If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

         ( \( ( (?>[^()]+) | (?1) )* \) )

       We have put the pattern into parentheses, and caused the  recursion  to
       refer to them instead of the whole pattern.

       In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be
       tricky. This is made easier by the use of relative references. (A  Perl
       5.10  feature.)   Instead  of  (?1)  in the pattern above you can write
       (?-2) to refer to the second most recently opened parentheses preceding
       the  recursion.  In  other  words,  a  negative number counts capturing
       parentheses leftwards from the point at which it is encountered.

       It is also possible to refer to  subsequently  opened  parentheses,  by
       writing  references  such  as (?+2). However, these cannot be recursive
       because the reference is not inside the  parentheses  that  are  refer-
       enced.  They  are  always  "subroutine" calls, as described in the next
       section.

       An alternative approach is to use named parentheses instead.  The  Perl
       syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

         (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )

       If there is more than one subpattern with the same name,  the  earliest
       one is used.

       This  particular  example pattern that we have been looking at contains
       nested unlimited repeats, and so the use of atomic grouping for  match-
       ing  strings  of non-parentheses is important when applying the pattern
       to strings that do not match. For example, when this pattern is applied
       to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields "no match" quickly. However, if atomic grouping is not used,
       the match runs for a very long time indeed because there  are  so  many
       different  ways  the  + and * repeats can carve up the subject, and all
       have to be tested before failure can be reported.

       At the end of a match, the values set for any capturing subpatterns are
       those from the outermost level of the recursion at which the subpattern
       value is set.  If you want to obtain  intermediate  values,  a  callout
       function  can be used (see below and the pcrecallout documentation). If
       the pattern above is matched against

         (ab(cd)ef)

       the value for the capturing parentheses is  "ef",  which  is  the  last
       value  taken  on at the top level. If additional parentheses are added,
       giving

         \( ( ( (?>[^()]+) | (?R) )* ) \)
            ^                        ^
            ^                        ^

       the string they capture is "ab(cd)ef", the contents of  the  top  level
       parentheses.  If there are more than 15 capturing parentheses in a pat-
       tern, PCRE has to obtain extra memory to store data during a recursion,
       which  it  does  by  using pcre_malloc, freeing it via pcre_free after-
       wards. If  no  memory  can  be  obtained,  the  match  fails  with  the
       PCRE_ERROR_NOMEMORY error.

       Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In  this  pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and  non-recursive  cases.
       The (?R) item is the actual recursive call.


SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern reference (either by number or
       by name) is used outside the parentheses to which it refers,  it  oper-
       ates  like a subroutine in a programming language. The "called" subpat-
       tern may be defined before or after the reference. A numbered reference
       can be absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

       Like recursive subpatterns, a "subroutine" call is always treated as an
       atomic  group. That is, once it has matched some of the subject string,
       it is never re-entered, even if it contains  untried  alternatives  and
       there is a subsequent matching failure.

       When  a  subpattern is used as a subroutine, processing options such as
       case-independence are fixed when the subpattern is defined. They cannot
       be changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl  code to be obeyed in the middle of matching a regular expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an external function by putting its entry point in the global  variable
       pcre_callout.   By default, this variable contains NULL, which disables
       all calling out.

       Within a regular expression, (?C) indicates the  points  at  which  the
       external  function  is  to be called. If you want to identify different
       callout points, you can put a number less than 256 after the letter  C.
       The  default  value is zero.  For example, this pattern has two callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically  installed  before each item in the pattern. They are all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set),  the  external function is called. It is provided with the number
       of the callout, the position in the pattern, and, optionally, one  item
       of  data  originally supplied by the caller of pcre_exec(). The callout
       function may cause matching to proceed, to backtrack, or to fail  alto-
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.


BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control  Verbs",
       which are described in the Perl documentation as "experimental and sub-
       ject to change or removal in a future version of Perl". It goes  on  to
       say:  "Their usage in production code should be noted to avoid problems
       during upgrades." The same remarks apply to the PCRE features described
       in this section.

       Since these verbs are specifically related to backtracking, they can be
       used only when the pattern is to be matched  using  pcre_exec(),  which
       uses  a  backtracking  algorithm. They cause an error if encountered by
       pcre_dfa_exec().

       The new verbs make use of what was previously invalid syntax: an  open-
       ing parenthesis followed by an asterisk. In Perl, they are generally of
       the form (*VERB:ARG) but PCRE does not support the use of arguments, so
       its  general  form is just (*VERB). Any number of these verbs may occur
       in a pattern. There are two kinds:

   Verbs that act immediately

       The following verbs act as soon as they are encountered:

          (*ACCEPT)

       This verb causes the match to end successfully, skipping the  remainder
       of  the pattern. When inside a recursion, only the innermost pattern is
       ended immediately. PCRE differs  from  Perl  in  what  happens  if  the
       (*ACCEPT)  is inside capturing parentheses. In Perl, the data so far is
       captured: in PCRE no data is captured. For example:

         A(A|B(*ACCEPT)|C)D

       This matches "AB", "AAD", or "ACD", but when it matches "AB",  no  data
       is captured.

         (*FAIL) or (*F)

       This  verb  causes the match to fail, forcing backtracking to occur. It
       is equivalent to (?!) but easier to read. The Perl documentation  notes
       that  it  is  probably  useful only when combined with (?{}) or (??{}).
       Those are, of course, Perl features that are not present in  PCRE.  The
       nearest  equivalent is the callout feature, as for example in this pat-
       tern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout  is  taken
       before each backtrack happens (in this example, 10 times).

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues with what follows, but if there is no subsequent match, a  fail-
       ure  is  forced.   The  verbs  differ  in  exactly what kind of failure
       occurs.

