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# Line 2  Line 2 
2  This file contains a concatenation of the PCRE man pages, converted to plain  This file contains a concatenation of the PCRE man pages, converted to plain
3  text format for ease of searching with a text editor, or for use on systems  text format for ease of searching with a text editor, or for use on systems
4  that do not have a man page processor. The small individual files that give  that do not have a man page processor. The small individual files that give
5  synopses of each function in the library have not been included. There are  synopses of each function in the library have not been included. Neither has
6  separate text files for the pcregrep and pcretest commands.  the pcredemo program. There are separate text files for the pcregrep and
7    pcretest commands.
8  -----------------------------------------------------------------------------  -----------------------------------------------------------------------------
9    
10    
# Line 18  INTRODUCTION Line 19  INTRODUCTION
19    
20         The  PCRE  library is a set of functions that implement regular expres-         The  PCRE  library is a set of functions that implement regular expres-
21         sion pattern matching using the same syntax and semantics as Perl, with         sion pattern matching using the same syntax and semantics as Perl, with
22         just  a  few  differences. Certain features that appeared in Python and         just  a few differences. Some features that appeared in Python and PCRE
23         PCRE before they appeared in Perl are also available using  the  Python         before they appeared in Perl are also available using the  Python  syn-
24         syntax.  There is also some support for certain .NET and Oniguruma syn-         tax,  there  is  some  support for one or two .NET and Oniguruma syntax
25         tax items, and there is an option for  requesting  some  minor  changes         items, and there is an option for requesting some  minor  changes  that
26         that give better JavaScript compatibility.         give better JavaScript compatibility.
27    
28         The  current  implementation of PCRE (release 7.x) corresponds approxi-         The  current implementation of PCRE corresponds approximately with Perl
29         mately with Perl 5.10, including support for UTF-8 encoded strings  and         5.10, including support for UTF-8 encoded strings and  Unicode  general
30         Unicode general category properties. However, UTF-8 and Unicode support         category  properties.  However,  UTF-8  and  Unicode  support has to be
31         has to be explicitly enabled; it is not the default. The Unicode tables         explicitly enabled; it is not the default. The  Unicode  tables  corre-
32         correspond to Unicode release 5.0.0.         spond to Unicode release 5.2.0.
33    
34         In  addition to the Perl-compatible matching function, PCRE contains an         In  addition to the Perl-compatible matching function, PCRE contains an
35         alternative matching function that matches the same  compiled  patterns         alternative function that matches the same compiled patterns in a  dif-
36         in  a different way. In certain circumstances, the alternative function         ferent way. In certain circumstances, the alternative function has some
37         has some advantages. For a discussion of the two  matching  algorithms,         advantages.  For a discussion of the two matching algorithms,  see  the
38         see the pcrematching page.         pcrematching page.
39    
40         PCRE  is  written  in C and released as a C library. A number of people         PCRE  is  written  in C and released as a C library. A number of people
41         have written wrappers and interfaces of various kinds.  In  particular,         have written wrappers and interfaces of various kinds.  In  particular,
# Line 54  INTRODUCTION Line 55  INTRODUCTION
55         library is built. The pcre_config() function makes it  possible  for  a         library is built. The pcre_config() function makes it  possible  for  a
56         client  to  discover  which  features are available. The features them-         client  to  discover  which  features are available. The features them-
57         selves are described in the pcrebuild page. Documentation about  build-         selves are described in the pcrebuild page. Documentation about  build-
58         ing  PCRE for various operating systems can be found in the README file         ing  PCRE  for various operating systems can be found in the README and
59         in the source distribution.         NON-UNIX-USE files in the source distribution.
60    
61         The library contains a number of undocumented  internal  functions  and         The library contains a number of undocumented  internal  functions  and
62         data  tables  that  are  used by more than one of the exported external         data  tables  that  are  used by more than one of the exported external
# Line 71  USER DOCUMENTATION Line 72  USER DOCUMENTATION
72         The user documentation for PCRE comprises a number  of  different  sec-         The user documentation for PCRE comprises a number  of  different  sec-
73         tions.  In the "man" format, each of these is a separate "man page". In         tions.  In the "man" format, each of these is a separate "man page". In
74         the HTML format, each is a separate page, linked from the  index  page.         the HTML format, each is a separate page, linked from the  index  page.
75         In  the  plain text format, all the sections are concatenated, for ease         In  the  plain  text format, all the sections, except the pcredemo sec-
76         of searching. The sections are as follows:         tion, are concatenated, for ease of searching. The sections are as fol-
77           lows:
78    
79           pcre              this document           pcre              this document
80           pcre-config       show PCRE installation configuration information           pcre-config       show PCRE installation configuration information
# Line 81  USER DOCUMENTATION Line 83  USER DOCUMENTATION
83           pcrecallout       details of the callout feature           pcrecallout       details of the callout feature
84           pcrecompat        discussion of Perl compatibility           pcrecompat        discussion of Perl compatibility
85           pcrecpp           details of the C++ wrapper           pcrecpp           details of the C++ wrapper
86             pcredemo          a demonstration C program that uses PCRE
87           pcregrep          description of the pcregrep command           pcregrep          description of the pcregrep command
88           pcrematching      discussion of the two matching algorithms           pcrematching      discussion of the two matching algorithms
89           pcrepartial       details of the partial matching facility           pcrepartial       details of the partial matching facility
90           pcrepattern       syntax and semantics of supported           pcrepattern       syntax and semantics of supported
91                               regular expressions                               regular expressions
          pcresyntax        quick syntax reference  
92           pcreperform       discussion of performance issues           pcreperform       discussion of performance issues
93           pcreposix         the POSIX-compatible C API           pcreposix         the POSIX-compatible C API
94           pcreprecompile    details of saving and re-using precompiled patterns           pcreprecompile    details of saving and re-using precompiled patterns
95           pcresample        discussion of the sample program           pcresample        discussion of the pcredemo program
96           pcrestack         discussion of stack usage           pcrestack         discussion of stack usage
97             pcresyntax        quick syntax reference
98           pcretest          description of the pcretest testing command           pcretest          description of the pcretest testing command
99    
100         In  addition,  in the "man" and HTML formats, there is a short page for         In  addition,  in the "man" and HTML formats, there is a short page for
# Line 136  UTF-8 AND UNICODE PROPERTY SUPPORT Line 139  UTF-8 AND UNICODE PROPERTY SUPPORT
139    
140         In  order  process  UTF-8 strings, you must build PCRE to include UTF-8         In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
141         support in the code, and, in addition,  you  must  call  pcre_compile()         support in the code, and, in addition,  you  must  call  pcre_compile()
142         with  the PCRE_UTF8 option flag. When you do this, both the pattern and         with  the  PCRE_UTF8  option  flag,  or the pattern must start with the
143         any subject strings that are matched against it are  treated  as  UTF-8         sequence (*UTF8). When either of these is the case,  both  the  pattern
144         strings instead of just strings of bytes.         and  any  subject  strings  that  are matched against it are treated as
145           UTF-8 strings instead of strings of 1-byte characters.
146    
147         If  you compile PCRE with UTF-8 support, but do not use it at run time,         If you compile PCRE with UTF-8 support, but do not use it at run  time,
148         the library will be a bit bigger, but the additional run time  overhead         the  library will be a bit bigger, but the additional run time overhead
149         is limited to testing the PCRE_UTF8 flag occasionally, so should not be         is limited to testing the PCRE_UTF8 flag occasionally, so should not be
150         very big.         very big.
151    
152         If PCRE is built with Unicode character property support (which implies         If PCRE is built with Unicode character property support (which implies
153         UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-         UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
154         ported.  The available properties that can be tested are limited to the         ported.  The available properties that can be tested are limited to the
155         general  category  properties such as Lu for an upper case letter or Nd         general category properties such as Lu for an upper case letter  or  Nd
156         for a decimal number, the Unicode script names such as Arabic  or  Han,         for  a  decimal number, the Unicode script names such as Arabic or Han,
157         and  the  derived  properties  Any  and L&. A full list is given in the         and the derived properties Any and L&. A full  list  is  given  in  the
158         pcrepattern documentation. Only the short names for properties are sup-         pcrepattern documentation. Only the short names for properties are sup-
159         ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-         ported. For example, \p{L} matches a letter. Its Perl synonym,  \p{Let-
160         ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may         ter},  is  not  supported.   Furthermore,  in Perl, many properties may
161         optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE         optionally be prefixed by "Is", for compatibility with Perl  5.6.  PCRE
162         does not support this.         does not support this.
163    
164     Validity of UTF-8 strings     Validity of UTF-8 strings
165    
166         When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and         When  you  set  the  PCRE_UTF8 flag, the strings passed as patterns and
167         subjects are (by default) checked for validity on entry to the relevant         subjects are (by default) checked for validity on entry to the relevant
168         functions. From release 7.3 of PCRE, the check is according  the  rules         functions.  From  release 7.3 of PCRE, the check is according the rules
169         of  RFC  3629, which are themselves derived from the Unicode specifica-         of RFC 3629, which are themselves derived from the  Unicode  specifica-
170         tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which         tion.  Earlier  releases  of PCRE followed the rules of RFC 2279, which
171         allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current         allows the full range of 31-bit values (0 to 0x7FFFFFFF).  The  current
172         check allows only values in the range U+0 to U+10FFFF, excluding U+D800         check allows only values in the range U+0 to U+10FFFF, excluding U+D800
173         to U+DFFF.         to U+DFFF.
174    
175         The  excluded  code  points are the "Low Surrogate Area" of Unicode, of         The excluded code points are the "Low Surrogate Area"  of  Unicode,  of
176         which the Unicode Standard says this: "The Low Surrogate Area does  not         which  the Unicode Standard says this: "The Low Surrogate Area does not
177         contain  any  character  assignments,  consequently  no  character code         contain any  character  assignments,  consequently  no  character  code
178         charts or namelists are provided for this area. Surrogates are reserved         charts or namelists are provided for this area. Surrogates are reserved
179         for  use  with  UTF-16 and then must be used in pairs." The code points         for use with UTF-16 and then must be used in pairs."  The  code  points
180         that are encoded by UTF-16 pairs  are  available  as  independent  code         that  are  encoded  by  UTF-16  pairs are available as independent code
181         points  in  the  UTF-8  encoding.  (In other words, the whole surrogate         points in the UTF-8 encoding. (In  other  words,  the  whole  surrogate
182         thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)         thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
183    
184         If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return         If  an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error return
185         (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know         (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
186         that your strings are valid, and therefore want to skip these checks in         that your strings are valid, and therefore want to skip these checks in
187         order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at         order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
188         compile time or at run time, PCRE assumes that the pattern  or  subject         compile  time  or at run time, PCRE assumes that the pattern or subject
189         it  is  given  (respectively)  contains only valid UTF-8 codes. In this         it is given (respectively) contains only valid  UTF-8  codes.  In  this
190         case, it does not diagnose an invalid UTF-8 string.         case, it does not diagnose an invalid UTF-8 string.
191    
192         If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,         If  you  pass  an  invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
193         what  happens  depends on why the string is invalid. If the string con-         what happens depends on why the string is invalid. If the  string  con-
194         forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a         forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
195         string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,         string of characters in the range 0  to  0x7FFFFFFF.  In  other  words,
196         apart from the initial validity test, PCRE (when in UTF-8 mode) handles         apart from the initial validity test, PCRE (when in UTF-8 mode) handles
197         strings  according  to  the more liberal rules of RFC 2279. However, if         strings according to the more liberal rules of RFC  2279.  However,  if
198         the string does not even conform to RFC 2279, the result is  undefined.         the  string does not even conform to RFC 2279, the result is undefined.
199         Your program may crash.         Your program may crash.
200    
201         If  you  want  to  process  strings  of  values  in the full range 0 to         If you want to process strings  of  values  in  the  full  range  0  to
202         0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can         0x7FFFFFFF,  encoded in a UTF-8-like manner as per the old RFC, you can
203         set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in         set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
204         this situation, you will have to apply your own validity check.         this situation, you will have to apply your own validity check.
205    
206     General comments about UTF-8 mode     General comments about UTF-8 mode
207    
208         1. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a         1.  An  unbraced  hexadecimal  escape sequence (such as \xb3) matches a
209         two-byte UTF-8 character if the value is greater than 127.         two-byte UTF-8 character if the value is greater than 127.
210    
211         2.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8         2. Octal numbers up to \777 are recognized, and  match  two-byte  UTF-8
212         characters for values greater than \177.         characters for values greater than \177.
213    
214         3. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-         3.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
215         vidual bytes, for example: \x{100}{3}.         vidual bytes, for example: \x{100}{3}.
216    
217         4.  The dot metacharacter matches one UTF-8 character instead of a sin-         4. The dot metacharacter matches one UTF-8 character instead of a  sin-
218         gle byte.         gle byte.
219    
220         5. The escape sequence \C can be used to match a single byte  in  UTF-8         5.  The  escape sequence \C can be used to match a single byte in UTF-8
221         mode,  but  its  use can lead to some strange effects. This facility is         mode, but its use can lead to some strange effects.  This  facility  is
222         not available in the alternative matching function, pcre_dfa_exec().         not available in the alternative matching function, pcre_dfa_exec().
223    
224         6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly         6.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
225         test  characters of any code value, but the characters that PCRE recog-         test characters of any code value, but the characters that PCRE  recog-
226         nizes as digits, spaces, or word characters  remain  the  same  set  as         nizes  as  digits,  spaces,  or  word characters remain the same set as
227         before, all with values less than 256. This remains true even when PCRE         before, all with values less than 256. This remains true even when PCRE
228         includes Unicode property support, because to do otherwise  would  slow         includes  Unicode  property support, because to do otherwise would slow
229         down  PCRE in many common cases. If you really want to test for a wider         down PCRE in many common cases. If you really want to test for a  wider
230         sense of, say, "digit", you must use Unicode  property  tests  such  as         sense  of,  say,  "digit",  you must use Unicode property tests such as
231         \p{Nd}.         \p{Nd}. Note that this also applies to \b, because  it  is  defined  in
232           terms of \w and \W.
233    
234         7.  Similarly,  characters that match the POSIX named character classes         7.  Similarly,  characters that match the POSIX named character classes
235         are all low-valued characters.         are all low-valued characters.
# Line 258  AUTHOR Line 263  AUTHOR
263    
264  REVISION  REVISION
265    
266         Last updated: 12 April 2008         Last updated: 01 March 2010
267         Copyright (c) 1997-2008 University of Cambridge.         Copyright (c) 1997-2010 University of Cambridge.
268  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
269    
270    
# Line 277  PCRE BUILD-TIME OPTIONS Line 282  PCRE BUILD-TIME OPTIONS
282         script,  where the optional features are selected or deselected by pro-         script,  where the optional features are selected or deselected by pro-
283         viding options to configure before running the make  command.  However,         viding options to configure before running the make  command.  However,
284         the  same  options  can be selected in both Unix-like and non-Unix-like         the  same  options  can be selected in both Unix-like and non-Unix-like
285         environments using the GUI facility of  CMakeSetup  if  you  are  using         environments using the GUI facility of cmake-gui if you are using CMake
286         CMake instead of configure to build PCRE.         instead of configure to build PCRE.
287    
288           There  is  a  lot more information about building PCRE in non-Unix-like
289           environments in the file called NON_UNIX_USE, which is part of the PCRE
290           distribution.  You  should consult this file as well as the README file
291           if you are building in a non-Unix-like environment.
292    
293         The complete list of options for configure (which includes the standard         The complete list of options for configure (which includes the standard
294         ones such as the  selection  of  the  installation  directory)  can  be         ones  such  as  the  selection  of  the  installation directory) can be
295         obtained by running         obtained by running
296    
297           ./configure --help           ./configure --help
298    
299         The  following  sections  include  descriptions  of options whose names         The following sections include  descriptions  of  options  whose  names
300         begin with --enable or --disable. These settings specify changes to the         begin with --enable or --disable. These settings specify changes to the
301         defaults  for  the configure command. Because of the way that configure         defaults for the configure command. Because of the way  that  configure
302         works, --enable and --disable always come in pairs, so  the  complemen-         works,  --enable  and --disable always come in pairs, so the complemen-
303         tary  option always exists as well, but as it specifies the default, it         tary option always exists as well, but as it specifies the default,  it
304         is not described.         is not described.
305    
306    
# Line 307  C++ SUPPORT Line 317  C++ SUPPORT
317    
318  UTF-8 SUPPORT  UTF-8 SUPPORT
319    
320         To build PCRE with support for UTF-8 character strings, add         To build PCRE with support for UTF-8 Unicode character strings, add
321    
322           --enable-utf8           --enable-utf8
323    
324         to the configure command. Of itself, this  does  not  make  PCRE  treat         to  the  configure  command.  Of  itself, this does not make PCRE treat
325         strings  as UTF-8. As well as compiling PCRE with this option, you also         strings as UTF-8. As well as compiling PCRE with this option, you  also
326         have have to set the PCRE_UTF8 option when you call the  pcre_compile()         have  have to set the PCRE_UTF8 option when you call the pcre_compile()
327         function.         or pcre_compile2() functions.
328    
329           If you set --enable-utf8 when compiling in an EBCDIC environment,  PCRE
330           expects its input to be either ASCII or UTF-8 (depending on the runtime
331           option). It is not possible to support both EBCDIC and UTF-8  codes  in
332           the  same  version  of  the  library.  Consequently,  --enable-utf8 and
333           --enable-ebcdic are mutually exclusive.
334    
335    
336  UNICODE CHARACTER PROPERTY SUPPORT  UNICODE CHARACTER PROPERTY SUPPORT
337    
338         UTF-8  support allows PCRE to process character values greater than 255         UTF-8 support allows PCRE to process character values greater than  255
339         in the strings that it handles. On its own, however, it does  not  pro-         in  the  strings that it handles. On its own, however, it does not pro-
340         vide any facilities for accessing the properties of such characters. If         vide any facilities for accessing the properties of such characters. If
341         you want to be able to use the pattern escapes \P, \p,  and  \X,  which         you  want  to  be able to use the pattern escapes \P, \p, and \X, which
342         refer to Unicode character properties, you must add         refer to Unicode character properties, you must add
343    
344           --enable-unicode-properties           --enable-unicode-properties
345    
346         to  the configure command. This implies UTF-8 support, even if you have         to the configure command. This implies UTF-8 support, even if you  have
347         not explicitly requested it.         not explicitly requested it.
348    
349         Including Unicode property support adds around 30K  of  tables  to  the         Including  Unicode  property  support  adds around 30K of tables to the
350         PCRE  library.  Only  the general category properties such as Lu and Nd         PCRE library. Only the general category properties such as  Lu  and  Nd
351         are supported. Details are given in the pcrepattern documentation.         are supported. Details are given in the pcrepattern documentation.
352    
353    
354  CODE VALUE OF NEWLINE  CODE VALUE OF NEWLINE
355    
356         By default, PCRE interprets character 10 (linefeed, LF)  as  indicating         By  default,  PCRE interprets the linefeed (LF) character as indicating
357         the  end  of  a line. This is the normal newline character on Unix-like         the end of a line. This is the normal newline  character  on  Unix-like
358         systems. You can compile PCRE to use character 13 (carriage return, CR)         systems.  You  can compile PCRE to use carriage return (CR) instead, by
359         instead, by adding         adding
360    
361           --enable-newline-is-cr           --enable-newline-is-cr
362    
363         to  the  configure  command.  There  is  also  a --enable-newline-is-lf         to the  configure  command.  There  is  also  a  --enable-newline-is-lf
364         option, which explicitly specifies linefeed as the newline character.         option, which explicitly specifies linefeed as the newline character.
365    
366         Alternatively, you can specify that line endings are to be indicated by         Alternatively, you can specify that line endings are to be indicated by
# Line 356  CODE VALUE OF NEWLINE Line 372  CODE VALUE OF NEWLINE
372    
373           --enable-newline-is-anycrlf           --enable-newline-is-anycrlf
374    
375         which  causes  PCRE  to recognize any of the three sequences CR, LF, or         which causes PCRE to recognize any of the three sequences  CR,  LF,  or
376         CRLF as indicating a line ending. Finally, a fifth option, specified by         CRLF as indicating a line ending. Finally, a fifth option, specified by
377    
378           --enable-newline-is-any           --enable-newline-is-any
# Line 416  HANDLING VERY LARGE PATTERNS Line 432  HANDLING VERY LARGE PATTERNS
432         nation metacharacter). By default, two-byte values are used  for  these         nation metacharacter). By default, two-byte values are used  for  these
433         offsets,  leading  to  a  maximum size for a compiled pattern of around         offsets,  leading  to  a  maximum size for a compiled pattern of around
434         64K. This is sufficient to handle all but the most  gigantic  patterns.         64K. This is sufficient to handle all but the most  gigantic  patterns.
435         Nevertheless,  some  people do want to process enormous patterns, so it         Nevertheless,  some  people do want to process truyl enormous patterns,
436         is possible to compile PCRE to use three-byte or four-byte  offsets  by         so it is possible to compile PCRE to use three-byte or  four-byte  off-
437         adding a setting such as         sets by adding a setting such as
438    
439           --with-link-size=3           --with-link-size=3
440    
# Line 445  AVOIDING EXCESSIVE STACK USAGE Line 461  AVOIDING EXCESSIVE STACK USAGE
461         to the configure command. With this configuration, PCRE  will  use  the         to the configure command. With this configuration, PCRE  will  use  the
462         pcre_stack_malloc  and pcre_stack_free variables to call memory manage-         pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
463         ment functions. By default these point to malloc() and free(), but  you         ment functions. By default these point to malloc() and free(), but  you
464         can replace the pointers so that your own functions are used.         can replace the pointers so that your own functions are used instead.
465    
466         Separate  functions  are  provided  rather  than  using pcre_malloc and         Separate  functions  are  provided  rather  than  using pcre_malloc and
467         pcre_free because the  usage  is  very  predictable:  the  block  sizes         pcre_free because the  usage  is  very  predictable:  the  block  sizes
# Line 453  AVOIDING EXCESSIVE STACK USAGE Line 469  AVOIDING EXCESSIVE STACK USAGE
469         reverse order. A calling program might be able to  implement  optimized         reverse order. A calling program might be able to  implement  optimized
470         functions  that  perform  better  than  malloc()  and free(). PCRE runs         functions  that  perform  better  than  malloc()  and free(). PCRE runs
471         noticeably more slowly when built in this way. This option affects only         noticeably more slowly when built in this way. This option affects only
472         the   pcre_exec()   function;   it   is   not   relevant  for  the  the         the pcre_exec() function; it is not relevant for pcre_dfa_exec().
        pcre_dfa_exec() function.  
473    
474    
475  LIMITING PCRE RESOURCE USAGE  LIMITING PCRE RESOURCE USAGE
476    
477         Internally, PCRE has a function called match(), which it calls  repeat-         Internally,  PCRE has a function called match(), which it calls repeat-
478         edly   (sometimes   recursively)  when  matching  a  pattern  with  the         edly  (sometimes  recursively)  when  matching  a  pattern   with   the
479         pcre_exec() function. By controlling the maximum number of  times  this         pcre_exec()  function.  By controlling the maximum number of times this
480         function  may be called during a single matching operation, a limit can         function may be called during a single matching operation, a limit  can
481         be placed on the resources used by a single call  to  pcre_exec().  The         be  placed  on  the resources used by a single call to pcre_exec(). The
482         limit  can be changed at run time, as described in the pcreapi documen-         limit can be changed at run time, as described in the pcreapi  documen-
483         tation. The default is 10 million, but this can be changed by adding  a         tation.  The default is 10 million, but this can be changed by adding a
484         setting such as         setting such as
485    
486           --with-match-limit=500000           --with-match-limit=500000
487    
488         to   the   configure  command.  This  setting  has  no  effect  on  the         to  the  configure  command.  This  setting  has  no  effect   on   the
489         pcre_dfa_exec() matching function.         pcre_dfa_exec() matching function.
490    
491         In some environments it is desirable to limit the  depth  of  recursive         In  some  environments  it is desirable to limit the depth of recursive
492         calls of match() more strictly than the total number of calls, in order         calls of match() more strictly than the total number of calls, in order
493         to restrict the maximum amount of stack (or heap,  if  --disable-stack-         to  restrict  the maximum amount of stack (or heap, if --disable-stack-
494         for-recursion is specified) that is used. A second limit controls this;         for-recursion is specified) that is used. A second limit controls this;
495         it defaults to the value that  is  set  for  --with-match-limit,  which         it  defaults  to  the  value  that is set for --with-match-limit, which
496         imposes  no  additional constraints. However, you can set a lower limit         imposes no additional constraints. However, you can set a  lower  limit
497         by adding, for example,         by adding, for example,
498    
499           --with-match-limit-recursion=10000           --with-match-limit-recursion=10000
500    
501         to the configure command. This value can  also  be  overridden  at  run         to  the  configure  command.  This  value can also be overridden at run
502         time.         time.
503    
504    
505  CREATING CHARACTER TABLES AT BUILD TIME  CREATING CHARACTER TABLES AT BUILD TIME
506    
507         PCRE  uses fixed tables for processing characters whose code values are         PCRE uses fixed tables for processing characters whose code values  are
508         less than 256. By default, PCRE is built with a set of tables that  are         less  than 256. By default, PCRE is built with a set of tables that are
509         distributed  in  the  file pcre_chartables.c.dist. These tables are for         distributed in the file pcre_chartables.c.dist. These  tables  are  for
510         ASCII codes only. If you add         ASCII codes only. If you add
511    
512           --enable-rebuild-chartables           --enable-rebuild-chartables
513    
514         to the configure command, the distributed tables are  no  longer  used.         to  the  configure  command, the distributed tables are no longer used.
515         Instead,  a  program  called dftables is compiled and run. This outputs         Instead, a program called dftables is compiled and  run.  This  outputs
516         the source for new set of tables, created in the default locale of your         the source for new set of tables, created in the default locale of your
517         C runtime system. (This method of replacing the tables does not work if         C runtime system. (This method of replacing the tables does not work if
518         you are cross compiling, because dftables is run on the local host.  If         you  are cross compiling, because dftables is run on the local host. If
519         you  need  to  create alternative tables when cross compiling, you will         you need to create alternative tables when cross  compiling,  you  will
520         have to do so "by hand".)         have to do so "by hand".)
521    
522    
523  USING EBCDIC CODE  USING EBCDIC CODE
524    
525         PCRE assumes by default that it will run in an  environment  where  the         PCRE  assumes  by  default that it will run in an environment where the
526         character  code  is  ASCII  (or Unicode, which is a superset of ASCII).         character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
527         This is the case for most computer operating systems.  PCRE  can,  how-         This  is  the  case for most computer operating systems. PCRE can, how-
528         ever, be compiled to run in an EBCDIC environment by adding         ever, be compiled to run in an EBCDIC environment by adding
529    
530           --enable-ebcdic           --enable-ebcdic
531    
532         to the configure command. This setting implies --enable-rebuild-charta-         to the configure command. This setting implies --enable-rebuild-charta-
533         bles. You should only use it if you know that  you  are  in  an  EBCDIC         bles.  You  should  only  use  it if you know that you are in an EBCDIC
534         environment (for example, an IBM mainframe operating system).         environment (for example,  an  IBM  mainframe  operating  system).  The
535           --enable-ebcdic option is incompatible with --enable-utf8.
536    
537    
538  PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT  PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
# Line 542  PCRETEST OPTION FOR LIBREADLINE SUPPORT Line 558  PCRETEST OPTION FOR LIBREADLINE SUPPORT
558         to  the  configure  command,  pcretest  is  linked with the libreadline         to  the  configure  command,  pcretest  is  linked with the libreadline
559         library, and when its input is from a terminal, it reads it  using  the         library, and when its input is from a terminal, it reads it  using  the
560         readline() function. This provides line-editing and history facilities.         readline() function. This provides line-editing and history facilities.
561         Note that libreadline is GPL-licenced, so if you distribute a binary of         Note that libreadline is GPL-licensed, so if you distribute a binary of
562         pcretest linked in this way, there may be licensing issues.         pcretest linked in this way, there may be licensing issues.
563    
564         Setting  this  option  causes  the -lreadline option to be added to the         Setting  this  option  causes  the -lreadline option to be added to the
# Line 578  AUTHOR Line 594  AUTHOR
594    
595  REVISION  REVISION
596    
597         Last updated: 13 April 2008         Last updated: 29 September 2009
598         Copyright (c) 1997-2008 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
599  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
600    
601    
# Line 666  THE ALTERNATIVE MATCHING ALGORITHM Line 682  THE ALTERNATIVE MATCHING ALGORITHM
682         though  it is not implemented as a traditional finite state machine (it         though  it is not implemented as a traditional finite state machine (it
683         keeps multiple states active simultaneously).         keeps multiple states active simultaneously).
684    
685           Although the general principle of this matching algorithm  is  that  it
686           scans  the subject string only once, without backtracking, there is one
687           exception: when a lookaround assertion is encountered,  the  characters
688           following  or  preceding  the  current  point  have to be independently
689           inspected.
690    
691         The scan continues until either the end of the subject is  reached,  or         The scan continues until either the end of the subject is  reached,  or
692         there  are  no more unterminated paths. At this point, terminated paths         there  are  no more unterminated paths. At this point, terminated paths
693         represent the different matching possibilities (if there are none,  the         represent the different matching possibilities (if there are none,  the
694         match  has  failed).   Thus,  if there is more than one possible match,         match  has  failed).   Thus,  if there is more than one possible match,
695         this algorithm finds all of them, and in particular, it finds the long-         this algorithm finds all of them, and in particular, it finds the long-
696         est.  In PCRE, there is an option to stop the algorithm after the first         est.  There  is  an  option to stop the algorithm after the first match
697         match (which is necessarily the shortest) has been found.         (which is necessarily the shortest) is found.
698    
699         Note that all the matches that are found start at the same point in the         Note that all the matches that are found start at the same point in the
700         subject. If the pattern         subject. If the pattern
# Line 742  ADVANTAGES OF THE ALTERNATIVE ALGORITHM Line 764  ADVANTAGES OF THE ALTERNATIVE ALGORITHM
764         more than one match using the standard algorithm, you have to do kludgy         more than one match using the standard algorithm, you have to do kludgy
765         things with callouts.         things with callouts.
766    
767         2.  There is much better support for partial matching. The restrictions         2.  Because  the  alternative  algorithm  scans the subject string just
        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  
768         once, and never needs to backtrack, it is possible to  pass  very  long         once, and never needs to backtrack, it is possible to  pass  very  long
769         subject  strings  to  the matching function in several pieces, checking         subject  strings  to  the matching function in several pieces, checking
770         for partial matching each time.         for partial matching each time.  The  pcrepartial  documentation  gives
771           details of partial matching.
772    
773    
774  DISADVANTAGES OF THE ALTERNATIVE ALGORITHM  DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
775    
776         The alternative algorithm suffers from a number of disadvantages:         The alternative algorithm suffers from a number of disadvantages:
777    
778         1. It is substantially slower than  the  standard  algorithm.  This  is         1.  It  is  substantially  slower  than the standard algorithm. This is
779         partly  because  it has to search for all possible matches, but is also         partly because it has to search for all possible matches, but  is  also
780         because it is less susceptible to optimization.         because it is less susceptible to optimization.
781    
782         2. Capturing parentheses and back references are not supported.         2. Capturing parentheses and back references are not supported.
# Line 777  AUTHOR Line 794  AUTHOR
794    
795  REVISION  REVISION
796    
797         Last updated: 19 April 2008         Last updated: 29 September 2009
798         Copyright (c) 1997-2008 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
799  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
800    
801    
# Line 889  PCRE API OVERVIEW Line 906  PCRE API OVERVIEW
906         pcre_exec() are used for compiling and matching regular expressions  in         pcre_exec() are used for compiling and matching regular expressions  in
907         a  Perl-compatible  manner. A sample program that demonstrates the sim-         a  Perl-compatible  manner. A sample program that demonstrates the sim-
908         plest way of using them is provided in the file  called  pcredemo.c  in         plest way of using them is provided in the file  called  pcredemo.c  in
909         the  source distribution. The pcresample documentation describes how to         the PCRE source distribution. A listing of this program is given in the
910         compile and run it.         pcredemo documentation, and the pcresample documentation describes  how
911           to compile and run it.
912    
913         A second matching function, pcre_dfa_exec(), which is not Perl-compati-         A second matching function, pcre_dfa_exec(), which is not Perl-compati-
914         ble,  is  also provided. This uses a different algorithm for the match-         ble, is also provided. This uses a different algorithm for  the  match-
915         ing. The alternative algorithm finds all possible matches (at  a  given         ing.  The  alternative algorithm finds all possible matches (at a given
916         point  in  the subject), and scans the subject just once. However, this         point in the subject), and scans the subject just  once  (unless  there
917         algorithm does not return captured substrings. A description of the two         are  lookbehind  assertions).  However,  this algorithm does not return
918         matching  algorithms and their advantages and disadvantages is given in         captured substrings. A description of the two matching  algorithms  and
919         the pcrematching documentation.         their  advantages  and disadvantages is given in the pcrematching docu-
920           mentation.
921    
922         In addition to the main compiling and  matching  functions,  there  are         In addition to the main compiling and  matching  functions,  there  are
923         convenience functions for extracting captured substrings from a subject         convenience functions for extracting captured substrings from a subject
# Line 999  MULTITHREADING Line 1018  MULTITHREADING
1018         pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the         pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1019         callout function pointed to by pcre_callout, are shared by all threads.         callout function pointed to by pcre_callout, are shared by all threads.
1020    
1021         The compiled form of a regular expression is not altered during  match-         The  compiled form of a regular expression is not altered during match-
1022         ing, so the same compiled pattern can safely be used by several threads         ing, so the same compiled pattern can safely be used by several threads
1023         at once.         at once.
1024    
# Line 1007  MULTITHREADING Line 1026  MULTITHREADING
1026  SAVING PRECOMPILED PATTERNS FOR LATER USE  SAVING PRECOMPILED PATTERNS FOR LATER USE
1027    
1028         The compiled form of a regular expression can be saved and re-used at a         The compiled form of a regular expression can be saved and re-used at a
1029         later  time,  possibly by a different program, and even on a host other         later time, possibly by a different program, and even on a  host  other
1030         than the one on which  it  was  compiled.  Details  are  given  in  the         than  the  one  on  which  it  was  compiled.  Details are given in the
1031         pcreprecompile  documentation.  However, compiling a regular expression         pcreprecompile documentation. However, compiling a  regular  expression
1032         with one version of PCRE for use with a different version is not  guar-         with  one version of PCRE for use with a different version is not guar-
1033         anteed to work and may cause crashes.         anteed to work and may cause crashes.
1034    
1035    
# Line 1018  CHECKING BUILD-TIME OPTIONS Line 1037  CHECKING BUILD-TIME OPTIONS
1037    
1038         int pcre_config(int what, void *where);         int pcre_config(int what, void *where);
1039    
1040         The  function pcre_config() makes it possible for a PCRE client to dis-         The function pcre_config() makes it possible for a PCRE client to  dis-
1041         cover which optional features have been compiled into the PCRE library.         cover which optional features have been compiled into the PCRE library.
1042         The  pcrebuild documentation has more details about these optional fea-         The pcrebuild documentation has more details about these optional  fea-
1043         tures.         tures.
1044    
1045         The first argument for pcre_config() is an  integer,  specifying  which         The  first  argument  for pcre_config() is an integer, specifying which
1046         information is required; the second argument is a pointer to a variable         information is required; the second argument is a pointer to a variable
1047         into which the information is  placed.  The  following  information  is         into  which  the  information  is  placed. The following information is
1048         available:         available:
1049    
1050           PCRE_CONFIG_UTF8           PCRE_CONFIG_UTF8
1051    
1052         The  output is an integer that is set to one if UTF-8 support is avail-         The output is an integer that is set to one if UTF-8 support is  avail-
1053         able; otherwise it is set to zero.         able; otherwise it is set to zero.
1054    
1055           PCRE_CONFIG_UNICODE_PROPERTIES           PCRE_CONFIG_UNICODE_PROPERTIES
1056    
1057         The output is an integer that is set to  one  if  support  for  Unicode         The  output  is  an  integer  that is set to one if support for Unicode
1058         character properties is available; otherwise it is set to zero.         character properties is available; otherwise it is set to zero.