         (*COMMIT)

       This verb causes the whole match to fail outright if the  rest  of  the
       pattern  does  not match. Even if the pattern is unanchored, no further
       attempts to find a match by advancing the start point take place.  Once
       (*COMMIT)  has been passed, pcre_exec() is committed to finding a match
       at the current starting point, or not at all. For example:

         a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as  a  kind
       of dynamic anchor, or "I've started, so I must finish."

         (*PRUNE)

       This  verb causes the match to fail at the current position if the rest
       of the pattern does not match. If the pattern is unanchored, the normal
       "bumpalong"  advance to the next starting character then happens. Back-
       tracking can occur as usual to the left of (*PRUNE), or  when  matching
       to  the right of (*PRUNE), but if there is no match to the right, back-
       tracking cannot cross (*PRUNE).  In simple cases, the use  of  (*PRUNE)
       is just an alternative to an atomic group or possessive quantifier, but
       there are some uses of (*PRUNE) that cannot be expressed in  any  other
       way.

         (*SKIP)

       This  verb  is like (*PRUNE), except that if the pattern is unanchored,
       the "bumpalong" advance is not to the next character, but to the  posi-
       tion  in  the  subject where (*SKIP) was encountered. (*SKIP) signifies
       that whatever text was matched leading up to it cannot  be  part  of  a
       successful match. Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect in this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".

         (*THEN)

       This verb causes a skip to the next alternation if the rest of the pat-
       tern does not match. That is, it cancels pending backtracking, but only
       within  the  current  alternation.  Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further  items
       after  the  end  of  the group if FOO succeeds); on failure the matcher
       skips to the second alternative and tries COND2,  without  backtracking
       into  COND1.  If  (*THEN)  is  used outside of any alternation, it acts
       exactly like (*PRUNE).


SEE ALSO

       pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 17 September 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCRESYNTAX(3)                                                    PCRESYNTAX(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE are described in  the  pcrepattern  documentation.  This
       document contains just a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal


CHARACTERS

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any character
         \e         escape (hex 1B)
         \f         formfeed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \ddd       character with octal code ddd, or backreference
         \xhh       character with hex code hh
         \x{hhh..}  character with hex code hhh..


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one byte, even in UTF-8 mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal whitespace character
         \H         a character that is not a horizontal whitespace character
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a whitespace character
         \S         a character that is not a whitespace character
         \v         a vertical whitespace character
         \V         a character that is not a vertical whitespace character
         \w         a "word" character
         \W         a "non-word" character
         \X         an extended Unicode sequence

       In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.


GENERAL CATEGORY PROPERTY CODES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


SCRIPT NAMES FOR \p AND \P

       Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
       Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
       Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
       gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,
       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
       Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       whitespace
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In PCRE, POSIX character set names recognize only ASCII characters. You
       can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                      also after internal newline in multiline mode
         \A          start of subject
         $           end of subject
                      also before newline at end of subject
                      also before internal newline in multiline mode
         \Z          end of subject
                      also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


MATCH POINT RESET

         \K          reset start of match


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)          capturing group
         (?<name>...)   named capturing group (Perl)
         (?'name'...)   named capturing group (Perl)
         (?P<name>...)  named capturing group (Python)
         (?:...)        non-capturing group
         (?|...)        non-capturing group; reset group numbers for
                         capturing groups in each alternative


ATOMIC GROUPS

         (?>...)        atomic, non-capturing group


COMMENT

         (?#....)       comment (not nestable)


OPTION SETTING

         (?i)           caseless
         (?J)           allow duplicate names
         (?m)           multiline
         (?s)           single line (dotall)
         (?U)           default ungreedy (lazy)
         (?x)           extended (ignore white space)
         (?-...)        unset option(s)


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)        positive look ahead
         (?!...)        negative look ahead
         (?<=...)       positive look behind
         (?<!...)       negative look behind

       Each top-level branch of a look behind must be of a fixed length.


BACKREFERENCES

         \n             reference by number (can be ambiguous)
         \gn            reference by number
         \g{n}          reference by number
         \g{-n}         relative reference by number
         \k<name>       reference by name (Perl)
         \k'name'       reference by name (Perl)
         \g{name}       reference by name (Perl)
         \k{name}       reference by name (.NET)
         (?P=name)      reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)           recurse whole pattern
         (?n)           call subpattern by absolute number
         (?+n)          call subpattern by relative number
         (?-n)          call subpattern by relative number
         (?&name)       call subpattern by name (Perl)
         (?P>name)      call subpattern by name (Python)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)...       absolute reference condition
         (?(+n)...      relative reference condition
         (?(-n)...      relative reference condition
         (?(<name>)...  named reference condition (Perl)
         (?('name')...  named reference condition (Perl)
         (?(name)...    named reference condition (PCRE)
         (?(R)...       overall recursion condition
         (?(Rn)...      specific group recursion condition
         (?(R&name)...  specific recursion condition
         (?(DEFINE)...  define subpattern for reference
         (?(assert)...  assertion condition


BACKTRACKING CONTROL

       The following act immediately they are reached:

         (*ACCEPT)      force successful match
         (*FAIL)        force backtrack; synonym (*F)

       The following act only when a subsequent match failure causes  a  back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)      overall failure, no advance of starting point
         (*PRUNE)       advance to next starting character
         (*SKIP)        advance start to current matching position
         (*THEN)        local failure, backtrack to next alternation


NEWLINE CONVENTIONS

       These  are  recognized only at the very start of the pattern or after a
       (*BSR_...) option.

         (*CR)
         (*LF)
         (*CRLF)
         (*ANYCRLF)
         (*ANY)


WHAT \R MATCHES

       These are recognized only at the very start of the pattern or  after  a
       (*...) option that sets the newline convention.