1059    
1060           PCRE_CONFIG_NEWLINE           PCRE_CONFIG_NEWLINE
1061    
1062         The  output  is  an integer whose value specifies the default character         The output is an integer whose value specifies  the  default  character
1063         sequence that is recognized as meaning "newline". The four values  that         sequence  that is recognized as meaning "newline". The four values that
1064         are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,         are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1065         and -1 for ANY. The default should normally be  the  standard  sequence         and  -1  for  ANY.  Though they are derived from ASCII, the same values
1066         for your operating system.         are returned in EBCDIC environments. The default should normally corre-
1067           spond to the standard sequence for your operating system.
1068    
1069           PCRE_CONFIG_BSR           PCRE_CONFIG_BSR
1070    
# Line 1071  CHECKING BUILD-TIME OPTIONS Line 1091  CHECKING BUILD-TIME OPTIONS
1091    
1092           PCRE_CONFIG_MATCH_LIMIT           PCRE_CONFIG_MATCH_LIMIT
1093    
1094         The output is an integer that gives the default limit for the number of         The output is a long integer that gives the default limit for the  num-
1095         internal  matching  function  calls in a pcre_exec() execution. Further         ber  of  internal  matching  function calls in a pcre_exec() execution.
1096         details are given with pcre_exec() below.         Further details are given with pcre_exec() below.
1097    
1098           PCRE_CONFIG_MATCH_LIMIT_RECURSION           PCRE_CONFIG_MATCH_LIMIT_RECURSION
1099    
1100         The output is an integer that gives the default limit for the depth  of         The output is a long integer that gives the default limit for the depth
1101         recursion  when calling the internal matching function in a pcre_exec()         of   recursion  when  calling  the  internal  matching  function  in  a
1102         execution. Further details are given with pcre_exec() below.         pcre_exec() execution.  Further  details  are  given  with  pcre_exec()
1103           below.
1104    
1105           PCRE_CONFIG_STACKRECURSE           PCRE_CONFIG_STACKRECURSE
1106    
1107         The output is an integer that is set to one if internal recursion  when         The  output is an integer that is set to one if internal recursion when
1108         running pcre_exec() is implemented by recursive function calls that use         running pcre_exec() is implemented by recursive function calls that use
1109         the stack to remember their state. This is the usual way that  PCRE  is         the  stack  to remember their state. This is the usual way that PCRE is
1110         compiled. The output is zero if PCRE was compiled to use blocks of data         compiled. The output is zero if PCRE was compiled to use blocks of data
1111         on the  heap  instead  of  recursive  function  calls.  In  this  case,         on  the  heap  instead  of  recursive  function  calls.  In  this case,
1112         pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory         pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
1113         blocks on the heap, thus avoiding the use of the stack.         blocks on the heap, thus avoiding the use of the stack.
1114    
1115    
# Line 1105  COMPILING A PATTERN Line 1126  COMPILING A PATTERN
1126    
1127         Either of the functions pcre_compile() or pcre_compile2() can be called         Either of the functions pcre_compile() or pcre_compile2() can be called
1128         to compile a pattern into an internal form. The only difference between         to compile a pattern into an internal form. The only difference between
1129         the two interfaces is that pcre_compile2() has an additional  argument,         the  two interfaces is that pcre_compile2() has an additional argument,
1130         errorcodeptr, via which a numerical error code can be returned.         errorcodeptr, via which a numerical error  code  can  be  returned.  To
1131           avoid  too  much repetition, we refer just to pcre_compile() below, but
1132           the information applies equally to pcre_compile2().
1133    
1134         The pattern is a C string terminated by a binary zero, and is passed in         The pattern is a C string terminated by a binary zero, and is passed in
1135         the pattern argument. A pointer to a single block  of  memory  that  is         the  pattern  argument.  A  pointer to a single block of memory that is
1136         obtained  via  pcre_malloc is returned. This contains the compiled code         obtained via pcre_malloc is returned. This contains the  compiled  code
1137         and related data. The pcre type is defined for the returned block; this         and related data. The pcre type is defined for the returned block; this
1138         is a typedef for a structure whose contents are not externally defined.         is a typedef for a structure whose contents are not externally defined.
1139         It is up to the caller to free the memory (via pcre_free) when it is no         It is up to the caller to free the memory (via pcre_free) when it is no
1140         longer required.         longer required.
1141    
1142         Although  the compiled code of a PCRE regex is relocatable, that is, it         Although the compiled code of a PCRE regex is relocatable, that is,  it
1143         does not depend on memory location, the complete pcre data block is not         does not depend on memory location, the complete pcre data block is not
1144         fully  relocatable, because it may contain a copy of the tableptr argu-         fully relocatable, because it may contain a copy of the tableptr  argu-
1145         ment, which is an address (see below).         ment, which is an address (see below).
1146    
1147         The options argument contains various bit settings that affect the com-         The options argument contains various bit settings that affect the com-
1148         pilation.  It  should be zero if no options are required. The available         pilation. It should be zero if no options are required.  The  available
1149         options are described below. Some of them, in  particular,  those  that         options  are  described  below. Some of them (in particular, those that
1150         are  compatible  with  Perl,  can also be set and unset from within the         are compatible with Perl, but some others as well) can also be set  and
1151         pattern (see the detailed description  in  the  pcrepattern  documenta-         unset  from  within  the  pattern  (see the detailed description in the
1152         tion).  For  these options, the contents of the options argument speci-         pcrepattern documentation). For those options that can be different  in
1153         fies their initial settings at the start of compilation and  execution.         different  parts  of  the pattern, the contents of the options argument
1154         The  PCRE_ANCHORED  and PCRE_NEWLINE_xxx options can be set at the time         specifies their settings at the start of compilation and execution. The
1155         of matching as well as at compile time.         PCRE_ANCHORED, PCRE_BSR_xxx, and PCRE_NEWLINE_xxx options can be set at
1156           the time of matching as well as at compile time.
1157    
1158         If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,         If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
1159         if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and         if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
1160         sets the variable pointed to by errptr to point to a textual error mes-         sets the variable pointed to by errptr to point to a textual error mes-
1161         sage. This is a static string that is part of the library. You must not         sage. This is a static string that is part of the library. You must not
1162         try to free it. The offset from the start of the pattern to the charac-         try to free it. The byte offset from the start of the  pattern  to  the
1163         ter where the error was discovered is placed in the variable pointed to         character  that  was  being  processed when the error was discovered is
1164         by erroffset, which must not be NULL. If it is, an immediate  error  is         placed in the variable pointed to by erroffset, which must not be NULL.
1165         given.         If  it  is,  an  immediate error is given. Some errors are not detected
1166           until checks are carried out when the whole pattern has  been  scanned;
1167           in this case the offset is set to the end of the pattern.
1168    
1169         If  pcre_compile2()  is  used instead of pcre_compile(), and the error-         If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
1170         codeptr argument is not NULL, a non-zero error code number is  returned         codeptr argument is not NULL, a non-zero error code number is  returned
# Line 1335  COMPILING A PATTERN Line 1361  COMPILING A PATTERN
1361         and are therefore ignored.         and are therefore ignored.
1362    
1363         The newline option that is set at compile time becomes the default that         The newline option that is set at compile time becomes the default that
1364         is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.         is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1365    
1366           PCRE_NO_AUTO_CAPTURE           PCRE_NO_AUTO_CAPTURE
1367    
1368         If this option is set, it disables the use of numbered capturing paren-         If this option is set, it disables the use of numbered capturing paren-
1369         theses  in the pattern. Any opening parenthesis that is not followed by         theses in the pattern. Any opening parenthesis that is not followed  by
1370         ? behaves as if it were followed by ?: but named parentheses can  still         ?  behaves as if it were followed by ?: but named parentheses can still
1371         be  used  for  capturing  (and  they acquire numbers in the usual way).         be used for capturing (and they acquire  numbers  in  the  usual  way).
1372         There is no equivalent of this option in Perl.         There is no equivalent of this option in Perl.
1373    
1374           PCRE_UNGREEDY           PCRE_UNGREEDY
1375    
1376         This option inverts the "greediness" of the quantifiers  so  that  they         This  option  inverts  the "greediness" of the quantifiers so that they
1377         are  not greedy by default, but become greedy if followed by "?". It is         are not greedy by default, but become greedy if followed by "?". It  is
1378         not compatible with Perl. It can also be set by a (?U)  option  setting         not  compatible  with Perl. It can also be set by a (?U) option setting
1379         within the pattern.         within the pattern.
1380    
1381           PCRE_UTF8           PCRE_UTF8
1382    
1383         This  option  causes PCRE to regard both the pattern and the subject as         This option causes PCRE to regard both the pattern and the  subject  as
1384         strings of UTF-8 characters instead of single-byte  character  strings.         strings  of  UTF-8 characters instead of single-byte character strings.
1385         However,  it is available only when PCRE is built to include UTF-8 sup-         However, it is available only when PCRE is built to include UTF-8  sup-
1386         port. If not, the use of this option provokes an error. Details of  how         port.  If not, the use of this option provokes an error. Details of how
1387         this  option  changes the behaviour of PCRE are given in the section on         this option changes the behaviour of PCRE are given in the  section  on
1388         UTF-8 support in the main pcre page.         UTF-8 support in the main pcre page.
1389    
1390           PCRE_NO_UTF8_CHECK           PCRE_NO_UTF8_CHECK
1391    
1392         When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is         When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1393         automatically  checked.  There  is  a  discussion about the validity of         automatically checked. There is a  discussion  about  the  validity  of
1394         UTF-8 strings in the main pcre page. If an invalid  UTF-8  sequence  of         UTF-8  strings  in  the main pcre page. If an invalid UTF-8 sequence of
1395         bytes  is  found,  pcre_compile() returns an error. If you already know         bytes is found, pcre_compile() returns an error. If  you  already  know
1396         that your pattern is valid, and you want to skip this check for perfor-         that your pattern is valid, and you want to skip this check for perfor-
1397         mance  reasons,  you  can set the PCRE_NO_UTF8_CHECK option. When it is         mance reasons, you can set the PCRE_NO_UTF8_CHECK option.  When  it  is
1398         set, the effect of passing an invalid UTF-8  string  as  a  pattern  is         set,  the  effect  of  passing  an invalid UTF-8 string as a pattern is
1399         undefined.  It  may  cause your program to crash. Note that this option         undefined. It may cause your program to crash. Note  that  this  option
1400         can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress  the         can  also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1401         UTF-8 validity checking of subject strings.         UTF-8 validity checking of subject strings.
1402    
1403    
1404  COMPILATION ERROR CODES  COMPILATION ERROR CODES
1405    
1406         The  following  table  lists  the  error  codes than may be returned by         The following table lists the error  codes  than  may  be  returned  by
1407         pcre_compile2(), along with the error messages that may be returned  by         pcre_compile2(),  along with the error messages that may be returned by
1408         both  compiling functions. As PCRE has developed, some error codes have         both compiling functions. As PCRE has developed, some error codes  have
1409         fallen out of use. To avoid confusion, they have not been re-used.         fallen out of use. To avoid confusion, they have not been re-used.
1410    
1411            0  no error            0  no error
# Line 1435  COMPILATION ERROR CODES Line 1461  COMPILATION ERROR CODES
1461           50  [this code is not in use]           50  [this code is not in use]
1462           51  octal value is greater than \377 (not in UTF-8 mode)           51  octal value is greater than \377 (not in UTF-8 mode)
1463           52  internal error: overran compiling workspace           52  internal error: overran compiling workspace
1464           53  internal  error:  previously-checked  referenced  subpattern  not           53   internal  error:  previously-checked  referenced  subpattern not
1465         found         found
1466           54  DEFINE group contains more than one branch           54  DEFINE group contains more than one branch
1467           55  repeating a DEFINE group is not allowed           55  repeating a DEFINE group is not allowed
# Line 1450  COMPILATION ERROR CODES Line 1476  COMPILATION ERROR CODES
1476           63  digit expected after (?+           63  digit expected after (?+
1477           64  ] is an invalid data character in JavaScript compatibility mode           64  ] is an invalid data character in JavaScript compatibility mode
1478    
1479         The  numbers  32  and 10000 in errors 48 and 49 are defaults; different         The numbers 32 and 10000 in errors 48 and 49  are  defaults;  different
1480         values may be used if the limits were changed when PCRE was built.         values may be used if the limits were changed when PCRE was built.
1481    
1482    
# Line 1459  STUDYING A PATTERN Line 1485  STUDYING A PATTERN
1485         pcre_extra *pcre_study(const pcre *code, int options         pcre_extra *pcre_study(const pcre *code, int options
1486              const char **errptr);              const char **errptr);
1487    
1488         If a compiled pattern is going to be used several times,  it  is  worth         If  a  compiled  pattern is going to be used several times, it is worth
1489         spending more time analyzing it in order to speed up the time taken for         spending more time analyzing it in order to speed up the time taken for
1490         matching. The function pcre_study() takes a pointer to a compiled  pat-         matching.  The function pcre_study() takes a pointer to a compiled pat-
1491         tern as its first argument. If studying the pattern produces additional         tern as its first argument. If studying the pattern produces additional
1492         information that will help speed up matching,  pcre_study()  returns  a         information  that  will  help speed up matching, pcre_study() returns a
1493         pointer  to a pcre_extra block, in which the study_data field points to         pointer to a pcre_extra block, in which the study_data field points  to
1494         the results of the study.         the results of the study.
1495    
1496         The  returned  value  from  pcre_study()  can  be  passed  directly  to         The  returned  value  from  pcre_study()  can  be  passed  directly  to
1497         pcre_exec().  However,  a  pcre_extra  block also contains other fields         pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block  also  con-
1498         that can be set by the caller before the block  is  passed;  these  are         tains  other  fields  that can be set by the caller before the block is
1499         described below in the section on matching a pattern.         passed; these are described below in the section on matching a pattern.
1500    
1501         If  studying  the  pattern  does not produce any additional information         If studying the  pattern  does  not  produce  any  useful  information,
1502         pcre_study() returns NULL. In that circumstance, if the calling program         pcre_study() returns NULL. In that circumstance, if the calling program
1503         wants  to  pass  any of the other fields to pcre_exec(), it must set up         wants  to  pass  any  of   the   other   fields   to   pcre_exec()   or
1504         its own pcre_extra block.         pcre_dfa_exec(), it must set up its own pcre_extra block.
1505    
1506         The second argument of pcre_study() contains option bits.  At  present,         The  second  argument of pcre_study() contains option bits. At present,
1507         no options are defined, and this argument should always be zero.         no options are defined, and this argument should always be zero.
1508    
1509         The  third argument for pcre_study() is a pointer for an error message.         The third argument for pcre_study() is a pointer for an error  message.
1510         If studying succeeds (even if no data is  returned),  the  variable  it         If  studying  succeeds  (even  if no data is returned), the variable it
1511         points  to  is  set  to NULL. Otherwise it is set to point to a textual         points to is set to NULL. Otherwise it is set to  point  to  a  textual
1512         error message. This is a static string that is part of the library. You         error message. This is a static string that is part of the library. You
1513         must  not  try  to  free it. You should test the error pointer for NULL         must not try to free it. You should test the  error  pointer  for  NULL
1514         after calling pcre_study(), to be sure that it has run successfully.         after calling pcre_study(), to be sure that it has run successfully.
1515    
1516         This is a typical call to pcre_study():         This is a typical call to pcre_study():
# Line 1495  STUDYING A PATTERN Line 1521  STUDYING A PATTERN
1521             0,              /* no options exist */             0,              /* no options exist */
1522             &error);        /* set to NULL or points to a message */             &error);        /* set to NULL or points to a message */
1523    
1524         At present, studying a pattern is useful only for non-anchored patterns         Studying a pattern does two things: first, a lower bound for the length
1525         that  do not have a single fixed starting character. A bitmap of possi-         of subject string that is needed to match the pattern is computed. This
1526         ble starting bytes is created.         does not mean that there are any strings of that length that match, but
1527           it does guarantee that no shorter strings match. The value is  used  by
1528           pcre_exec()  and  pcre_dfa_exec()  to  avoid  wasting time by trying to
1529           match strings that are shorter than the lower bound. You can  find  out
1530           the value in a calling program via the pcre_fullinfo() function.
1531    
1532           Studying a pattern is also useful for non-anchored patterns that do not
1533           have a single fixed starting character. A bitmap of  possible  starting
1534           bytes  is  created. This speeds up finding a position in the subject at
1535           which to start matching.
1536    
1537    
1538  LOCALE SUPPORT  LOCALE SUPPORT
# Line 1663  INFORMATION ABOUT A PATTERN Line 1698  INFORMATION ABOUT A PATTERN
1698         example, for the pattern /^a\d+z\d+/ the returned value is "z", but for         example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1699         /^a\dz\d/ the returned value is -1.         /^a\dz\d/ the returned value is -1.
1700    
1701             PCRE_INFO_MINLENGTH
1702    
1703           If the pattern was studied and a minimum length  for  matching  subject
1704           strings  was  computed,  its  value is returned. Otherwise the returned
1705           value is -1. The value is a number of characters, not bytes  (this  may
1706           be  relevant in UTF-8 mode). The fourth argument should point to an int
1707           variable. A non-negative value is a lower bound to the  length  of  any
1708           matching  string.  There  may not be any strings of that length that do
1709           actually match, but every string that does match is at least that long.
1710    
1711           PCRE_INFO_NAMECOUNT           PCRE_INFO_NAMECOUNT
1712           PCRE_INFO_NAMEENTRYSIZE           PCRE_INFO_NAMEENTRYSIZE
1713           PCRE_INFO_NAMETABLE           PCRE_INFO_NAMETABLE
# Line 1684  INFORMATION ABOUT A PATTERN Line 1729  INFORMATION ABOUT A PATTERN
1729         a pointer to the first entry of the table  (a  pointer  to  char).  The         a pointer to the first entry of the table  (a  pointer  to  char).  The
1730         first two bytes of each entry are the number of the capturing parenthe-         first two bytes of each entry are the number of the capturing parenthe-
1731         sis, most significant byte first. The rest of the entry is  the  corre-         sis, most significant byte first. The rest of the entry is  the  corre-
1732         sponding  name,  zero  terminated. The names are in alphabetical order.         sponding name, zero terminated.
1733         When PCRE_DUPNAMES is set, duplicate names are in order of their paren-  
1734         theses  numbers.  For  example,  consider the following pattern (assume         The  names are in alphabetical order. Duplicate names may appear if (?|
1735         PCRE_EXTENDED is  set,  so  white  space  -  including  newlines  -  is         is used to create multiple groups with the same number, as described in
1736         ignored):         the  section  on  duplicate subpattern numbers in the pcrepattern page.
1737           Duplicate names for subpatterns with different  numbers  are  permitted
1738           only  if  PCRE_DUPNAMES  is  set. In all cases of duplicate names, they
1739           appear in the table in the order in which they were found in  the  pat-
1740           tern.  In  the  absence  of (?| this is the order of increasing number;
1741           when (?| is used this is not necessarily the case because later subpat-
1742           terns may have lower numbers.
1743    
1744           As  a  simple  example of the name/number table, consider the following
1745           pattern (assume PCRE_EXTENDED is set, so white space -  including  new-
1746           lines - is ignored):
1747    
1748           (?<date> (?<year>(\d\d)?\d\d) -           (?<date> (?<year>(\d\d)?\d\d) -
1749           (?<month>\d\d) - (?<day>\d\d) )           (?<month>\d\d) - (?<day>\d\d) )
# Line 1709  INFORMATION ABOUT A PATTERN Line 1764  INFORMATION ABOUT A PATTERN
1764    
1765           PCRE_INFO_OKPARTIAL           PCRE_INFO_OKPARTIAL
1766    
1767         Return 1 if the pattern can be used for partial matching, otherwise  0.         Return 1  if  the  pattern  can  be  used  for  partial  matching  with
1768         The  fourth  argument  should point to an int variable. The pcrepartial         pcre_exec(),  otherwise  0.  The fourth argument should point to an int
1769         documentation lists the restrictions that apply to patterns  when  par-         variable. From  release  8.00,  this  always  returns  1,  because  the
1770         tial matching is used.         restrictions  that  previously  applied  to  partial matching have been
1771           lifted. The pcrepartial documentation gives details of  partial  match-
1772           ing.
1773    
1774           PCRE_INFO_OPTIONS           PCRE_INFO_OPTIONS
1775    
# Line 1749  INFORMATION ABOUT A PATTERN Line 1806  INFORMATION ABOUT A PATTERN
1806         Return the size of the data block pointed to by the study_data field in         Return the size of the data block pointed to by the study_data field in
1807         a pcre_extra block. That is,  it  is  the  value  that  was  passed  to         a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1808         pcre_malloc() when PCRE was getting memory into which to place the data         pcre_malloc() when PCRE was getting memory into which to place the data
1809         created by pcre_study(). The fourth argument should point to  a  size_t         created by pcre_study(). If pcre_extra is NULL, or there  is  no  study
1810           data,  zero  is  returned. The fourth argument should point to a size_t
1811         variable.         variable.
1812    
1813    
# Line 1757  OBSOLETE INFO FUNCTION Line 1815  OBSOLETE INFO FUNCTION
1815    
1816         int pcre_info(const pcre *code, int *optptr, int *firstcharptr);         int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1817    
1818         The  pcre_info()  function is now obsolete because its interface is too         The pcre_info() function is now obsolete because its interface  is  too
1819         restrictive to return all the available data about a compiled  pattern.         restrictive  to return all the available data about a compiled pattern.
1820         New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of         New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of
1821         pcre_info() is the number of capturing subpatterns, or one of the  fol-         pcre_info()  is the number of capturing subpatterns, or one of the fol-
1822         lowing negative numbers:         lowing negative numbers:
1823    
1824           PCRE_ERROR_NULL       the argument code was NULL           PCRE_ERROR_NULL       the argument code was NULL
1825           PCRE_ERROR_BADMAGIC   the "magic number" was not found           PCRE_ERROR_BADMAGIC   the "magic number" was not found
1826    
1827         If  the  optptr  argument is not NULL, a copy of the options with which         If the optptr argument is not NULL, a copy of the  options  with  which
1828         the pattern was compiled is placed in the integer  it  points  to  (see         the  pattern  was  compiled  is placed in the integer it points to (see
1829         PCRE_INFO_OPTIONS above).         PCRE_INFO_OPTIONS above).
1830    
1831         If  the  pattern  is  not anchored and the firstcharptr argument is not         If the pattern is not anchored and the  firstcharptr  argument  is  not
1832         NULL, it is used to pass back information about the first character  of         NULL,  it is used to pass back information about the first character of
1833         any matched string (see PCRE_INFO_FIRSTBYTE above).         any matched string (see PCRE_INFO_FIRSTBYTE above).
1834    
1835    
# Line 1779  REFERENCE COUNTS Line 1837  REFERENCE COUNTS
1837    
1838         int pcre_refcount(pcre *code, int adjust);         int pcre_refcount(pcre *code, int adjust);
1839    
1840         The  pcre_refcount()  function is used to maintain a reference count in         The pcre_refcount() function is used to maintain a reference  count  in
1841         the data block that contains a compiled pattern. It is provided for the         the data block that contains a compiled pattern. It is provided for the
1842         benefit  of  applications  that  operate  in an object-oriented manner,         benefit of applications that  operate  in  an  object-oriented  manner,
1843         where different parts of the application may be using the same compiled         where different parts of the application may be using the same compiled
1844         pattern, but you want to free the block when they are all done.         pattern, but you want to free the block when they are all done.
1845    
1846         When a pattern is compiled, the reference count field is initialized to         When a pattern is compiled, the reference count field is initialized to
1847         zero.  It is changed only by calling this function, whose action is  to         zero.   It is changed only by calling this function, whose action is to
1848         add  the  adjust  value  (which may be positive or negative) to it. The         add the adjust value (which may be positive or  negative)  to  it.  The
1849         yield of the function is the new value. However, the value of the count         yield of the function is the new value. However, the value of the count
1850         is  constrained to lie between 0 and 65535, inclusive. If the new value         is constrained to lie between 0 and 65535, inclusive. If the new  value
1851         is outside these limits, it is forced to the appropriate limit value.         is outside these limits, it is forced to the appropriate limit value.
1852    
1853         Except when it is zero, the reference count is not correctly  preserved         Except  when it is zero, the reference count is not correctly preserved
1854         if  a  pattern  is  compiled on one host and then transferred to a host         if a pattern is compiled on one host and then  transferred  to  a  host
1855         whose byte-order is different. (This seems a highly unlikely scenario.)         whose byte-order is different. (This seems a highly unlikely scenario.)
1856    
1857    
# Line 1805  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1863  MATCHING A PATTERN: THE TRADITIONAL FUNC
1863    
1864         The  function pcre_exec() is called to match a subject string against a         The  function pcre_exec() is called to match a subject string against a
1865         compiled pattern, which is passed in the code argument. If the  pattern         compiled pattern, which is passed in the code argument. If the  pattern
1866         has been studied, the result of the study should be passed in the extra         was  studied,  the  result  of  the study should be passed in the extra
1867         argument. This function is the main matching facility of  the  library,         argument. This function is the main matching facility of  the  library,
1868         and it operates in a Perl-like manner. For specialist use there is also         and it operates in a Perl-like manner. For specialist use there is also
1869         an alternative matching function, which is described below in the  sec-         an alternative matching function, which is described below in the  sec-
# Line 1864  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1922  MATCHING A PATTERN: THE TRADITIONAL FUNC
1922         The match_limit field provides a means of preventing PCRE from using up         The match_limit field provides a means of preventing PCRE from using up
1923         a  vast amount of resources when running patterns that are not going to         a  vast amount of resources when running patterns that are not going to
1924         match, but which have a very large number  of  possibilities  in  their         match, but which have a very large number  of  possibilities  in  their
1925         search  trees.  The  classic  example  is  the  use of nested unlimited         search  trees. The classic example is a pattern that uses nested unlim-
1926         repeats.         ited repeats.
1927    
1928         Internally, PCRE uses a function called match() which it calls  repeat-         Internally, PCRE uses a function called match() which it calls  repeat-
1929         edly  (sometimes  recursively). The limit set by match_limit is imposed         edly  (sometimes  recursively). The limit set by match_limit is imposed
# Line 1887  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1945  MATCHING A PATTERN: THE TRADITIONAL FUNC
1945         the  total number of calls, because not all calls to match() are recur-         the  total number of calls, because not all calls to match() are recur-
1946         sive.  This limit is of use only if it is set smaller than match_limit.         sive.  This limit is of use only if it is set smaller than match_limit.
1947    
1948         Limiting  the  recursion  depth  limits the amount of stack that can be         Limiting the recursion depth limits the amount of  stack  that  can  be
1949         used, or, when PCRE has been compiled to use memory on the heap instead         used, or, when PCRE has been compiled to use memory on the heap instead
1950         of the stack, the amount of heap memory that can be used.         of the stack, the amount of heap memory that can be used.
1951    
1952         The  default  value  for  match_limit_recursion can be set when PCRE is         The default value for match_limit_recursion can be  set  when  PCRE  is
1953         built; the default default  is  the  same  value  as  the  default  for         built;  the  default  default  is  the  same  value  as the default for
1954         match_limit.  You can override the default by suppling pcre_exec() with         match_limit. You can override the default by suppling pcre_exec()  with
1955         a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and         a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
1956         PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the         PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
1957         limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.         limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1958    
1959         The pcre_callout field is used in conjunction with the  "callout"  fea-         The  callout_data  field is used in conjunction with the "callout" fea-
1960         ture, which is described in the pcrecallout documentation.         ture, and is described in the pcrecallout documentation.
1961    
1962         The  tables  field  is  used  to  pass  a  character  tables pointer to         The tables field  is  used  to  pass  a  character  tables  pointer  to
1963         pcre_exec(); this overrides the value that is stored with the  compiled         pcre_exec();  this overrides the value that is stored with the compiled
1964         pattern.  A  non-NULL value is stored with the compiled pattern only if         pattern. A non-NULL value is stored with the compiled pattern  only  if
1965         custom tables were supplied to pcre_compile() via  its  tableptr  argu-         custom  tables  were  supplied to pcre_compile() via its tableptr argu-
1966         ment.  If NULL is passed to pcre_exec() using this mechanism, it forces         ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1967         PCRE's internal tables to be used. This facility is  helpful  when  re-         PCRE's  internal  tables  to be used. This facility is helpful when re-
1968         using  patterns  that  have been saved after compiling with an external         using patterns that have been saved after compiling  with  an  external
1969         set of tables, because the external tables  might  be  at  a  different         set  of  tables,  because  the  external tables might be at a different
1970         address  when  pcre_exec() is called. See the pcreprecompile documenta-         address when pcre_exec() is called. See the  pcreprecompile  documenta-
1971         tion for a discussion of saving compiled patterns for later use.         tion for a discussion of saving compiled patterns for later use.
1972    
1973     Option bits for pcre_exec()     Option bits for pcre_exec()
1974    
1975         The unused bits of the options argument for pcre_exec() must  be  zero.         The  unused  bits of the options argument for pcre_exec() must be zero.
1976         The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,         The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
1977         PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and         PCRE_NOTBOL,    PCRE_NOTEOL,    PCRE_NOTEMPTY,   PCRE_NOTEMPTY_ATSTART,
1978         PCRE_PARTIAL.         PCRE_NO_START_OPTIMIZE,  PCRE_NO_UTF8_CHECK,   PCRE_PARTIAL_SOFT,   and
1979           PCRE_PARTIAL_HARD.
1980    
1981           PCRE_ANCHORED           PCRE_ANCHORED
1982    
# Line 1997  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2056  MATCHING A PATTERN: THE TRADITIONAL FUNC
2056    
2057           a?b?           a?b?
2058    
2059         is applied to a string not beginning with "a" or "b",  it  matches  the         is applied to a string not beginning with "a" or  "b",  it  matches  an
2060         empty  string at the start of the subject. With PCRE_NOTEMPTY set, this         empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
2061         match is not valid, so PCRE searches further into the string for occur-         match is not valid, so PCRE searches further into the string for occur-
2062         rences of "a" or "b".         rences of "a" or "b".
2063    
2064         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-           PCRE_NOTEMPTY_ATSTART
2065         cial case of a pattern match of the empty  string  within  its  split()  
2066         function,  and  when  using  the /g modifier. It is possible to emulate         This  is  like PCRE_NOTEMPTY, except that an empty string match that is
2067         Perl's behaviour after matching a null string by first trying the match         not at the start of  the  subject  is  permitted.  If  the  pattern  is
2068         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then         anchored, such a match can occur only if the pattern contains \K.
2069         if that fails by advancing the starting offset (see below)  and  trying  
2070         an ordinary match again. There is some code that demonstrates how to do         Perl     has    no    direct    equivalent    of    PCRE_NOTEMPTY    or
2071         this in the pcredemo.c sample program.         PCRE_NOTEMPTY_ATSTART, but it does make a special  case  of  a  pattern
2072           match  of  the empty string within its split() function, and when using
2073           the /g modifier. It is  possible  to  emulate  Perl's  behaviour  after
2074           matching a null string by first trying the match again at the same off-
2075           set with PCRE_NOTEMPTY_ATSTART and  PCRE_ANCHORED,  and  then  if  that
2076           fails, by advancing the starting offset (see below) and trying an ordi-
2077           nary match again. There is some code that demonstrates how to  do  this
2078           in the pcredemo sample program.
2079    
2080             PCRE_NO_START_OPTIMIZE
2081    
2082           There  are a number of optimizations that pcre_exec() uses at the start
2083           of a match, in order to speed up the process. For  example,  if  it  is
2084           known  that  a  match must start with a specific character, it searches
2085           the subject for that character, and fails immediately if it cannot find
2086           it,  without actually running the main matching function. When callouts
2087           are in use, these optimizations can cause  them  to  be  skipped.  This
2088           option  disables  the  "start-up" optimizations, causing performance to
2089           suffer, but ensuring that the callouts do occur.
2090    
2091           PCRE_NO_UTF8_CHECK           PCRE_NO_UTF8_CHECK
2092    
# Line 2033  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2110  MATCHING A PATTERN: THE TRADITIONAL FUNC
2110         value  of startoffset that does not point to the start of a UTF-8 char-         value  of startoffset that does not point to the start of a UTF-8 char-
2111         acter, is undefined. Your program may crash.         acter, is undefined. Your program may crash.
2112    
2113           PCRE_PARTIAL           PCRE_PARTIAL_HARD
2114             PCRE_PARTIAL_SOFT
2115    
2116         This option turns on the  partial  matching  feature.  If  the  subject         These options turn on the partial matching feature. For backwards  com-
2117         string  fails to match the pattern, but at some point during the match-         patibility,  PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2118         ing process the end of the subject was reached (that  is,  the  subject         match occurs if the end of the subject string is reached  successfully,
2119         partially  matches  the  pattern and the failure to match occurred only         but  there  are not enough subject characters to complete the match. If
2120         because there were not enough subject characters), pcre_exec()  returns         this happens when PCRE_PARTIAL_HARD  is  set,  pcre_exec()  immediately
2121         PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is         returns  PCRE_ERROR_PARTIAL.  Otherwise,  if  PCRE_PARTIAL_SOFT is set,
2122         used, there are restrictions on what may appear in the  pattern.  These         matching continues by testing any other alternatives. Only if they  all
2123         are discussed in the pcrepartial documentation.         fail  is  PCRE_ERROR_PARTIAL  returned (instead of PCRE_ERROR_NOMATCH).
2124           The portion of the string that was inspected when the partial match was
2125           found  is  set  as  the first matching string. There is a more detailed
2126           discussion in the pcrepartial documentation.
2127    
2128     The string to be matched by pcre_exec()     The string to be matched by pcre_exec()
2129    
2130         The  subject string is passed to pcre_exec() as a pointer in subject, a         The subject string is passed to pcre_exec() as a pointer in subject,  a
2131         length in length, and a starting byte offset in startoffset.  In  UTF-8         length (in bytes) in length, and a starting byte offset in startoffset.
2132         mode,  the  byte  offset  must point to the start of a UTF-8 character.         In UTF-8 mode, the byte offset must point to the start of a UTF-8 char-
2133         Unlike the pattern string, the subject may contain binary  zero  bytes.         acter.  Unlike  the pattern string, the subject may contain binary zero
2134         When  the starting offset is zero, the search for a match starts at the         bytes. When the starting offset is zero, the search for a match  starts
2135         beginning of the subject, and this is by far the most common case.         at  the  beginning  of  the subject, and this is by far the most common
2136           case.
2137    
2138         A non-zero starting offset is useful when searching for  another  match         A non-zero starting offset is useful when searching for  another  match
2139         in  the same subject by calling pcre_exec() again after a previous suc-         in  the same subject by calling pcre_exec() again after a previous suc-
# Line 2087  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2169  MATCHING A PATTERN: THE TRADITIONAL FUNC
2169         string.  PCRE  supports several other kinds of parenthesized subpattern         string.  PCRE  supports several other kinds of parenthesized subpattern
2170         that do not cause substrings to be captured.         that do not cause substrings to be captured.
2171    
2172         Captured substrings are returned to the caller via a vector of  integer         Captured substrings are returned to the caller via a vector of integers
2173         offsets  whose  address is passed in ovector. The number of elements in         whose  address is passed in ovector. The number of elements in the vec-
2174         the vector is passed in ovecsize, which must be a non-negative  number.         tor is passed in ovecsize, which must be a non-negative  number.  Note:
2175         Note: this argument is NOT the size of ovector in bytes.         this argument is NOT the size of ovector in bytes.
2176    
2177         The  first  two-thirds of the vector is used to pass back captured sub-         The  first  two-thirds of the vector is used to pass back captured sub-
2178         strings, each substring using a pair of integers. The  remaining  third         strings, each substring using a pair of integers. The  remaining  third
2179         of  the  vector is used as workspace by pcre_exec() while matching cap-         of  the  vector is used as workspace by pcre_exec() while matching cap-
2180         turing subpatterns, and is not available for passing back  information.         turing subpatterns, and is not available for passing back  information.
2181         The  length passed in ovecsize should always be a multiple of three. If         The  number passed in ovecsize should always be a multiple of three. If
2182         it is not, it is rounded down.         it is not, it is rounded down.