         (*BSR_ANYCRLF)
         (*BSR_UNICODE)


CALLOUTS

         (?C)      callout
         (?Cn)     callout with data n


SEE ALSO

       pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 14 November 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREPARTIAL(3)                                                  PCREPARTIAL(3)


NAME
       PCRE - Perl-compatible regular expressions


PARTIAL MATCHING IN PCRE

       In  normal  use  of  PCRE,  if  the  subject  string  that is passed to
       pcre_exec() or pcre_dfa_exec() matches as far as it goes,  but  is  too
       short  to  match  the  entire  pattern, PCRE_ERROR_NOMATCH is returned.
       There are circumstances where it might be helpful to  distinguish  this
       case from other cases in which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error as soon as a mistake is made, possibly beeping and not
       reflecting the character that has been typed. This  immediate  feedback
       is  likely  to  be a better user interface than a check that is delayed
       until the entire string has been entered.

       PCRE supports the concept of partial matching by means of the PCRE_PAR-
       TIAL   option,   which   can   be   set  when  calling  pcre_exec()  or
       pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
       PCRE_ERROR_NOMATCH  is converted into PCRE_ERROR_PARTIAL if at any time
       during the matching process the last part of the subject string matched
       part  of  the  pattern. Unfortunately, for non-anchored matching, it is
       not possible to obtain the position of the start of the partial  match.
       No captured data is set when PCRE_ERROR_PARTIAL is returned.

       When   PCRE_PARTIAL   is  set  for  pcre_dfa_exec(),  the  return  code
       PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the  end  of
       the  subject is reached, there have been no complete matches, but there
       is still at least one matching possibility. The portion of  the  string
       that provided the partial match is set as the first matching string.

       Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
       the last literal byte in a pattern, and abandons  matching  immediately
       if  such a byte is not present in the subject string. This optimization
       cannot be used for a subject string that might match only partially.


RESTRICTED PATTERNS FOR PCRE_PARTIAL

       Because of the way certain internal optimizations  are  implemented  in
       the  pcre_exec()  function, the PCRE_PARTIAL option cannot be used with
       all patterns. These restrictions do not apply when  pcre_dfa_exec()  is
       used.  For pcre_exec(), repeated single characters such as

         a{2,4}

       and repeated single metasequences such as

         \d+

       are  not permitted if the maximum number of occurrences is greater than
       one.  Optional items such as \d? (where the maximum is one) are permit-
       ted.   Quantifiers  with any values are permitted after parentheses, so
       the invalid examples above can be coded thus:

         (a){2,4}
         (\d)+

       These constructions run more slowly, but for the kinds  of  application
       that  are  envisaged  for this facility, this is not felt to be a major
       restriction.

       If PCRE_PARTIAL is set for a pattern  that  does  not  conform  to  the
       restrictions,  pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
       (-13).  You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo()  to
       find out if a compiled pattern can be used for partial matching.


EXAMPLE OF PARTIAL MATCHING USING PCRETEST

       If  the  escape  sequence  \P  is  present in a pcretest data line, the
       PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
       uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P
          0: 25jun04
          1: jun
         data> 25dec3\P
         Partial match
         data> 3ju\P
         Partial match
         data> 3juj\P
         No match
         data> j\P
         No match

       The  first  data  string  is  matched completely, so pcretest shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete  pattern,  but  the first two are partial matches. The same test,
       using pcre_dfa_exec() matching (by means of the  \D  escape  sequence),
       produces the following output:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P\D
          0: 25jun04
         data> 23dec3\P\D
         Partial match: 23dec3
         data> 3ju\P\D
         Partial match: 3ju
         data> 3juj\P\D
         No match
         data> j\P\D
         No match

       Notice  that in this case the portion of the string that was matched is
       made available.


MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()

       When a partial match has been found using pcre_dfa_exec(), it is possi-
       ble  to  continue  the  match  by providing additional subject data and
       calling pcre_dfa_exec() again with the same  compiled  regular  expres-
       sion, this time setting the PCRE_DFA_RESTART option. You must also pass
       the same working space as before, because this is where details of  the
       previous  partial  match are stored. Here is an example using pcretest,
       using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
       \D are as above):

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\P\D
         Partial match: 23ja
         data> n05\R\D
          0: n05

       The  first  call has "23ja" as the subject, and requests partial match-
       ing; the second call  has  "n05"  as  the  subject  for  the  continued
       (restarted)  match.   Notice  that when the match is complete, only the
       last part is shown; PCRE does  not  retain  the  previously  partially-
       matched  string. It is up to the calling program to do that if it needs
       to.

       You can set PCRE_PARTIAL  with  PCRE_DFA_RESTART  to  continue  partial
       matching over multiple segments. This facility can be used to pass very
       long subject strings to pcre_dfa_exec(). However, some care  is  needed
       for certain types of pattern.

       1.  If  the  pattern contains tests for the beginning or end of a line,
       you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options,  as  appropri-
       ate,  when  the subject string for any call does not contain the begin-
       ning or end of a line.

       2. If the pattern contains backward assertions (including  \b  or  \B),
       you  need  to  arrange for some overlap in the subject strings to allow
       for this. For example, you could pass the subject in  chunks  that  are
       500  bytes long, but in a buffer of 700 bytes, with the starting offset
       set to 200 and the previous 200 bytes at the start of the buffer.

       3. Matching a subject string that is split into multiple segments  does
       not  always produce exactly the same result as matching over one single
       long string.  The difference arises when there  are  multiple  matching
       possibilities,  because a partial match result is given only when there
       are no completed matches in a call to pcre_dfa_exec(). This means  that
       as  soon  as  the  shortest match has been found, continuation to a new
       subject segment is no longer possible.  Consider this pcretest example:

           re> /dog(sbody)?/
         data> do\P\D
         Partial match: do
         data> gsb\R\P\D
          0: g
         data> dogsbody\D
          0: dogsbody
          1: dog

       The  pattern matches the words "dog" or "dogsbody". When the subject is
       presented in several parts ("do" and "gsb" being  the  first  two)  the
       match  stops  when "dog" has been found, and it is not possible to con-
       tinue. On the other hand,  if  "dogsbody"  is  presented  as  a  single
       string, both matches are found.

       Because  of  this  phenomenon,  it does not usually make sense to end a
       pattern that is going to be matched in this way with a variable repeat.

       4. Patterns that contain alternatives at the top level which do not all
       start with the same pattern item may not work as expected. For example,
       consider this pattern:

         1234|3789

       If  the  first  part of the subject is "ABC123", a partial match of the
       first alternative is found at offset 3. There is no partial  match  for
       the second alternative, because such a match does not start at the same
       point in the subject string. Attempting to  continue  with  the  string
       "789" does not yield a match because only those alternatives that match
       at one point in the subject are remembered. The problem arises  because
       the  start  of the second alternative matches within the first alterna-
       tive. There is no problem with anchored patterns or patterns such as:

         1234|ABCD

       where no string can be a partial match for both alternatives.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 04 June 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREPRECOMPILE(3)                                            PCREPRECOMPILE(3)


NAME
       PCRE - Perl-compatible regular expressions


SAVING AND RE-USING PRECOMPILED PCRE PATTERNS

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit more complicated.

       If you save compiled patterns to a file, you can copy them to a differ-
       ent  host  and  run them there. This works even if the new host has the
       opposite endianness to the one on which  the  patterns  were  compiled.
       There  may  be a small performance penalty, but it should be insignifi-
       cant. However, compiling regular expressions with one version  of  PCRE
       for  use  with  a  different  version is not guaranteed to work and may
       cause crashes.


SAVING A COMPILED PATTERN
       The value returned by pcre_compile() points to a single block of memory
       that  holds  the compiled pattern and associated data. You can find the
       length of this block in bytes by calling pcre_fullinfo() with an  argu-
       ment  of  PCRE_INFO_SIZE. You can then save the data in any appropriate
       manner. Here is sample code that compiles a pattern and writes it to  a
       file. It assumes that the variable fd refers to a file that is open for
       output:

         int erroroffset, rc, size;
         char *error;
         pcre *re;

         re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

       In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

       Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

       If  the pattern has been studied, it is also possible to save the study
       data in a similar way to the compiled  pattern  itself.  When  studying
       generates  additional  information, pcre_study() returns a pointer to a
       pcre_extra data block. Its format is defined in the section on matching
       a  pattern in the pcreapi documentation. The study_data field points to
       the binary study data,  and  this  is  what  you  must  save  (not  the
       pcre_extra  block itself). The length of the study data can be obtained
       by calling pcre_fullinfo() with  an  argument  of  PCRE_INFO_STUDYSIZE.
       Remember  to check that pcre_study() did return a non-NULL value before
       trying to save the study data.


RE-USING A PRECOMPILED PATTERN

       Re-using a precompiled pattern is straightforward. Having  reloaded  it
       into   main   memory,   you   pass   its   pointer  to  pcre_exec()  or
       pcre_dfa_exec() in the usual way. This  should  work  even  on  another
       host,  and  even  if  that  host has the opposite endianness to the one
       where the pattern was compiled.

       However, if you passed a pointer to custom character  tables  when  the
       pattern  was  compiled  (the  tableptr argument of pcre_compile()), you
       must now pass a similar  pointer  to  pcre_exec()  or  pcre_dfa_exec(),
       because  the  value  saved  with the compiled pattern will obviously be
       nonsense. A field in a pcre_extra() block is used to pass this data, as
       described  in the section on matching a pattern in the pcreapi documen-
       tation.

       If you did not provide custom character tables  when  the  pattern  was
       compiled,  the  pointer  in  the compiled pattern is NULL, which causes
       pcre_exec() to use PCRE's internal tables. Thus, you  do  not  need  to
       take any special action at run time in this case.

       If  you  saved study data with the compiled pattern, you need to create
       your own pcre_extra data block and set the study_data field to point to
       the  reloaded  study  data. You must also set the PCRE_EXTRA_STUDY_DATA
       bit in the flags field to indicate that study  data  is  present.  Then
       pass  the  pcre_extra  block  to  pcre_exec() or pcre_dfa_exec() in the
       usual way.


COMPATIBILITY WITH DIFFERENT PCRE RELEASES

       In general, it is safest to  recompile  all  saved  patterns  when  you
       update  to  a new PCRE release, though not all updates actually require
       this. Recompiling is definitely needed for release 7.2.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 13 June 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREPERFORM(3)                                                  PCREPERFORM(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


MEMORY USAGE

       Patterns are compiled by PCRE into a reasonably efficient byte code, so
       that most simple patterns do not use much memory. However, there is one
       case where memory usage can be unexpectedly large. When a parenthesized
       subpattern has a quantifier with a minimum greater than 1 and/or a lim-
       ited  maximum,  the  whole subpattern is repeated in the compiled code.
       For example, the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

       For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses 51K bytes when compiled. When PCRE is compiled  with  its  default
       internal  pointer  size of two bytes, the size limit on a compiled pat-
       tern is 64K, and this is reached with the above pattern  if  the  outer
       repetition is increased from 3 to 4. PCRE can be compiled to use larger
       internal pointers and thus handle larger compiled patterns, but  it  is
       better to try to rewrite your pattern to use less memory if you can.

       One  way  of reducing the memory usage for such patterns is to make use
       of PCRE's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to 18K, and indeed it remains under 20K
       even  with the outer repetition increased to 100. However, this pattern
       is not exactly equivalent, because the "subroutine" calls  are  treated
       as  atomic groups into which there can be no backtracking if there is a
       subsequent matching failure. Therefore, PCRE cannot  do  this  kind  of
       rewriting  automatically.   Furthermore,  there is a noticeable loss of
       speed when executing the modified pattern. Nevertheless, if the  atomic
       grouping  is  not  a  problem and the loss of speed is acceptable, this
       kind of rewriting will allow you to process patterns that  PCRE  cannot
       otherwise handle.