2183    
2184         When a match is successful, information about  captured  substrings  is         When a match is successful, information about  captured  substrings  is
2185         returned  in  pairs  of integers, starting at the beginning of ovector,         returned  in  pairs  of integers, starting at the beginning of ovector,
2186         and continuing up to two-thirds of its length at the  most.  The  first         and continuing up to two-thirds of its length at the  most.  The  first
2187         element of a pair is set to the offset of the first character in a sub-         element  of  each pair is set to the byte offset of the first character
2188         string, and the second is set to the  offset  of  the  first  character         in a substring, and the second is set to the byte offset of  the  first
2189         after  the  end  of  a  substring. The first pair, ovector[0] and ovec-         character  after  the end of a substring. Note: these values are always
2190         tor[1], identify the portion of  the  subject  string  matched  by  the         byte offsets, even in UTF-8 mode. They are not character counts.
2191         entire  pattern.  The next pair is used for the first capturing subpat-  
2192         tern, and so on. The value returned by pcre_exec() is one more than the         The first pair of integers, ovector[0]  and  ovector[1],  identify  the
2193         highest numbered pair that has been set. For example, if two substrings         portion  of  the subject string matched by the entire pattern. The next
2194         have been captured, the returned value is 3. If there are no  capturing         pair is used for the first capturing subpattern, and so on.  The  value
2195         subpatterns,  the return value from a successful match is 1, indicating         returned by pcre_exec() is one more than the highest numbered pair that
2196         that just the first pair of offsets has been set.         has been set.  For example, if two substrings have been  captured,  the
2197           returned  value is 3. If there are no capturing subpatterns, the return
2198           value from a successful match is 1, indicating that just the first pair
2199           of offsets has been set.
2200    
2201         If a capturing subpattern is matched repeatedly, it is the last portion         If a capturing subpattern is matched repeatedly, it is the last portion
2202         of the string that it matched that is returned.         of the string that it matched that is returned.
2203    
2204         If  the vector is too small to hold all the captured substring offsets,         If the vector is too small to hold all the captured substring  offsets,
2205         it is used as far as possible (up to two-thirds of its length), and the         it is used as far as possible (up to two-thirds of its length), and the
2206         function  returns a value of zero. In particular, if the substring off-         function returns a value of zero. If the substring offsets are  not  of
2207         sets are not of interest, pcre_exec() may be called with ovector passed         interest,  pcre_exec()  may  be  called with ovector passed as NULL and
2208         as  NULL  and  ovecsize  as zero. However, if the pattern contains back         ovecsize as zero. However, if the pattern contains back references  and
2209         references and the ovector is not big enough to  remember  the  related         the  ovector is not big enough to remember the related substrings, PCRE
2210         substrings,  PCRE has to get additional memory for use during matching.         has to get additional memory for use during matching. Thus it  is  usu-
2211         Thus it is usually advisable to supply an ovector.         ally advisable to supply an ovector.
2212    
2213         The pcre_info() function can be used to find  out  how  many  capturing         The pcre_fullinfo() function can be used to find out how many capturing
2214         subpatterns  there  are  in  a  compiled pattern. The smallest size for         subpatterns there are in a compiled  pattern.  The  smallest  size  for
2215         ovector that will allow for n captured substrings, in addition  to  the         ovector  that  will allow for n captured substrings, in addition to the
2216         offsets of the substring matched by the whole pattern, is (n+1)*3.         offsets of the substring matched by the whole pattern, is (n+1)*3.
2217    
2218         It  is  possible for capturing subpattern number n+1 to match some part         It is possible for capturing subpattern number n+1 to match  some  part
2219         of the subject when subpattern n has not been used at all. For example,         of the subject when subpattern n has not been used at all. For example,
2220         if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the         if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
2221         return from the function is 4, and subpatterns 1 and 3 are matched, but         return from the function is 4, and subpatterns 1 and 3 are matched, but
2222         2  is  not.  When  this happens, both values in the offset pairs corre-         2 is not. When this happens, both values in  the  offset  pairs  corre-
2223         sponding to unused subpatterns are set to -1.         sponding to unused subpatterns are set to -1.
2224    
2225         Offset values that correspond to unused subpatterns at the end  of  the         Offset  values  that correspond to unused subpatterns at the end of the
2226         expression  are  also  set  to  -1. For example, if the string "abc" is         expression are also set to -1. For example,  if  the  string  "abc"  is
2227         matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not         matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2228         matched.  The  return  from the function is 2, because the highest used         matched. The return from the function is 2, because  the  highest  used
2229         capturing subpattern number is 1. However, you can refer to the offsets         capturing subpattern number is 1. However, you can refer to the offsets
2230         for  the  second  and third capturing subpatterns if you wish (assuming         for the second and third capturing subpatterns if  you  wish  (assuming
2231         the vector is large enough, of course).         the vector is large enough, of course).
2232    
2233         Some convenience functions are provided  for  extracting  the  captured         Some  convenience  functions  are  provided for extracting the captured
2234         substrings as separate strings. These are described below.         substrings as separate strings. These are described below.
2235    
2236     Error return values from pcre_exec()     Error return values from pcre_exec()
2237    
2238         If  pcre_exec()  fails, it returns a negative number. The following are         If pcre_exec() fails, it returns a negative number. The  following  are
2239         defined in the header file:         defined in the header file:
2240    
2241           PCRE_ERROR_NOMATCH        (-1)           PCRE_ERROR_NOMATCH        (-1)
# Line 2159  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2244  MATCHING A PATTERN: THE TRADITIONAL FUNC
2244    
2245           PCRE_ERROR_NULL           (-2)           PCRE_ERROR_NULL           (-2)
2246    
2247         Either code or subject was passed as NULL,  or  ovector  was  NULL  and         Either  code  or  subject  was  passed as NULL, or ovector was NULL and
2248         ovecsize was not zero.         ovecsize was not zero.
2249    
2250           PCRE_ERROR_BADOPTION      (-3)           PCRE_ERROR_BADOPTION      (-3)
# Line 2168  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2253  MATCHING A PATTERN: THE TRADITIONAL FUNC
2253    
2254           PCRE_ERROR_BADMAGIC       (-4)           PCRE_ERROR_BADMAGIC       (-4)
2255    
2256         PCRE  stores a 4-byte "magic number" at the start of the compiled code,         PCRE stores a 4-byte "magic number" at the start of the compiled  code,
2257         to catch the case when it is passed a junk pointer and to detect when a         to catch the case when it is passed a junk pointer and to detect when a
2258         pattern that was compiled in an environment of one endianness is run in         pattern that was compiled in an environment of one endianness is run in
2259         an environment with the other endianness. This is the error  that  PCRE         an  environment  with the other endianness. This is the error that PCRE
2260         gives when the magic number is not present.         gives when the magic number is not present.
2261    
2262           PCRE_ERROR_UNKNOWN_OPCODE (-5)           PCRE_ERROR_UNKNOWN_OPCODE (-5)
2263    
2264         While running the pattern match, an unknown item was encountered in the         While running the pattern match, an unknown item was encountered in the
2265         compiled pattern. This error could be caused by a bug  in  PCRE  or  by         compiled  pattern.  This  error  could be caused by a bug in PCRE or by
2266         overwriting of the compiled pattern.         overwriting of the compiled pattern.
2267    
2268           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2269    
2270         If  a  pattern contains back references, but the ovector that is passed         If a pattern contains back references, but the ovector that  is  passed
2271         to pcre_exec() is not big enough to remember the referenced substrings,         to pcre_exec() is not big enough to remember the referenced substrings,
2272         PCRE  gets  a  block of memory at the start of matching to use for this         PCRE gets a block of memory at the start of matching to  use  for  this
2273         purpose. If the call via pcre_malloc() fails, this error is given.  The         purpose.  If the call via pcre_malloc() fails, this error is given. The
2274         memory is automatically freed at the end of matching.         memory is automatically freed at the end of matching.
2275    
2276           PCRE_ERROR_NOSUBSTRING    (-7)           PCRE_ERROR_NOSUBSTRING    (-7)
2277    
2278         This  error is used by the pcre_copy_substring(), pcre_get_substring(),         This error is used by the pcre_copy_substring(),  pcre_get_substring(),
2279         and  pcre_get_substring_list()  functions  (see  below).  It  is  never         and  pcre_get_substring_list()  functions  (see  below).  It  is  never
2280         returned by pcre_exec().         returned by pcre_exec().
2281    
2282           PCRE_ERROR_MATCHLIMIT     (-8)           PCRE_ERROR_MATCHLIMIT     (-8)
2283    
2284         The  backtracking  limit,  as  specified  by the match_limit field in a         The backtracking limit, as specified by  the  match_limit  field  in  a
2285         pcre_extra structure (or defaulted) was reached.  See  the  description         pcre_extra  structure  (or  defaulted) was reached. See the description
2286         above.         above.
2287    
2288           PCRE_ERROR_CALLOUT        (-9)           PCRE_ERROR_CALLOUT        (-9)
2289    
2290         This error is never generated by pcre_exec() itself. It is provided for         This error is never generated by pcre_exec() itself. It is provided for
2291         use by callout functions that want to yield a distinctive  error  code.         use  by  callout functions that want to yield a distinctive error code.
2292         See the pcrecallout documentation for details.         See the pcrecallout documentation for details.
2293    
2294           PCRE_ERROR_BADUTF8        (-10)           PCRE_ERROR_BADUTF8        (-10)
2295    
2296         A  string  that contains an invalid UTF-8 byte sequence was passed as a         A string that contains an invalid UTF-8 byte sequence was passed  as  a
2297         subject.         subject.
2298    
2299           PCRE_ERROR_BADUTF8_OFFSET (-11)           PCRE_ERROR_BADUTF8_OFFSET (-11)
2300    
2301         The UTF-8 byte sequence that was passed as a subject was valid, but the         The UTF-8 byte sequence that was passed as a subject was valid, but the
2302         value  of startoffset did not point to the beginning of a UTF-8 charac-         value of startoffset did not point to the beginning of a UTF-8  charac-
2303         ter.         ter.
2304    
2305           PCRE_ERROR_PARTIAL        (-12)           PCRE_ERROR_PARTIAL        (-12)
2306    
2307         The subject string did not match, but it did match partially.  See  the         The  subject  string did not match, but it did match partially. See the
2308         pcrepartial documentation for details of partial matching.         pcrepartial documentation for details of partial matching.
2309    
2310           PCRE_ERROR_BADPARTIAL     (-13)           PCRE_ERROR_BADPARTIAL     (-13)
2311    
2312         The  PCRE_PARTIAL  option  was  used with a compiled pattern containing         This code is no longer in  use.  It  was  formerly  returned  when  the
2313         items that are not supported for partial matching. See the  pcrepartial         PCRE_PARTIAL  option  was used with a compiled pattern containing items
2314         documentation for details of partial matching.         that were  not  supported  for  partial  matching.  From  release  8.00
2315           onwards, there are no restrictions on partial matching.
2316    
2317           PCRE_ERROR_INTERNAL       (-14)           PCRE_ERROR_INTERNAL       (-14)
2318    
# Line 2235  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2321  MATCHING A PATTERN: THE TRADITIONAL FUNC
2321    
2322           PCRE_ERROR_BADCOUNT       (-15)           PCRE_ERROR_BADCOUNT       (-15)
2323    
2324         This error is given if the value of the ovecsize argument is  negative.         This error is given if the value of the ovecsize argument is negative.
2325    
2326           PCRE_ERROR_RECURSIONLIMIT (-21)           PCRE_ERROR_RECURSIONLIMIT (-21)
2327    
2328         The internal recursion limit, as specified by the match_limit_recursion         The internal recursion limit, as specified by the match_limit_recursion
2329         field in a pcre_extra structure (or defaulted)  was  reached.  See  the         field  in  a  pcre_extra  structure (or defaulted) was reached. See the
2330         description above.         description above.
2331    
2332           PCRE_ERROR_BADNEWLINE     (-23)           PCRE_ERROR_BADNEWLINE     (-23)
# Line 2263  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER Line 2349  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2349         int pcre_get_substring_list(const char *subject,         int pcre_get_substring_list(const char *subject,
2350              int *ovector, int stringcount, const char ***listptr);              int *ovector, int stringcount, const char ***listptr);
2351    
2352         Captured  substrings  can  be  accessed  directly  by using the offsets         Captured substrings can be  accessed  directly  by  using  the  offsets
2353         returned by pcre_exec() in  ovector.  For  convenience,  the  functions         returned  by  pcre_exec()  in  ovector.  For convenience, the functions
2354         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2355         string_list() are provided for extracting captured substrings  as  new,         string_list()  are  provided for extracting captured substrings as new,
2356         separate,  zero-terminated strings. These functions identify substrings         separate, zero-terminated strings. These functions identify  substrings
2357         by number. The next section describes functions  for  extracting  named         by  number.  The  next section describes functions for extracting named
2358         substrings.         substrings.
2359    
2360         A  substring that contains a binary zero is correctly extracted and has         A substring that contains a binary zero is correctly extracted and  has
2361         a further zero added on the end, but the result is not, of course, a  C         a  further zero added on the end, but the result is not, of course, a C
2362         string.   However,  you  can  process such a string by referring to the         string.  However, you can process such a string  by  referring  to  the
2363         length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-         length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
2364         string().  Unfortunately, the interface to pcre_get_substring_list() is         string().  Unfortunately, the interface to pcre_get_substring_list() is
2365         not adequate for handling strings containing binary zeros, because  the         not  adequate for handling strings containing binary zeros, because the
2366         end of the final string is not independently indicated.         end of the final string is not independently indicated.
2367    
2368         The  first  three  arguments  are the same for all three of these func-         The first three arguments are the same for all  three  of  these  func-
2369         tions: subject is the subject string that has  just  been  successfully         tions:  subject  is  the subject string that has just been successfully
2370         matched, ovector is a pointer to the vector of integer offsets that was         matched, ovector is a pointer to the vector of integer offsets that was
2371         passed to pcre_exec(), and stringcount is the number of substrings that         passed to pcre_exec(), and stringcount is the number of substrings that
2372         were  captured  by  the match, including the substring that matched the         were captured by the match, including the substring  that  matched  the
2373         entire regular expression. This is the value returned by pcre_exec() if         entire regular expression. This is the value returned by pcre_exec() if
2374         it  is greater than zero. If pcre_exec() returned zero, indicating that         it is greater than zero. If pcre_exec() returned zero, indicating  that
2375         it ran out of space in ovector, the value passed as stringcount  should         it  ran out of space in ovector, the value passed as stringcount should
2376         be the number of elements in the vector divided by three.         be the number of elements in the vector divided by three.
2377    
2378         The  functions pcre_copy_substring() and pcre_get_substring() extract a         The functions pcre_copy_substring() and pcre_get_substring() extract  a
2379         single substring, whose number is given as  stringnumber.  A  value  of         single  substring,  whose  number  is given as stringnumber. A value of
2380         zero  extracts  the  substring that matched the entire pattern, whereas         zero extracts the substring that matched the  entire  pattern,  whereas
2381         higher values  extract  the  captured  substrings.  For  pcre_copy_sub-         higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
2382         string(),  the  string  is  placed  in buffer, whose length is given by         string(), the string is placed in buffer,  whose  length  is  given  by
2383         buffersize, while for pcre_get_substring() a new  block  of  memory  is         buffersize,  while  for  pcre_get_substring()  a new block of memory is
2384         obtained  via  pcre_malloc,  and its address is returned via stringptr.         obtained via pcre_malloc, and its address is  returned  via  stringptr.
2385         The yield of the function is the length of the  string,  not  including         The  yield  of  the function is the length of the string, not including
2386         the terminating zero, or one of these error codes:         the terminating zero, or one of these error codes:
2387    
2388           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2389    
2390         The  buffer  was too small for pcre_copy_substring(), or the attempt to         The buffer was too small for pcre_copy_substring(), or the  attempt  to
2391         get memory failed for pcre_get_substring().         get memory failed for pcre_get_substring().
2392    
2393           PCRE_ERROR_NOSUBSTRING    (-7)           PCRE_ERROR_NOSUBSTRING    (-7)
2394    
2395         There is no substring whose number is stringnumber.         There is no substring whose number is stringnumber.
2396    
2397         The pcre_get_substring_list()  function  extracts  all  available  sub-         The  pcre_get_substring_list()  function  extracts  all  available sub-
2398         strings  and  builds  a list of pointers to them. All this is done in a         strings and builds a list of pointers to them. All this is  done  in  a
2399         single block of memory that is obtained via pcre_malloc. The address of         single block of memory that is obtained via pcre_malloc. The address of
2400         the  memory  block  is returned via listptr, which is also the start of         the memory block is returned via listptr, which is also  the  start  of
2401         the list of string pointers. The end of the list is marked  by  a  NULL         the  list  of  string pointers. The end of the list is marked by a NULL
2402         pointer.  The  yield  of  the function is zero if all went well, or the         pointer. The yield of the function is zero if all  went  well,  or  the
2403         error code         error code
2404    
2405           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2406    
2407         if the attempt to get the memory block failed.         if the attempt to get the memory block failed.
2408    
2409         When any of these functions encounter a substring that is unset,  which         When  any of these functions encounter a substring that is unset, which
2410         can  happen  when  capturing subpattern number n+1 matches some part of         can happen when capturing subpattern number n+1 matches  some  part  of
2411         the subject, but subpattern n has not been used at all, they return  an         the  subject, but subpattern n has not been used at all, they return an
2412         empty string. This can be distinguished from a genuine zero-length sub-         empty string. This can be distinguished from a genuine zero-length sub-
2413         string by inspecting the appropriate offset in ovector, which is  nega-         string  by inspecting the appropriate offset in ovector, which is nega-
2414         tive for unset substrings.         tive for unset substrings.
2415    
2416         The  two convenience functions pcre_free_substring() and pcre_free_sub-         The two convenience functions pcre_free_substring() and  pcre_free_sub-
2417         string_list() can be used to free the memory  returned  by  a  previous         string_list()  can  be  used  to free the memory returned by a previous
2418         call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-         call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2419         tively. They do nothing more than  call  the  function  pointed  to  by         tively.  They  do  nothing  more  than  call the function pointed to by
2420         pcre_free,  which  of course could be called directly from a C program.         pcre_free, which of course could be called directly from a  C  program.
2421         However, PCRE is used in some situations where it is linked via a  spe-         However,  PCRE is used in some situations where it is linked via a spe-
2422         cial   interface  to  another  programming  language  that  cannot  use         cial  interface  to  another  programming  language  that  cannot   use
2423         pcre_free directly; it is for these cases that the functions  are  pro-         pcre_free  directly;  it is for these cases that the functions are pro-
2424         vided.         vided.