PROCESSING TIME

       Certain  items  in regular expression patterns are processed more effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]   than   a   set   of  single-character  alternatives  such  as
       (a|e|i|o|u). In general, the simplest construction  that  provides  the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains a lot of useful general discussion  about  optimizing  regular
       expressions  for  efficient  performance.  This document contains a few
       observations about PCRE.

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a  character's  property.
       If  you  can  find  an  alternative pattern that does not use character
       properties, it will probably be faster.

       When a pattern begins with .* not in  parentheses,  or  in  parentheses
       that are not the subject of a backreference, and the PCRE_DOTALL option
       is set, the pattern is implicitly anchored by PCRE, since it can  match
       only  at  the start of a subject string. However, if PCRE_DOTALL is not
       set, PCRE cannot make this optimization, because  the  .  metacharacter
       does  not then match a newline, and if the subject string contains new-
       lines, the pattern may match from the character  immediately  following
       one of them instead of from the very start. For example, the pattern

         .*second

       matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to do this, PCRE has to retry the match starting after every newline in
       the subject.

       If you are using such a pattern with subject strings that do  not  con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or starting the pattern with ^.* or ^.*? to indicate  explicit  anchor-
       ing.  That saves PCRE from having to scan along the subject looking for
       a newline to restart at.

       Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

         ^(a+)*

       This can match "aaaa" in 16 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail, PCRE has in
       principle  to  try  every  possible  variation,  and  this  can take an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before  embarking  on  the  standard
       matching  procedure,  PCRE checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 06 March 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCREPOSIX(3)                                                      PCREPOSIX(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF POSIX API

       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION

       This  set  of  functions provides a POSIX-style API to the PCRE regular
       expression package. See the pcreapi documentation for a description  of
       PCRE's native API, which contains much additional functionality.

       The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h  header  file,  and  on  Unix systems the library itself is
       called pcreposix.a, so can be accessed by  adding  -lpcreposix  to  the
       command  for  linking  an application that uses them. Because the POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I have implemented only those option bits that can be reasonably mapped
       to PCRE native options. In addition, the option REG_EXTENDED is defined
       with the value zero. This has no effect, but since  programs  that  are
       written  to  the  POSIX interface often use it, this makes it easier to
       slot in PCRE as a replacement library. Other POSIX options are not even
       defined.

       When  PCRE  is  called  via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE options, as described below. "POSIX-like in  style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-byte encoding  domains  it  is  probably
       even less compatible.

       The  header for these functions is supplied as pcreposix.h to avoid any
       potential clash with other POSIX  libraries.  It  can,  of  course,  be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two structure types, regex_t for  compiled  internal  forms,  and  reg-
       match_t  for  returning  captured substrings. It also defines some con-
       stants whose names start  with  "REG_";  these  are  used  for  setting
       options and identifying error codes.


COMPILING A PATTERN

       The  function regcomp() is called to compile a pattern into an internal
       form. The pattern is a C string terminated by a  binary  zero,  and  is
       passed  in  the  argument  pattern. The preg argument is a pointer to a
       regex_t structure that is used as a base for storing information  about
       the compiled regular expression.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The PCRE_DOTALL option is set when the regular expression is passed for
       compilation to the native function. Note that REG_DOTALL is not part of
       the POSIX standard.

         REG_ICASE

       The PCRE_CASELESS option is set when the regular expression  is  passed
       for compilation to the native function.

         REG_NEWLINE

       The  PCRE_MULTILINE option is set when the regular expression is passed
       for compilation to the native function. Note that this does  not  mimic
       the  defined  POSIX  behaviour  for REG_NEWLINE (see the following sec-
       tion).

         REG_NOSUB

       The PCRE_NO_AUTO_CAPTURE option is set when the regular  expression  is
       passed for compilation to the native function. In addition, when a pat-
       tern that is compiled with this flag is passed to regexec() for  match-
       ing,  the  nmatch  and  pmatch  arguments  are ignored, and no captured
       strings are returned.

         REG_UTF8

       The PCRE_UTF8 option is set when the regular expression is  passed  for
       compilation  to the native function. This causes the pattern itself and
       all data strings used for matching it to be treated as  UTF-8  strings.
       Note that REG_UTF8 is not part of the POSIX standard.

       In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the  regex  is  compiled  with  PCRE  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they  aren't)  or
       by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE to obey  POSIX  semantics,  but
       then  PCRE was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching  newline  characters  in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl,  there  is
       no way to stop newline from matching [^a].

       The   default  POSIX  newline  handling  can  be  obtained  by  setting
       PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to  make  PCRE
       behave exactly as for the REG_NEWLINE action.


MATCHING A PATTERN

       The  function  regexec()  is  called  to  match a compiled pattern preg
       against a given string, which is terminated by a zero byte, subject  to
       the options in eflags. These can be:

         REG_NOTBOL

       The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEOL

       The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       regexec() are ignored.

       Otherwise,the portion of the string that was matched, and also any cap-
       tured substrings, are returned via the pmatch argument, which points to
       an  array  of nmatch structures of type regmatch_t, containing the mem-
       bers rm_so and rm_eo. These contain the offset to the  first  character
       of  each  substring and the offset to the first character after the end
       of each substring, respectively. The 0th element of the vector  relates
       to  the  entire portion of string that was matched; subsequent elements
       relate to the capturing subpatterns of the regular  expression.  Unused
       entries in the array have both structure members set to -1.

       A  successful  match  yields  a  zero  return;  various error codes are
       defined in the header file, of  which  REG_NOMATCH  is  the  "expected"
       failure code.