2425    
2426    
# Line 2353  EXTRACTING CAPTURED SUBSTRINGS BY NAME Line 2439  EXTRACTING CAPTURED SUBSTRINGS BY NAME
2439              int stringcount, const char *stringname,              int stringcount, const char *stringname,
2440              const char **stringptr);              const char **stringptr);
2441    
2442         To  extract a substring by name, you first have to find associated num-         To extract a substring by name, you first have to find associated  num-
2443         ber.  For example, for this pattern         ber.  For example, for this pattern
2444    
2445           (a+)b(?<xxx>\d+)...           (a+)b(?<xxx>\d+)...
# Line 2362  EXTRACTING CAPTURED SUBSTRINGS BY NAME Line 2448  EXTRACTING CAPTURED SUBSTRINGS BY NAME
2448         be unique (PCRE_DUPNAMES was not set), you can find the number from the         be unique (PCRE_DUPNAMES was not set), you can find the number from the
2449         name by calling pcre_get_stringnumber(). The first argument is the com-         name by calling pcre_get_stringnumber(). The first argument is the com-
2450         piled pattern, and the second is the name. The yield of the function is         piled pattern, and the second is the name. The yield of the function is
2451         the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no         the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2452         subpattern of that name.         subpattern of that name.
2453    
2454         Given the number, you can extract the substring directly, or use one of         Given the number, you can extract the substring directly, or use one of
2455         the functions described in the previous section. For convenience, there         the functions described in the previous section. For convenience, there
2456         are also two functions that do the whole job.         are also two functions that do the whole job.
2457    
2458         Most    of    the    arguments   of   pcre_copy_named_substring()   and         Most   of   the   arguments    of    pcre_copy_named_substring()    and
2459         pcre_get_named_substring() are the same  as  those  for  the  similarly         pcre_get_named_substring()  are  the  same  as  those for the similarly
2460         named  functions  that extract by number. As these are described in the         named functions that extract by number. As these are described  in  the
2461         previous section, they are not re-described here. There  are  just  two         previous  section,  they  are not re-described here. There are just two
2462         differences:         differences:
2463    
2464         First,  instead  of a substring number, a substring name is given. Sec-         First, instead of a substring number, a substring name is  given.  Sec-
2465         ond, there is an extra argument, given at the start, which is a pointer         ond, there is an extra argument, given at the start, which is a pointer
2466         to  the compiled pattern. This is needed in order to gain access to the         to the compiled pattern. This is needed in order to gain access to  the
2467         name-to-number translation table.         name-to-number translation table.
2468    
2469         These functions call pcre_get_stringnumber(), and if it succeeds,  they         These  functions call pcre_get_stringnumber(), and if it succeeds, they
2470         then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-         then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
2471         ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the         ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
2472         behaviour may not be what you want (see the next section).         behaviour may not be what you want (see the next section).
2473    
2474           Warning: If the pattern uses the (?| feature to set up multiple subpat-
2475           terns  with  the  same number, as described in the section on duplicate
2476           subpattern numbers in the pcrepattern page, you  cannot  use  names  to
2477           distinguish  the  different subpatterns, because names are not included
2478           in the compiled code. The matching process uses only numbers. For  this
2479           reason,  the  use of different names for subpatterns of the same number
2480           causes an error at compile time.
2481    
2482    
2483  DUPLICATE SUBPATTERN NAMES  DUPLICATE SUBPATTERN NAMES
2484    
2485         int pcre_get_stringtable_entries(const pcre *code,         int pcre_get_stringtable_entries(const pcre *code,
2486              const char *name, char **first, char **last);              const char *name, char **first, char **last);
2487    
2488         When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for         When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
2489         subpatterns are not required to  be  unique.  Normally,  patterns  with         subpatterns  are not required to be unique. (Duplicate names are always
2490         duplicate  names  are such that in any one match, only one of the named         allowed for subpatterns with the same number, created by using the  (?|
2491         subpatterns participates. An example is shown in the pcrepattern  docu-         feature.  Indeed,  if  such subpatterns are named, they are required to
2492         mentation.         use the same names.)
2493    
2494           Normally, patterns with duplicate names are such that in any one match,
2495           only  one of the named subpatterns participates. An example is shown in
2496           the pcrepattern documentation.
2497    
2498         When    duplicates   are   present,   pcre_copy_named_substring()   and         When   duplicates   are   present,   pcre_copy_named_substring()    and
2499         pcre_get_named_substring() return the first substring corresponding  to         pcre_get_named_substring()  return the first substring corresponding to
2500         the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING         the given name that is set. If  none  are  set,  PCRE_ERROR_NOSUBSTRING
2501         (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()         (-7)  is  returned;  no  data  is returned. The pcre_get_stringnumber()
2502         function  returns one of the numbers that are associated with the name,         function returns one of the numbers that are associated with the  name,
2503         but it is not defined which it is.         but it is not defined which it is.
2504    
2505         If you want to get full details of all captured substrings for a  given         If  you want to get full details of all captured substrings for a given
2506         name,  you  must  use  the pcre_get_stringtable_entries() function. The         name, you must use  the  pcre_get_stringtable_entries()  function.  The
2507         first argument is the compiled pattern, and the second is the name. The         first argument is the compiled pattern, and the second is the name. The
2508         third  and  fourth  are  pointers to variables which are updated by the         third and fourth are pointers to variables which  are  updated  by  the
2509         function. After it has run, they point to the first and last entries in         function. After it has run, they point to the first and last entries in
2510         the  name-to-number  table  for  the  given  name.  The function itself         the name-to-number table  for  the  given  name.  The  function  itself
2511         returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if         returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2512         there  are none. The format of the table is described above in the sec-         there are none. The format of the table is described above in the  sec-
2513         tion entitled Information about a  pattern.   Given  all  the  relevant         tion  entitled  Information  about  a  pattern.  Given all the relevant
2514         entries  for the name, you can extract each of their numbers, and hence         entries for the name, you can extract each of their numbers, and  hence
2515         the captured data, if any.         the captured data, if any.
2516    
2517    
2518  FINDING ALL POSSIBLE MATCHES  FINDING ALL POSSIBLE MATCHES
2519    
2520         The traditional matching function uses a  similar  algorithm  to  Perl,         The  traditional  matching  function  uses a similar algorithm to Perl,
2521         which stops when it finds the first match, starting at a given point in         which stops when it finds the first match, starting at a given point in
2522         the subject. If you want to find all possible matches, or  the  longest         the  subject.  If you want to find all possible matches, or the longest
2523         possible  match,  consider using the alternative matching function (see         possible match, consider using the alternative matching  function  (see
2524         below) instead. If you cannot use the alternative function,  but  still         below)  instead.  If you cannot use the alternative function, but still
2525         need  to  find all possible matches, you can kludge it up by making use         need to find all possible matches, you can kludge it up by  making  use
2526         of the callout facility, which is described in the pcrecallout documen-         of the callout facility, which is described in the pcrecallout documen-
2527         tation.         tation.
2528    
2529         What you have to do is to insert a callout right at the end of the pat-         What you have to do is to insert a callout right at the end of the pat-
2530         tern.  When your callout function is called, extract and save the  cur-         tern.   When your callout function is called, extract and save the cur-
2531         rent  matched  substring.  Then  return  1, which forces pcre_exec() to         rent matched substring. Then return  1,  which  forces  pcre_exec()  to
2532         backtrack and try other alternatives. Ultimately, when it runs  out  of         backtrack  and  try other alternatives. Ultimately, when it runs out of
2533         matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.         matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2534    
2535    
# Line 2442  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2540  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2540              int options, int *ovector, int ovecsize,              int options, int *ovector, int ovecsize,
2541              int *workspace, int wscount);              int *workspace, int wscount);
2542    
2543         The  function  pcre_dfa_exec()  is  called  to  match  a subject string         The function pcre_dfa_exec()  is  called  to  match  a  subject  string
2544         against a compiled pattern, using a matching algorithm that  scans  the         against  a  compiled pattern, using a matching algorithm that scans the
2545         subject  string  just  once, and does not backtrack. This has different         subject string just once, and does not backtrack.  This  has  different
2546         characteristics to the normal algorithm, and  is  not  compatible  with         characteristics  to  the  normal  algorithm, and is not compatible with
2547         Perl.  Some  of the features of PCRE patterns are not supported. Never-         Perl. Some of the features of PCRE patterns are not  supported.  Never-
2548         theless, there are times when this kind of matching can be useful.  For         theless,  there are times when this kind of matching can be useful. For
2549         a discussion of the two matching algorithms, see the pcrematching docu-         a discussion of the two matching algorithms, and  a  list  of  features
2550         mentation.         that  pcre_dfa_exec() does not support, see the pcrematching documenta-
2551           tion.
2552    
2553         The arguments for the pcre_dfa_exec() function  are  the  same  as  for         The arguments for the pcre_dfa_exec() function  are  the  same  as  for
2554         pcre_exec(), plus two extras. The ovector argument is used in a differ-         pcre_exec(), plus two extras. The ovector argument is used in a differ-
# Line 2484  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2583  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2583    
2584         The unused bits of the options argument  for  pcre_dfa_exec()  must  be         The unused bits of the options argument  for  pcre_dfa_exec()  must  be
2585         zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-         zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
2586         LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,         LINE_xxx,        PCRE_NOTBOL,        PCRE_NOTEOL,        PCRE_NOTEMPTY,
2587         PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last         PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, PCRE_PAR-
2588         three of these are the same as for pcre_exec(), so their description is         TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All  but  the  last
2589         not repeated here.         four  of  these  are  exactly  the  same  as  for pcre_exec(), so their
2590           description is not repeated here.
2591           PCRE_PARTIAL  
2592             PCRE_PARTIAL_HARD
2593         This  has  the  same general effect as it does for pcre_exec(), but the           PCRE_PARTIAL_SOFT
2594         details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for  
2595         pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into         These have the same general effect as they do for pcre_exec(), but  the
2596         PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have         details  are  slightly  different.  When  PCRE_PARTIAL_HARD  is set for
2597         been no complete matches, but there is still at least one matching pos-         pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of  the  sub-
2598         sibility. The portion of the string that provided the partial match  is         ject  is  reached  and there is still at least one matching possibility
2599         set as the first matching string.         that requires additional characters. This happens even if some complete
2600           matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
2601           code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
2602           of  the  subject  is  reached, there have been no complete matches, but
2603           there is still at least one matching possibility. The  portion  of  the
2604           string  that  was inspected when the longest partial match was found is
2605           set as the first matching string in both cases.
2606    
2607           PCRE_DFA_SHORTEST           PCRE_DFA_SHORTEST
2608    
2609         Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to         Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
2610         stop as soon as it has found one match. Because of the way the alterna-         stop as soon as it has found one match. Because of the way the alterna-
2611         tive  algorithm  works, this is necessarily the shortest possible match         tive algorithm works, this is necessarily the shortest  possible  match
2612         at the first possible matching point in the subject string.         at the first possible matching point in the subject string.
2613    
2614           PCRE_DFA_RESTART           PCRE_DFA_RESTART
2615    
2616         When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and         When pcre_dfa_exec() returns a partial match, it is possible to call it
2617         returns  a  partial  match, it is possible to call it again, with addi-         again, with additional subject characters, and have  it  continue  with
2618         tional subject characters, and have it continue with  the  same  match.         the  same match. The PCRE_DFA_RESTART option requests this action; when
2619         The  PCRE_DFA_RESTART  option requests this action; when it is set, the         it is set, the workspace and wscount options must  reference  the  same
2620         workspace and wscount options must reference the same vector as  before         vector  as  before  because data about the match so far is left in them
2621         because  data  about  the  match so far is left in them after a partial         after a partial match. There is more discussion of this facility in the
2622         match. There is more discussion of this  facility  in  the  pcrepartial         pcrepartial documentation.
        documentation.  
2623    
2624     Successful returns from pcre_dfa_exec()     Successful returns from pcre_dfa_exec()
2625    
# Line 2592  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2696  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2696  SEE ALSO  SEE ALSO
2697    
2698         pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-         pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
2699         tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).         tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2700    
2701    
2702  AUTHOR  AUTHOR
# Line 2604  AUTHOR Line 2708  AUTHOR
2708    
2709  REVISION  REVISION
2710    
2711         Last updated: 12 April 2008         Last updated: 03 October 2009
2712         Copyright (c) 1997-2008 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
2713  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2714    
2715    
# Line 2634  PCRE CALLOUTS Line 2738  PCRE CALLOUTS
2738    
2739           (?C1)abc(?C2)def           (?C1)abc(?C2)def
2740    
2741         If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is         If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() or
2742         called, PCRE automatically  inserts  callouts,  all  with  number  255,         pcre_compile2() is called, PCRE  automatically  inserts  callouts,  all
2743         before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is         with  number  255,  before  each  item  in the pattern. For example, if
2744         used with the pattern         PCRE_AUTO_CALLOUT is used with the pattern
2745    
2746           A(\d{2}|--)           A(\d{2}|--)
2747    
# Line 2656  PCRE CALLOUTS Line 2760  PCRE CALLOUTS
2760  MISSING CALLOUTS  MISSING CALLOUTS
2761    
2762         You  should  be  aware  that,  because of optimizations in the way PCRE         You  should  be  aware  that,  because of optimizations in the way PCRE
2763         matches patterns, callouts sometimes do not happen. For example, if the         matches patterns by default, callouts  sometimes  do  not  happen.  For
2764         pattern is         example, if the pattern is
2765    
2766           ab(?C4)cd           ab(?C4)cd
2767    
# Line 2666  MISSING CALLOUTS Line 2770  MISSING CALLOUTS
2770         ever  start,  and  the  callout is never reached. However, with "abyd",         ever  start,  and  the  callout is never reached. However, with "abyd",
2771         though the result is still no match, the callout is obeyed.         though the result is still no match, the callout is obeyed.
2772    
2773           If the pattern is studied, PCRE knows the minimum length of a  matching
2774           string,  and will immediately give a "no match" return without actually
2775           running a match if the subject is not long enough, or,  for  unanchored
2776           patterns, if it has been scanned far enough.
2777    
2778           You  can disable these optimizations by passing the PCRE_NO_START_OPTI-
2779           MIZE option to pcre_exec() or  pcre_dfa_exec().  This  slows  down  the
2780           matching  process,  but  does  ensure that callouts such as the example
2781           above are obeyed.
2782    
2783    
2784  THE CALLOUT INTERFACE  THE CALLOUT INTERFACE
2785    
# Line 2693  THE CALLOUT INTERFACE Line 2807  THE CALLOUT INTERFACE
2807         version number will change again in future  if  additional  fields  are         version number will change again in future  if  additional  fields  are
2808         added, but the intention is never to remove any of the existing fields.         added, but the intention is never to remove any of the existing fields.
2809    
2810         The callout_number field contains the number of the  callout,  as  com-         The  callout_number  field  contains the number of the callout, as com-
2811         piled  into  the pattern (that is, the number after ?C for manual call-         piled into the pattern (that is, the number after ?C for  manual  call-
2812         outs, and 255 for automatically generated callouts).         outs, and 255 for automatically generated callouts).
2813    
2814         The offset_vector field is a pointer to the vector of offsets that  was         The  offset_vector field is a pointer to the vector of offsets that was
2815         passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When         passed  by  the  caller  to  pcre_exec()   or   pcre_dfa_exec().   When
2816         pcre_exec() is used, the contents can be inspected in order to  extract         pcre_exec()  is used, the contents can be inspected in order to extract
2817         substrings  that  have  been  matched  so  far,  in the same way as for         substrings that have been matched so  far,  in  the  same  way  as  for
2818         extracting substrings after a match has completed. For  pcre_dfa_exec()         extracting  substrings after a match has completed. For pcre_dfa_exec()
2819         this field is not useful.         this field is not useful.
2820    
2821         The subject and subject_length fields contain copies of the values that         The subject and subject_length fields contain copies of the values that
2822         were passed to pcre_exec().         were passed to pcre_exec().
2823    
2824         The start_match field normally contains the offset within  the  subject         The  start_match  field normally contains the offset within the subject
2825         at  which  the  current  match  attempt started. However, if the escape         at which the current match attempt  started.  However,  if  the  escape
2826         sequence \K has been encountered, this value is changed to reflect  the         sequence  \K has been encountered, this value is changed to reflect the
2827         modified  starting  point.  If the pattern is not anchored, the callout         modified starting point. If the pattern is not  anchored,  the  callout
2828         function may be called several times from the same point in the pattern         function may be called several times from the same point in the pattern
2829         for different starting points in the subject.         for different starting points in the subject.
2830    
2831         The  current_position  field  contains the offset within the subject of         The current_position field contains the offset within  the  subject  of
2832         the current match pointer.         the current match pointer.
2833    
2834         When the pcre_exec() function is used, the capture_top  field  contains         When  the  pcre_exec() function is used, the capture_top field contains
2835         one  more than the number of the highest numbered captured substring so         one more than the number of the highest numbered captured substring  so
2836         far. If no substrings have been captured, the value of  capture_top  is         far.  If  no substrings have been captured, the value of capture_top is
2837         one.  This  is always the case when pcre_dfa_exec() is used, because it         one. This is always the case when pcre_dfa_exec() is used,  because  it
2838         does not support captured substrings.         does not support captured substrings.
2839    
2840         The capture_last field contains the number of the  most  recently  cap-         The  capture_last  field  contains the number of the most recently cap-
2841         tured  substring. If no substrings have been captured, its value is -1.         tured substring. If no substrings have been captured, its value is  -1.
2842         This is always the case when pcre_dfa_exec() is used.         This is always the case when pcre_dfa_exec() is used.
2843    
2844         The callout_data field contains a value that is passed  to  pcre_exec()         The  callout_data  field contains a value that is passed to pcre_exec()
2845         or  pcre_dfa_exec() specifically so that it can be passed back in call-         or pcre_dfa_exec() specifically so that it can be passed back in  call-
2846         outs. It is passed in the pcre_callout field  of  the  pcre_extra  data         outs.  It  is  passed  in the pcre_callout field of the pcre_extra data
2847         structure.  If  no such data was passed, the value of callout_data in a         structure. If no such data was passed, the value of callout_data  in  a
2848         pcre_callout block is NULL. There is a description  of  the  pcre_extra         pcre_callout  block  is  NULL. There is a description of the pcre_extra
2849         structure in the pcreapi documentation.         structure in the pcreapi documentation.
2850    
2851         The  pattern_position field is present from version 1 of the pcre_call-         The pattern_position field is present from version 1 of the  pcre_call-
2852         out structure. It contains the offset to the next item to be matched in         out structure. It contains the offset to the next item to be matched in
2853         the pattern string.         the pattern string.
2854    
2855         The  next_item_length field is present from version 1 of the pcre_call-         The next_item_length field is present from version 1 of the  pcre_call-
2856         out structure. It contains the length of the next item to be matched in         out structure. It contains the length of the next item to be matched in
2857         the  pattern  string. When the callout immediately precedes an alterna-         the pattern string. When the callout immediately precedes  an  alterna-
2858         tion bar, a closing parenthesis, or the end of the pattern, the  length         tion  bar, a closing parenthesis, or the end of the pattern, the length
2859         is  zero.  When the callout precedes an opening parenthesis, the length         is zero. When the callout precedes an opening parenthesis,  the  length
2860         is that of the entire subpattern.         is that of the entire subpattern.
2861    
2862         The pattern_position and next_item_length fields are intended  to  help         The  pattern_position  and next_item_length fields are intended to help
2863         in  distinguishing between different automatic callouts, which all have         in distinguishing between different automatic callouts, which all  have
2864         the same callout number. However, they are set for all callouts.         the same callout number. However, they are set for all callouts.
2865    
2866    
2867  RETURN VALUES  RETURN VALUES
2868    
2869         The external callout function returns an integer to PCRE. If the  value         The  external callout function returns an integer to PCRE. If the value
2870         is  zero,  matching  proceeds  as  normal. If the value is greater than         is zero, matching proceeds as normal. If  the  value  is  greater  than
2871         zero, matching fails at the current point, but  the  testing  of  other         zero,  matching  fails  at  the current point, but the testing of other
2872         matching possibilities goes ahead, just as if a lookahead assertion had         matching possibilities goes ahead, just as if a lookahead assertion had
2873         failed. If the value is less than zero, the  match  is  abandoned,  and         failed.  If  the  value  is less than zero, the match is abandoned, and
2874         pcre_exec() (or pcre_dfa_exec()) returns the negative value.         pcre_exec() or pcre_dfa_exec() returns the negative value.
2875    
2876         Negative   values   should   normally   be   chosen  from  the  set  of         Negative  values  should  normally  be   chosen   from   the   set   of
2877         PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-         PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2878         dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is         dard "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT  is
2879         reserved for use by callout functions; it will never be  used  by  PCRE         reserved  for  use  by callout functions; it will never be used by PCRE
2880         itself.         itself.
2881    
2882    
# Line 2775  AUTHOR Line 2889  AUTHOR
2889    
2890  REVISION  REVISION
2891    
2892         Last updated: 29 May 2007         Last updated: 29 September 2009
2893         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
2894  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2895    
2896    
# Line 2790  NAME Line 2904  NAME
2904  DIFFERENCES BETWEEN PCRE AND PERL  DIFFERENCES BETWEEN PCRE AND PERL
2905    
2906         This  document describes the differences in the ways that PCRE and Perl         This  document describes the differences in the ways that PCRE and Perl
2907         handle regular expressions. The differences described here  are  mainly         handle regular expressions. The differences  described  here  are  with
2908         with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain         respect to Perl 5.10.
        some features that are expected to be in the forthcoming Perl 5.10.  
2909    
2910         1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details         1.  PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2911         of  what  it does have are given in the section on UTF-8 support in the         of what it does have are given in the section on UTF-8 support  in  the
2912         main pcre page.         main pcre page.
2913    
2914         2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl         2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2915         permits  them,  but they do not mean what you might think. For example,         permits them, but they do not mean what you might think.  For  example,
2916         (?!a){3} does not assert that the next three characters are not "a". It         (?!a){3} does not assert that the next three characters are not "a". It
2917         just asserts that the next character is not "a" three times.         just asserts that the next character is not "a" three times.
2918    
2919         3.  Capturing  subpatterns  that occur inside negative lookahead asser-         3. Capturing subpatterns that occur inside  negative  lookahead  asser-
2920         tions are counted, but their entries in the offsets  vector  are  never         tions  are  counted,  but their entries in the offsets vector are never
2921         set.  Perl sets its numerical variables from any such patterns that are         set. Perl sets its numerical variables from any such patterns that  are
2922         matched before the assertion fails to match something (thereby succeed-         matched before the assertion fails to match something (thereby succeed-
2923         ing),  but  only  if the negative lookahead assertion contains just one         ing), but only if the negative lookahead assertion  contains  just  one
2924         branch.         branch.
2925    
2926         4. Though binary zero characters are supported in the  subject  string,         4.  Though  binary zero characters are supported in the subject string,
2927         they are not allowed in a pattern string because it is passed as a nor-         they are not allowed in a pattern string because it is passed as a nor-
2928         mal C string, terminated by zero. The escape sequence \0 can be used in         mal C string, terminated by zero. The escape sequence \0 can be used in
2929         the pattern to represent a binary zero.         the pattern to represent a binary zero.
2930    
2931         5.  The  following Perl escape sequences are not supported: \l, \u, \L,         5. The following Perl escape sequences are not supported: \l,  \u,  \L,
2932         \U, and \N. In fact these are implemented by Perl's general string-han-         \U, and \N. In fact these are implemented by Perl's general string-han-
2933         dling  and are not part of its pattern matching engine. If any of these         dling and are not part of its pattern matching engine. If any of  these
2934         are encountered by PCRE, an error is generated.         are encountered by PCRE, an error is generated.
2935    
2936         6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE         6.  The Perl escape sequences \p, \P, and \X are supported only if PCRE
2937         is  built  with Unicode character property support. The properties that         is built with Unicode character property support. The  properties  that
2938         can be tested with \p and \P are limited to the general category  prop-         can  be tested with \p and \P are limited to the general category prop-
2939         erties  such  as  Lu and Nd, script names such as Greek or Han, and the         erties such as Lu and Nd, script names such as Greek or  Han,  and  the
2940         derived properties Any and L&.         derived  properties  Any  and  L&. PCRE does support the Cs (surrogate)
2941           property, which Perl does not; the  Perl  documentation  says  "Because
2942           Perl hides the need for the user to understand the internal representa-
2943           tion of Unicode characters, there is no need to implement the  somewhat
2944           messy concept of surrogates."
2945    
2946         7. PCRE does support the \Q...\E escape for quoting substrings. Charac-         7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2947         ters  in  between  are  treated as literals. This is slightly different         ters in between are treated as literals.  This  is  slightly  different
2948         from Perl in that $ and @ are  also  handled  as  literals  inside  the         from  Perl  in  that  $  and  @ are also handled as literals inside the
2949         quotes.  In Perl, they cause variable interpolation (but of course PCRE         quotes. In Perl, they cause variable interpolation (but of course  PCRE
2950         does not have variables). Note the following examples:         does not have variables). Note the following examples:
2951    
2952             Pattern            PCRE matches      Perl matches             Pattern            PCRE matches      Perl matches
# Line 2839  DIFFERENCES BETWEEN PCRE AND PERL Line 2956  DIFFERENCES BETWEEN PCRE AND PERL
2956             \Qabc\$xyz\E       abc\$xyz          abc\$xyz             \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2957             \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz             \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2958    
2959         The \Q...\E sequence is recognized both inside  and  outside  character         The  \Q...\E  sequence  is recognized both inside and outside character
2960         classes.         classes.
2961    
2962         8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})         8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2963         constructions. However, there is support for recursive  patterns.  This         constructions.  However,  there is support for recursive patterns. This
2964         is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE         is not available in Perl 5.8, but it is in Perl 5.10.  Also,  the  PCRE
2965         "callout" feature allows an external function to be called during  pat-         "callout"  feature allows an external function to be called during pat-
2966         tern matching. See the pcrecallout documentation for details.         tern matching. See the pcrecallout documentation for details.
2967    
2968         9.  Subpatterns  that  are  called  recursively or as "subroutines" are         9. Subpatterns that are called  recursively  or  as  "subroutines"  are
2969         always treated as atomic groups in  PCRE.  This  is  like  Python,  but         always  treated  as  atomic  groups  in  PCRE. This is like Python, but
2970         unlike Perl.         unlike Perl. There is a discussion of an example that explains this  in
2971           more  detail  in  the section on recursion differences from Perl in the
2972         10.  There are some differences that are concerned with the settings of         pcrepattern page.
2973         captured strings when part of  a  pattern  is  repeated.  For  example,  
2974         matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2         10. There are some differences that are concerned with the settings  of
2975           captured  strings  when  part  of  a  pattern is repeated. For example,
2976           matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
2977         unset, but in PCRE it is set to "b".         unset, but in PCRE it is set to "b".
2978    
2979         11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),         11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
2980         (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in         (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but  only  in
2981         the forms without an  argument.  PCRE  does  not  support  (*MARK).  If         the forms without an argument. PCRE does not support (*MARK).
2982         (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-  
2983         ture group; this is different to Perl.         12.  PCRE's handling of duplicate subpattern numbers and duplicate sub-
2984           pattern names is not as general as Perl's. This is a consequence of the
2985         12. PCRE provides some extensions to the Perl regular expression facil-         fact the PCRE works internally just with numbers, using an external ta-
2986         ities.   Perl  5.10  will  include new features that are not in earlier         ble to translate between numbers and names. In  particular,  a  pattern
2987         versions, some of which (such as named parentheses) have been  in  PCRE         such  as  (?|(?<a>A)|(?<b)B),  where the two capturing parentheses have
2988         for some time. This list is with respect to Perl 5.10:         the same number but different names, is not supported,  and  causes  an
2989           error  at compile time. If it were allowed, it would not be possible to
2990         (a)  Although  lookbehind  assertions  must match fixed length strings,         distinguish which parentheses matched, because both names map  to  cap-
2991         each alternative branch of a lookbehind assertion can match a different         turing subpattern number 1. To avoid this confusing situation, an error
2992         length of string. Perl requires them all to have the same length.         is given at compile time.
2993    
2994           13. PCRE provides some extensions to the Perl regular expression facil-
2995           ities.   Perl  5.10  includes new features that are not in earlier ver-
2996           sions of Perl, some of which (such as named parentheses) have  been  in
2997           PCRE for some time. This list is with respect to Perl 5.10:
2998    
2999           (a)  Although  lookbehind  assertions  in  PCRE must match fixed length
3000           strings, each alternative branch of a lookbehind assertion can match  a
3001           different  length  of  string.  Perl requires them all to have the same
3002           length.
3003    
3004         (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $         (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
3005         meta-character matches only at the very end of the string.         meta-character matches only at the very end of the string.
3006    
3007         (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-         (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3008         cial meaning is faulted. Otherwise, like Perl, the backslash is quietly         cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3009         ignored.  (Perl can be made to issue a warning.)         ignored.  (Perl can be made to issue a warning.)
3010    
3011         (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-         (d)  If  PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3012         fiers is inverted, that is, by default they are not greedy, but if fol-         fiers is inverted, that is, by default they are not greedy, but if fol-
3013         lowed by a question mark they are.         lowed by a question mark they are.
3014    
3015         (e) PCRE_ANCHORED can be used at matching time to force a pattern to be         (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3016         tried only at the first matching position in the subject string.         tried only at the first matching position in the subject string.
3017    
3018         (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-         (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
3019         TURE options for pcre_exec() have no Perl equivalents.         and  PCRE_NO_AUTO_CAPTURE  options for pcre_exec() have no Perl equiva-
3020           lents.
3021    
3022         (g) The \R escape sequence can be restricted to match only CR,  LF,  or         (g) The \R escape sequence can be restricted to match only CR,  LF,  or
3023         CRLF by the PCRE_BSR_ANYCRLF option.         CRLF by the PCRE_BSR_ANYCRLF option.
# Line 2916  AUTHOR Line 3046  AUTHOR
3046    
3047  REVISION  REVISION
3048    
3049         Last updated: 11 September 2007         Last updated: 04 October 2009
3050         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
3051  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
3052    
3053    
# Line 2947  PCRE REGULAR EXPRESSION DETAILS Line 3077  PCRE REGULAR EXPRESSION DETAILS
3077    
3078         The original operation of PCRE was on strings of  one-byte  characters.         The original operation of PCRE was on strings of  one-byte  characters.
3079         However,  there is now also support for UTF-8 character strings. To use         However,  there is now also support for UTF-8 character strings. To use
3080         this, you must build PCRE to  include  UTF-8  support,  and  then  call         this, PCRE must be built to include UTF-8 support, and  you  must  call
3081         pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern         pcre_compile()  or  pcre_compile2() with the PCRE_UTF8 option. There is
3082         matching is mentioned in several places below. There is also a  summary         also a special sequence that can be given at the start of a pattern:
3083         of  UTF-8  features  in  the  section on UTF-8 support in the main pcre  
3084         page.           (*UTF8)
3085    
3086           Starting a pattern with this sequence  is  equivalent  to  setting  the
3087           PCRE_UTF8  option.  This  feature  is  not Perl-compatible. How setting
3088           UTF-8 mode affects pattern matching  is  mentioned  in  several  places
3089           below.  There  is  also  a  summary of UTF-8 features in the section on
3090           UTF-8 support in the main pcre page.
3091    
3092         The remainder of this document discusses the  patterns  that  are  sup-         The remainder of this document discusses the  patterns  that  are  sup-
3093         ported  by  PCRE when its main matching function, pcre_exec(), is used.         ported  by  PCRE when its main matching function, pcre_exec(), is used.
# Line 2981  NEWLINE CONVENTIONS Line 3117  NEWLINE CONVENTIONS
3117           (*ANYCRLF)   any of the three above           (*ANYCRLF)   any of the three above
3118           (*ANY)       all Unicode newline sequences           (*ANY)       all Unicode newline sequences
3119    
3120         These override the default and the options given to pcre_compile(). For         These  override  the default and the options given to pcre_compile() or
3121         example, on a Unix system where LF is the default newline sequence, the         pcre_compile2(). For example, on a Unix system where LF is the  default
3122         pattern         newline sequence, the pattern
3123    
3124           (*CR)a.b           (*CR)a.b
3125    
# Line 3055  CHARACTERS AND METACHARACTERS Line 3191  CHARACTERS AND METACHARACTERS
3191                    syntax)                    syntax)
3192           ]      terminates the character class           ]      terminates the character class
3193    
3194         The  following sections describe the use of each of the metacharacters.         The following sections describe the use of each of the metacharacters.
3195    
3196    
3197  BACKSLASH  BACKSLASH
3198    
3199         The backslash character has several uses. Firstly, if it is followed by         The backslash character has several uses. Firstly, if it is followed by
3200         a  non-alphanumeric  character,  it takes away any special meaning that         a non-alphanumeric character, it takes away any  special  meaning  that
3201         character may have. This  use  of  backslash  as  an  escape  character         character  may  have.  This  use  of  backslash  as an escape character
3202         applies both inside and outside character classes.         applies both inside and outside character classes.
3203    
3204         For  example,  if  you want to match a * character, you write \* in the         For example, if you want to match a * character, you write  \*  in  the
3205         pattern.  This escaping action applies whether  or  not  the  following         pattern.   This  escaping  action  applies whether or not the following
3206         character  would  otherwise be interpreted as a metacharacter, so it is         character would otherwise be interpreted as a metacharacter, so  it  is
3207         always safe to precede a non-alphanumeric  with  backslash  to  specify         always  safe  to  precede  a non-alphanumeric with backslash to specify
3208         that  it stands for itself. In particular, if you want to match a back-         that it stands for itself. In particular, if you want to match a  back-
3209         slash, you write \\.         slash, you write \\.
3210    
3211         If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in         If  a  pattern is compiled with the PCRE_EXTENDED option, whitespace in
3212         the  pattern (other than in a character class) and characters between a         the pattern (other than in a character class) and characters between  a
3213         # outside a character class and the next newline are ignored. An escap-         # outside a character class and the next newline are ignored. An escap-
3214         ing  backslash  can  be  used to include a whitespace or # character as         ing backslash can be used to include a whitespace  or  #  character  as
3215         part of the pattern.         part of the pattern.
3216    
3217         If you want to remove the special meaning from a  sequence  of  charac-         If  you  want  to remove the special meaning from a sequence of charac-
3218         ters,  you can do so by putting them between \Q and \E. This is differ-         ters, you can do so by putting them between \Q and \E. This is  differ-
3219         ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E         ent  from  Perl  in  that  $  and  @ are handled as literals in \Q...\E
3220         sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-         sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
3221         tion. Note the following examples:         tion. Note the following examples:
3222    
3223           Pattern            PCRE matches   Perl matches           Pattern            PCRE matches   Perl matches
# Line 3091  BACKSLASH Line 3227  BACKSLASH
3227           \Qabc\$xyz\E       abc\$xyz       abc\$xyz           \Qabc\$xyz\E       abc\$xyz       abc\$xyz
3228           \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz           \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
3229    
3230         The \Q...\E sequence is recognized both inside  and  outside  character         The  \Q...\E  sequence  is recognized both inside and outside character
3231         classes.         classes.
3232    
3233     Non-printing characters     Non-printing characters
3234    
3235         A second use of backslash provides a way of encoding non-printing char-         A second use of backslash provides a way of encoding non-printing char-
3236         acters in patterns in a visible manner. There is no restriction on  the         acters  in patterns in a visible manner. There is no restriction on the
3237         appearance  of non-printing characters, apart from the binary zero that         appearance of non-printing characters, apart from the binary zero  that
3238         terminates a pattern, but when a pattern  is  being  prepared  by  text         terminates  a  pattern,  but  when  a pattern is being prepared by text
3239         editing,  it  is  usually  easier  to  use  one of the following escape         editing, it is  often  easier  to  use  one  of  the  following  escape
3240         sequences than the binary character it represents:         sequences than the binary character it represents:
3241    
3242           \a        alarm, that is, the BEL character (hex 07)           \a        alarm, that is, the BEL character (hex 07)
# Line 3110  BACKSLASH Line 3246  BACKSLASH
3246           \n        linefeed (hex 0A)           \n        linefeed (hex 0A)
3247           \r        carriage return (hex 0D)           \r        carriage return (hex 0D)
3248           \t        tab (hex 09)           \t        tab (hex 09)
3249           \ddd      character with octal code ddd, or backreference           \ddd      character with octal code ddd, or back reference
3250           \xhh      character with hex code hh           \xhh      character with hex code hh
3251           \x{hhh..} character with hex code hhh..           \x{hhh..} character with hex code hhh..