ERROR MESSAGES

       The regerror() function maps a non-zero errorcode from either regcomp()
       or regexec() to a printable message. If preg is  not  NULL,  the  error
       should have arisen from the use of that structure. A message terminated
       by a binary zero is placed  in  errbuf.  The  length  of  the  message,
       including  the  zero, is limited to errbuf_size. The yield of the func-
       tion is the size of buffer needed to hold the whole message.


MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with  the preg structure. The function regfree() frees all such
       memory, after which preg may no longer be used as  a  compiled  expres-
       sion.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 06 March 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------


PCRECPP(3)                                                          PCRECPP(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF C++ WRAPPER

       #include <pcrecpp.h>


DESCRIPTION

       The  C++  wrapper  for PCRE was provided by Google Inc. Some additional
       functionality was added by Giuseppe Maxia. This brief man page was con-
       structed  from  the  notes  in the pcrecpp.h file, which should be con-
       sulted for further details.


MATCHING INTERFACE

       The "FullMatch" operation checks that supplied text matches a  supplied
       pattern  exactly.  If pointer arguments are supplied, it copies matched
       sub-strings that match sub-patterns into them.

         Example: successful match
            pcrecpp::RE re("h.*o");
            re.FullMatch("hello");

         Example: unsuccessful match (requires full match):
            pcrecpp::RE re("e");
            !re.FullMatch("hello");

         Example: creating a temporary RE object:
            pcrecpp::RE("h.*o").FullMatch("hello");

       You can pass in a "const char*" or a "string" for "text". The  examples
       below  tend to use a const char*. You can, as in the different examples
       above, store the RE object explicitly in a variable or use a  temporary
       RE  object.  The  examples below use one mode or the other arbitrarily.
       Either could correctly be used for any of these examples.

       You must supply extra pointer arguments to extract matched subpieces.

         Example: extracts "ruby" into "s" and 1234 into "i"
            int i;
            string s;
            pcrecpp::RE re("(\\w+):(\\d+)");
            re.FullMatch("ruby:1234", &s, &i);

         Example: does not try to extract any extra sub-patterns
            re.FullMatch("ruby:1234", &s);

         Example: does not try to extract into NULL
            re.FullMatch("ruby:1234", NULL, &i);

         Example: integer overflow causes failure
            !re.FullMatch("ruby:1234567891234", NULL, &i);

         Example: fails because there aren't enough sub-patterns:
            !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);

         Example: fails because string cannot be stored in integer
            !pcrecpp::RE("(.*)").FullMatch("ruby", &i);

       The provided pointer arguments can be pointers to  any  scalar  numeric
       type, or one of:

          string        (matched piece is copied to string)
          StringPiece   (StringPiece is mutated to point to matched piece)
          T             (where "bool T::ParseFrom(const char*, int)" exists)
          NULL          (the corresponding matched sub-pattern is not copied)

       The  function returns true iff all of the following conditions are sat-
       isfied:

         a. "text" matches "pattern" exactly;

         b. The number of matched sub-patterns is >= number of supplied
            pointers;

         c. The "i"th argument has a suitable type for holding the
            string captured as the "i"th sub-pattern. If you pass in
            void * NULL for the "i"th argument, or a non-void * NULL
            of the correct type, or pass fewer arguments than the
            number of sub-patterns, "i"th captured sub-pattern is
            ignored.

       CAVEAT: An optional sub-pattern that does  not  exist  in  the  matched
       string  is  assigned  the  empty  string. Therefore, the following will
       return false (because the empty string is not a valid number):

          int number;
          pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);

       The matching interface supports at most 16 arguments per call.  If  you
       need    more,    consider    using    the    more   general   interface
       pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.


QUOTING METACHARACTERS

       You can use the "QuoteMeta" operation to insert backslashes before  all
       potentially  meaningful  characters  in  a string. The returned string,
       used as a regular expression, will exactly match the original string.

         Example:
            string quoted = RE::QuoteMeta(unquoted);

       Note that it's legal to escape a character even if it  has  no  special
       meaning  in  a  regular expression -- so this function does that. (This
       also makes it identical to the perl function  of  the  same  name;  see
       "perldoc    -f    quotemeta".)    For   example,   "1.5-2.0?"   becomes
       "1\.5\-2\.0\?".


PARTIAL MATCHES

       You can use the "PartialMatch" operation when you want the  pattern  to
       match any substring of the text.

         Example: simple search for a string:
            pcrecpp::RE("ell").PartialMatch("hello");

         Example: find first number in a string:
            int number;
            pcrecpp::RE re("(\\d+)");
            re.PartialMatch("x*100 + 20", &number);
            assert(number == 100);


UTF-8 AND THE MATCHING INTERFACE

       By  default,  pattern  and text are plain text, one byte per character.
       The UTF8 flag, passed to  the  constructor,  causes  both  pattern  and
       string to be treated as UTF-8 text, still a byte stream but potentially
       multiple bytes per character. In practice, the text is likelier  to  be
       UTF-8  than  the pattern, but the match returned may depend on the UTF8
       flag, so always use it when matching UTF8 text. For example,  "."  will
       match  one  byte normally but with UTF8 set may match up to three bytes
       of a multi-byte character.

         Example:
            pcrecpp::RE_Options options;
            options.set_utf8();
            pcrecpp::RE re(utf8_pattern, options);
            re.FullMatch(utf8_string);

         Example: using the convenience function UTF8():
            pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
            re.FullMatch(utf8_string);

       NOTE: The UTF8 flag is ignored if pcre was not configured with the
             --enable-utf8 flag.


PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE

       PCRE defines some modifiers to  change  the  behavior  of  the  regular
       expression   engine.  The  C++  wrapper  defines  an  auxiliary  class,
       RE_Options, as a vehicle to pass such modifiers to  a  RE  class.  Cur-
       rently, the following modifiers are supported:

          modifier              description               Perl corresponding

          PCRE_CASELESS         case insensitive match      /i
          PCRE_MULTILINE        multiple lines match        /m
          PCRE_DOTALL           dot matches newlines        /s
          PCRE_DOLLAR_ENDONLY   $ matches only at end       N/A
          PCRE_EXTRA            strict escape parsing       N/A
          PCRE_EXTENDED         ignore whitespaces          /x
          PCRE_UTF8             handles UTF8 chars          built-in
          PCRE_UNGREEDY         reverses * and *?           N/A
          PCRE_NO_AUTO_CAPTURE  disables capturing parens   N/A (*)

       (*)  Both Perl and PCRE allow non capturing parentheses by means of the
       "?:" modifier within the pattern itself. e.g. (?:ab|cd) does  not  cap-
       ture, while (ab|cd) does.

       For  a  full  account on how each modifier works, please check the PCRE
       API reference page.

       For each modifier, there are two member functions whose  name  is  made
       out  of  the  modifier  in  lowercase,  without the "PCRE_" prefix. For
       instance, PCRE_CASELESS is handled by

         bool caseless()

       which returns true if the modifier is set, and

         RE_Options & set_caseless(bool)

       which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
       be  accessed  through  the  set_match_limit()  and match_limit() member
       functions. Setting match_limit to a non-zero value will limit the  exe-
       cution  of pcre to keep it from doing bad things like blowing the stack
       or taking an eternity to return a result.  A  value  of  5000  is  good
       enough  to stop stack blowup in a 2MB thread stack. Setting match_limit
       to  zero  disables  match  limiting.  Alternatively,   you   can   call
       match_limit_recursion()  which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
       limit how much  PCRE  recurses.  match_limit()  limits  the  number  of
       matches PCRE does; match_limit_recursion() limits the depth of internal
       recursion, and therefore the amount of stack that is used.

       Normally, to pass one or more modifiers to a RE class,  you  declare  a
       RE_Options object, set the appropriate options, and pass this object to
       a RE constructor. Example:

          RE_options opt;
          opt.set_caseless(true);
          if (RE("HELLO", opt).PartialMatch("hello world")) ...

       RE_options has two constructors. The default constructor takes no argu-
       ments  and creates a set of flags that are off by default. The optional
       parameter option_flags is to facilitate transfer of legacy code from  C
       programs.  This lets you do

          RE(pattern,
            RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);

       However, new code is better off doing

          RE(pattern,
            RE_Options().set_caseless(true).set_multiline(true))
              .PartialMatch(str);

       If you are going to pass one of the most used modifiers, there are some
       convenience functions that return a RE_Options class with the appropri-
       ate  modifier  already  set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
       and EXTENDED().

       If you need to set several options at once, and you don't  want  to  go
       through  the pains of declaring a RE_Options object and setting several
       options, there is a parallel method that give you such ability  on  the
       fly.  You  can  concatenate several set_xxxxx() member functions, since
       each of them returns a reference to its class object. For  example,  to
       pass  PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
       statement, you may write:

          RE(" ^ xyz \\s+ .* blah$",
            RE_Options()
              .set_caseless(true)
              .set_extended(true)
              .set_multiline(true)).PartialMatch(sometext);


SCANNING TEXT INCREMENTALLY

       The "Consume" operation may be useful if you want to  repeatedly  match
       regular expressions at the front of a string and skip over them as they
       match. This requires use of the "StringPiece" type, which represents  a
       sub-range  of  a  real  string.  Like RE, StringPiece is defined in the
       pcrecpp namespace.

         Example: read lines of the form "var = value" from a string.
            string contents = ...;                 // Fill string somehow
            pcrecpp::StringPiece input(contents);  // Wrap in a StringPiece

            string var;
            int value;
            pcrecpp::RE re("(\\w+) = (\\d+)\n");
            while (re.Consume(&input, &var, &value)) {
              ...;
            }

       Each successful call  to  "Consume"  will  set  "var/value",  and  also
       advance "input" so it points past the matched text.

       The  "FindAndConsume"  operation  is  similar to "Consume" but does not
       anchor your match at the beginning of  the  string.  For  example,  you
       could extract all words from a string by repeatedly calling

         pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)


PARSING HEX/OCTAL/C-RADIX NUMBERS

       By default, if you pass a pointer to a numeric value, the corresponding
       text is interpreted as a base-10  number.  You  can  instead  wrap  the
       pointer with a call to one of the operators Hex(), Octal(), or CRadix()
       to interpret the text in another base. The CRadix  operator  interprets
       C-style  "0"  (base-8)  and  "0x"  (base-16)  prefixes, but defaults to
       base-10.

         Example:
           int a, b, c, d;
           pcrecpp::RE re("(.*) (.*) (.*) (.*)");
           re.FullMatch("100 40 0100 0x40",
                        pcrecpp::Octal(&a), pcrecpp::Hex(&b),
                        pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));

       will leave 64 in a, b, c, and d.


REPLACING PARTS OF STRINGS

       You can replace the first match of "pattern" in "str"  with  "rewrite".
       Within  "rewrite",  backslash-escaped  digits (\1 to \9) can be used to
       insert text matching corresponding parenthesized group  from  the  pat-
       tern. \0 in "rewrite" refers to the entire matching text. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").Replace("d", &s);

       will  leave  "s" containing "yada dabba doo". The result is true if the
       pattern matches and a replacement occurs, false otherwise.

       GlobalReplace is like Replace except that it replaces  all  occurrences
       of  the  pattern  in  the string with the rewrite. Replacements are not
       subject to re-matching. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").GlobalReplace("d", &s);

       will leave "s" containing "yada dada doo". It  returns  the  number  of
       replacements made.

       Extract  is like Replace, except that if the pattern matches, "rewrite"
       is copied into "out" (an additional argument) with substitutions.   The
       non-matching  portions  of "text" are ignored. Returns true iff a match
       occurred and the extraction happened successfully;  if no match occurs,
       the string is left unaffected.


AUTHOR

       The C++ wrapper was contributed by Google Inc.
       Copyright (c) 2007 Google Inc.


REVISION

       Last updated: 12 November 2007
------------------------------------------------------------------------------


PCRESAMPLE(3)                                                    PCRESAMPLE(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE SAMPLE PROGRAM

       A simple, complete demonstration program, to get you started with using
       PCRE, is supplied in the file pcredemo.c in the PCRE distribution.

       The program compiles the regular expression that is its first argument,
       and  matches  it  against the subject string in its second argument. No
       PCRE options are set, and default character tables are used. If  match-
       ing  succeeds,  the  program  outputs  the  portion of the subject that
       matched, together with the contents of any captured substrings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       If PCRE is installed in the standard include  and  library  directories
       for  your  system, you should be able to compile the demonstration pro-
       gram using this command:

         gcc -o pcredemo pcredemo.c -lpcre

       If PCRE is installed elsewhere, you may need to add additional  options
       to  the  command line. For example, on a Unix-like system that has PCRE
       installed in /usr/local, you  can  compile  the  demonstration  program
       using a command like this:

         gcc -o pcredemo -I/usr/local/include pcredemo.c \
             -L/usr/local/lib -lpcre

       Once  you  have  compiled the demonstration program, you can run simple
       tests like this:

         ./pcredemo 'cat|dog' 'the cat sat on the mat'
         ./pcredemo -g 'cat|dog' 'the dog sat on the cat'

       Note that there is a  much  more  comprehensive  test  program,  called
       pcretest,  which  supports  many  more  facilities  for testing regular
       expressions and the PCRE library. The pcredemo program is provided as a
       simple coding example.

       On some operating systems (e.g. Solaris), when PCRE is not installed in
       the standard library directory, you may get an error like this when you
       try to run pcredemo:

         ld.so.1:  a.out:  fatal:  libpcre.so.0:  open failed: No such file or
       directory

       This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 23 January 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCRESTACK(3)                                                      PCRESTACK(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE DISCUSSION OF STACK USAGE

       When  you call pcre_exec(), it makes use of an internal function called
       match(). This calls itself recursively at branch points in the pattern,
       in  order to remember the state of the match so that it can back up and
       try a different alternative if the first one fails.  As  matching  pro-
       ceeds  deeper  and deeper into the tree of possibilities, the recursion
       depth increases.

       Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       The pcre_dfa_exec() function operates in an entirely different way, and
       hardly uses recursion at all. The limit on its complexity is the amount
       of  workspace  it  is  given.  The comments that follow do NOT apply to
       pcre_dfa_exec(); they are relevant only for pcre_exec().

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively. If the limit is exceeded, an error occurs.
       For details, see the section on  extra  data  for  pcre_exec()  in  the
       pcreapi documentation.

       Each  time  that match() is actually called recursively, it uses memory
       from the process stack. For certain kinds of  pattern  and  data,  very
       large  amounts of stack may be needed, despite the recognition of "tail
       recursion".  You can often reduce the amount of recursion,  and  there-
       fore  the  amount of stack used, by modifying the pattern that is being
       matched. Consider, for example, this pattern:

         ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet"  or  the
       end  of  the  data,  and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a  parenthesis  is  processed,  a  recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter. For a long string, a lot of stack is required. Consider  now  this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))+

       This  uses very much less stack, because runs of characters that do not
       contain "<" are "swallowed" in one item inside the parentheses.  Recur-
       sion  happens  only when a "<" character that is not followed by "inet"
       is encountered (and we assume this is relatively  rare).  A  possessive
       quantifier  is  used  to stop any backtracking into the runs of non-"<"
       characters, but that is not related to stack usage.

       This example shows that one way of avoiding stack problems when  match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

       In environments where stack memory is constrained, you  might  want  to
       compile  PCRE to use heap memory instead of stack for remembering back-
       up points. This makes it run a lot more slowly, however. Details of how
       to do this are given in the pcrebuild documentation. When built in this
       way, instead of using the stack, PCRE obtains and frees memory by call-
       ing  the  functions  that  are  pointed to by the pcre_stack_malloc and
       pcre_stack_free variables. By default,  these  point  to  malloc()  and
       free(),  but you can replace the pointers to cause PCRE to use your own
       functions. Since the block sizes are always the same,  and  are  always
       freed in reverse order, it may be possible to implement customized mem-
       ory handlers that are more efficient than the standard functions.

       In Unix-like environments, there is not often a problem with the  stack
       unless  very  long  strings  are  involved, though the default limit on
       stack size varies from system to system. Values from 8Mb  to  64Mb  are
       common. You can find your default limit by running the command:

         ulimit -s

       Unfortunately,  the  effect  of  running out of stack is often SIGSEGV,
       though sometimes a more explicit error message is given. You  can  nor-
       mally increase the limit on stack size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This  reads  the current limits (soft and hard) using getrlimit(), then
       attempts to increase the soft limit to  100Mb  using  setrlimit().  You
       must do this before calling pcre_exec().

       PCRE  has  an  internal  counter that can be used to limit the depth of
       recursion, and thus cause pcre_exec() to give an error code  before  it
       runs  out  of  stack. By default, the limit is very large, and unlikely
       ever to operate. It can be changed when PCRE is built, and it can  also
       be set when pcre_exec() is called. For details of these interfaces, see
       the pcrebuild and pcreapi documentation.

       As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion.  Thus,  if  you  want  to limit your stack usage to 8Mb, you
       should set the limit at 16000 recursions. A 64Mb stack,  on  the  other
       hand,  can  support around 128000 recursions. The pcretest test program
       has a command line option (-S) that can be used to increase the size of
       its stack.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 05 June 2007
       Copyright (c) 1997-2007 University of Cambridge.
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