3252    
3253         The precise effect of \cx is as follows: if x is a lower  case  letter,         The  precise  effect of \cx is as follows: if x is a lower case letter,
3254         it  is converted to upper case. Then bit 6 of the character (hex 40) is         it is converted to upper case. Then bit 6 of the character (hex 40)  is
3255         inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;         inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3256         becomes hex 7B.         becomes hex 7B.
3257    
3258         After  \x, from zero to two hexadecimal digits are read (letters can be         After \x, from zero to two hexadecimal digits are read (letters can  be
3259         in upper or lower case). Any number of hexadecimal  digits  may  appear         in  upper  or  lower case). Any number of hexadecimal digits may appear
3260         between  \x{  and  },  but the value of the character code must be less         between \x{ and }, but the value of the character  code  must  be  less
3261         than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,         than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3262         the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger         the maximum value in hexadecimal is 7FFFFFFF. Note that this is  bigger
3263         than the largest Unicode code point, which is 10FFFF.         than the largest Unicode code point, which is 10FFFF.
3264    
3265         If characters other than hexadecimal digits appear between \x{  and  },         If  characters  other than hexadecimal digits appear between \x{ and },
3266         or if there is no terminating }, this form of escape is not recognized.         or if there is no terminating }, this form of escape is not recognized.
3267         Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal         Instead,  the  initial  \x  will  be interpreted as a basic hexadecimal
3268         escape,  with  no  following  digits, giving a character whose value is         escape, with no following digits, giving a  character  whose  value  is
3269         zero.         zero.
3270    
3271         Characters whose value is less than 256 can be defined by either of the         Characters whose value is less than 256 can be defined by either of the
3272         two  syntaxes  for  \x. There is no difference in the way they are han-         two syntaxes for \x. There is no difference in the way  they  are  han-
3273         dled. For example, \xdc is exactly the same as \x{dc}.         dled. For example, \xdc is exactly the same as \x{dc}.
3274    
3275         After \0 up to two further octal digits are read. If  there  are  fewer         After  \0  up  to two further octal digits are read. If there are fewer
3276         than  two  digits,  just  those  that  are  present  are used. Thus the         than two digits, just  those  that  are  present  are  used.  Thus  the
3277         sequence \0\x\07 specifies two binary zeros followed by a BEL character         sequence \0\x\07 specifies two binary zeros followed by a BEL character
3278         (code  value 7). Make sure you supply two digits after the initial zero         (code value 7). Make sure you supply two digits after the initial  zero
3279         if the pattern character that follows is itself an octal digit.         if the pattern character that follows is itself an octal digit.
3280    
3281         The handling of a backslash followed by a digit other than 0 is compli-         The handling of a backslash followed by a digit other than 0 is compli-
3282         cated.  Outside a character class, PCRE reads it and any following dig-         cated.  Outside a character class, PCRE reads it and any following dig-
3283         its as a decimal number. If the number is less than  10,  or  if  there         its  as  a  decimal  number. If the number is less than 10, or if there
3284         have been at least that many previous capturing left parentheses in the         have been at least that many previous capturing left parentheses in the
3285         expression, the entire  sequence  is  taken  as  a  back  reference.  A         expression,  the  entire  sequence  is  taken  as  a  back reference. A
3286         description  of how this works is given later, following the discussion         description of how this works is given later, following the  discussion
3287         of parenthesized subpatterns.         of parenthesized subpatterns.
3288    
3289         Inside a character class, or if the decimal number is  greater  than  9         Inside  a  character  class, or if the decimal number is greater than 9
3290         and  there have not been that many capturing subpatterns, PCRE re-reads         and there have not been that many capturing subpatterns, PCRE  re-reads
3291         up to three octal digits following the backslash, and uses them to gen-         up to three octal digits following the backslash, and uses them to gen-
3292         erate  a data character. Any subsequent digits stand for themselves. In         erate a data character. Any subsequent digits stand for themselves.  In
3293         non-UTF-8 mode, the value of a character specified  in  octal  must  be         non-UTF-8  mode,  the  value  of a character specified in octal must be
3294         less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For         less than \400. In UTF-8 mode, values up to  \777  are  permitted.  For
3295         example:         example:
3296    
3297           \040   is another way of writing a space           \040   is another way of writing a space
# Line 3173  BACKSLASH Line 3309  BACKSLASH
3309           \81    is either a back reference, or a binary zero           \81    is either a back reference, or a binary zero
3310                     followed by the two characters "8" and "1"                     followed by the two characters "8" and "1"
3311    
3312         Note that octal values of 100 or greater must not be  introduced  by  a         Note  that  octal  values of 100 or greater must not be introduced by a
3313         leading zero, because no more than three octal digits are ever read.         leading zero, because no more than three octal digits are ever read.
3314    
3315         All the sequences that define a single character value can be used both         All the sequences that define a single character value can be used both
3316         inside and outside character classes. In addition, inside  a  character         inside  and  outside character classes. In addition, inside a character
3317         class,  the  sequence \b is interpreted as the backspace character (hex         class, the sequence \b is interpreted as the backspace  character  (hex
3318         08), and the sequences \R and \X are interpreted as the characters  "R"         08),  and the sequences \R and \X are interpreted as the characters "R"
3319         and  "X", respectively. Outside a character class, these sequences have         and "X", respectively. Outside a character class, these sequences  have
3320         different meanings (see below).         different meanings (see below).
3321    
3322     Absolute and relative back references     Absolute and relative back references
3323    
3324         The sequence \g followed by an unsigned or a negative  number,  option-         The  sequence  \g followed by an unsigned or a negative number, option-
3325         ally  enclosed  in braces, is an absolute or relative back reference. A         ally enclosed in braces, is an absolute or relative back  reference.  A
3326         named back reference can be coded as \g{name}. Back references are dis-         named back reference can be coded as \g{name}. Back references are dis-
3327         cussed later, following the discussion of parenthesized subpatterns.         cussed later, following the discussion of parenthesized subpatterns.
3328    
3329     Absolute and relative subroutine calls     Absolute and relative subroutine calls
3330    
3331         For  compatibility with Oniguruma, the non-Perl syntax \g followed by a         For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
3332         name or a number enclosed either in angle brackets or single quotes, is         name or a number enclosed either in angle brackets or single quotes, is
3333         an  alternative  syntax for referencing a subpattern as a "subroutine".         an alternative syntax for referencing a subpattern as  a  "subroutine".
3334         Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and         Details  are  discussed  later.   Note  that  \g{...} (Perl syntax) and
3335         \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back         \g<...> (Oniguruma syntax) are not synonymous. The  former  is  a  back
3336         reference; the latter is a subroutine call.         reference; the latter is a subroutine call.
3337    
3338     Generic character types     Generic character types
# Line 3216  BACKSLASH Line 3352  BACKSLASH
3352           \W     any "non-word" character           \W     any "non-word" character
3353    
3354         Each pair of escape sequences partitions the complete set of characters         Each pair of escape sequences partitions the complete set of characters
3355         into two disjoint sets. Any given character matches one, and only  one,         into  two disjoint sets. Any given character matches one, and only one,
3356         of each pair.         of each pair.
3357    
3358         These character type sequences can appear both inside and outside char-         These character type sequences can appear both inside and outside char-
3359         acter classes. They each match one character of the  appropriate  type.         acter  classes.  They each match one character of the appropriate type.
3360         If  the current matching point is at the end of the subject string, all         If the current matching point is at the end of the subject string,  all
3361         of them fail, since there is no character to match.         of them fail, since there is no character to match.
3362    
3363         For compatibility with Perl, \s does not match the VT  character  (code         For  compatibility  with Perl, \s does not match the VT character (code
3364         11).   This makes it different from the the POSIX "space" class. The \s         11).  This makes it different from the the POSIX "space" class. The  \s
3365         characters are HT (9), LF (10), FF (12), CR (13), and  space  (32).  If         characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If
3366         "use locale;" is included in a Perl script, \s may match the VT charac-         "use locale;" is included in a Perl script, \s may match the VT charac-
3367         ter. In PCRE, it never does.         ter. In PCRE, it never does.
3368    
3369         In UTF-8 mode, characters with values greater than 128 never match  \d,         In  UTF-8 mode, characters with values greater than 128 never match \d,
3370         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3371         code character property support is available.  These  sequences  retain         code  character  property  support is available. These sequences retain
3372         their original meanings from before UTF-8 support was available, mainly         their original meanings from before UTF-8 support was available, mainly
3373         for efficiency reasons.         for  efficiency  reasons. Note that this also affects \b, because it is
3374           defined in terms of \w and \W.
3375    
3376         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3377         the  other  sequences, these do match certain high-valued codepoints in         the  other  sequences, these do match certain high-valued codepoints in
# Line 3311  BACKSLASH Line 3448  BACKSLASH
3448           (*BSR_ANYCRLF)   CR, LF, or CRLF only           (*BSR_ANYCRLF)   CR, LF, or CRLF only
3449           (*BSR_UNICODE)   any Unicode newline sequence           (*BSR_UNICODE)   any Unicode newline sequence
3450    
3451         These override the default and the options given to pcre_compile(), but         These override the default and the options given to  pcre_compile()  or
3452         they can be overridden by options given to pcre_exec(). Note that these         pcre_compile2(),  but  they  can  be  overridden  by  options  given to
3453         special settings, which are not Perl-compatible, are recognized only at         pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
3454         the  very  start  of a pattern, and that they must be in upper case. If         are  not  Perl-compatible,  are  recognized only at the very start of a
3455         more than one of them is present, the last one is  used.  They  can  be         pattern, and that they must be in upper case. If more than one of  them
3456         combined  with  a  change of newline convention, for example, a pattern         is present, the last one is used. They can be combined with a change of
3457         can start with:         newline convention, for example, a pattern can start with:
3458    
3459           (*ANY)(*BSR_ANYCRLF)           (*ANY)(*BSR_ANYCRLF)
3460    
# Line 3351  BACKSLASH Line 3488  BACKSLASH
3488         Those that are not part of an identified script are lumped together  as         Those that are not part of an identified script are lumped together  as
3489         "Common". The current list of scripts is:         "Common". The current list of scripts is:
3490    
3491         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,         Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
3492         Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,         Buginese, Buhid, Canadian_Aboriginal, Carian, Cham,  Cherokee,  Common,
3493         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,         Coptic,   Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,  Egyp-
3494         Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-         tian_Hieroglyphs,  Ethiopic,  Georgian,  Glagolitic,   Gothic,   Greek,
3495         gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,         Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana, Impe-
3496         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,         rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
3497         Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,         Javanese,  Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
3498         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,         Latin,  Lepcha,  Limbu,  Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
3499         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.         Meetei_Mayek,  Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
3500           Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki,  Oriya,  Osmanya,
3501           Phags_Pa,  Phoenician,  Rejang,  Runic, Samaritan, Saurashtra, Shavian,
3502           Sinhala, Sundanese, Syloti_Nagri, Syriac,  Tagalog,  Tagbanwa,  Tai_Le,
3503           Tai_Tham,  Tai_Viet,  Tamil,  Telugu,  Thaana, Thai, Tibetan, Tifinagh,
3504           Ugaritic, Vai, Yi.
3505    
3506         Each  character has exactly one general category property, specified by         Each character has exactly one general category property, specified  by
3507         a two-letter abbreviation. For compatibility with Perl, negation can be         a two-letter abbreviation. For compatibility with Perl, negation can be
3508         specified  by  including a circumflex between the opening brace and the         specified by including a circumflex between the opening brace  and  the
3509         property name. For example, \p{^Lu} is the same as \P{Lu}.         property name. For example, \p{^Lu} is the same as \P{Lu}.
3510    
3511         If only one letter is specified with \p or \P, it includes all the gen-         If only one letter is specified with \p or \P, it includes all the gen-
3512         eral  category properties that start with that letter. In this case, in         eral category properties that start with that letter. In this case,  in
3513         the absence of negation, the curly brackets in the escape sequence  are         the  absence of negation, the curly brackets in the escape sequence are
3514         optional; these two examples have the same effect:         optional; these two examples have the same effect:
3515    
3516           \p{L}           \p{L}
# Line 3420  BACKSLASH Line 3562  BACKSLASH
3562           Zp    Paragraph separator           Zp    Paragraph separator
3563           Zs    Space separator           Zs    Space separator
3564    
3565         The  special property L& is also supported: it matches a character that         The special property L& is also supported: it matches a character  that
3566         has the Lu, Ll, or Lt property, in other words, a letter  that  is  not         has  the  Lu,  Ll, or Lt property, in other words, a letter that is not
3567         classified as a modifier or "other".         classified as a modifier or "other".
3568    
3569         The  Cs  (Surrogate)  property  applies only to characters in the range         The Cs (Surrogate) property applies only to  characters  in  the  range
3570         U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see         U+D800  to  U+DFFF. Such characters are not valid in UTF-8 strings (see
3571         RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-         RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3572         ing has been turned off (see the discussion  of  PCRE_NO_UTF8_CHECK  in         ing  has  been  turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3573         the pcreapi page).         the pcreapi page). Perl does not support the Cs property.
3574    
3575         The  long  synonyms  for  these  properties that Perl supports (such as         The long synonyms for  property  names  that  Perl  supports  (such  as
3576         \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix         \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
3577         any of these properties with "Is".         any of these properties with "Is".
3578    
3579         No character that is in the Unicode table has the Cn (unassigned) prop-         No character that is in the Unicode table has the Cn (unassigned) prop-
3580         erty.  Instead, this property is assumed for any code point that is not         erty.  Instead, this property is assumed for any code point that is not
3581         in the Unicode table.         in the Unicode table.
3582    
3583         Specifying  caseless  matching  does not affect these escape sequences.         Specifying caseless matching does not affect  these  escape  sequences.
3584         For example, \p{Lu} always matches only upper case letters.         For example, \p{Lu} always matches only upper case letters.
3585    
3586         The \X escape matches any number of Unicode  characters  that  form  an         The  \X  escape  matches  any number of Unicode characters that form an
3587         extended Unicode sequence. \X is equivalent to         extended Unicode sequence. \X is equivalent to
3588    
3589           (?>\PM\pM*)           (?>\PM\pM*)
3590    
3591         That  is,  it matches a character without the "mark" property, followed         That is, it matches a character without the "mark"  property,  followed
3592         by zero or more characters with the "mark"  property,  and  treats  the         by  zero  or  more  characters with the "mark" property, and treats the
3593         sequence  as  an  atomic group (see below).  Characters with the "mark"         sequence as an atomic group (see below).  Characters  with  the  "mark"
3594         property are typically accents that  affect  the  preceding  character.         property  are  typically  accents  that affect the preceding character.
3595         None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X         None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
3596         matches any one character.         matches any one character.
3597    
3598         Matching characters by Unicode property is not fast, because  PCRE  has         Matching  characters  by Unicode property is not fast, because PCRE has
3599         to  search  a  structure  that  contains data for over fifteen thousand         to search a structure that contains  data  for  over  fifteen  thousand
3600         characters. That is why the traditional escape sequences such as \d and         characters. That is why the traditional escape sequences such as \d and
3601         \w do not use Unicode properties in PCRE.         \w do not use Unicode properties in PCRE.
3602    
3603     Resetting the match start     Resetting the match start
3604    
3605         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3606         ously matched characters not  to  be  included  in  the  final  matched         ously  matched  characters  not  to  be  included  in the final matched
3607         sequence. For example, the pattern:         sequence. For example, the pattern:
3608    
3609           foo\Kbar           foo\Kbar
3610    
3611         matches  "foobar",  but reports that it has matched "bar". This feature         matches "foobar", but reports that it has matched "bar".  This  feature
3612         is similar to a lookbehind assertion (described  below).   However,  in         is  similar  to  a lookbehind assertion (described below).  However, in
3613         this  case, the part of the subject before the real match does not have         this case, the part of the subject before the real match does not  have
3614         to be of fixed length, as lookbehind assertions do. The use of \K  does         to  be of fixed length, as lookbehind assertions do. The use of \K does
3615         not  interfere  with  the setting of captured substrings.  For example,         not interfere with the setting of captured  substrings.   For  example,
3616         when the pattern         when the pattern
3617    
3618           (foo)\Kbar           (foo)\Kbar
3619    
3620         matches "foobar", the first substring is still set to "foo".         matches "foobar", the first substring is still set to "foo".
3621    
3622           Perl  documents  that  the  use  of  \K  within assertions is "not well
3623           defined". In PCRE, \K is acted upon  when  it  occurs  inside  positive
3624           assertions, but is ignored in negative assertions.
3625    
3626     Simple assertions     Simple assertions
3627    
3628         The final use of backslash is for certain simple assertions. An  asser-         The  final use of backslash is for certain simple assertions. An asser-
3629         tion  specifies a condition that has to be met at a particular point in         tion specifies a condition that has to be met at a particular point  in
3630         a match, without consuming any characters from the subject string.  The         a  match, without consuming any characters from the subject string. The
3631         use  of subpatterns for more complicated assertions is described below.         use of subpatterns for more complicated assertions is described  below.
3632         The backslashed assertions are:         The backslashed assertions are:
3633    
3634           \b     matches at a word boundary           \b     matches at a word boundary
# Line 3493  BACKSLASH Line 3639  BACKSLASH
3639           \z     matches only at the end of the subject           \z     matches only at the end of the subject
3640           \G     matches at the first matching position in the subject           \G     matches at the first matching position in the subject
3641    
3642         These assertions may not appear in character classes (but note that  \b         These  assertions may not appear in character classes (but note that \b
3643         has a different meaning, namely the backspace character, inside a char-         has a different meaning, namely the backspace character, inside a char-
3644         acter class).         acter class).
3645    
3646         A word boundary is a position in the subject string where  the  current         A  word  boundary is a position in the subject string where the current
3647         character  and  the previous character do not both match \w or \W (i.e.         character and the previous character do not both match \w or  \W  (i.e.
3648         one matches \w and the other matches \W), or the start or  end  of  the         one  matches  \w  and the other matches \W), or the start or end of the
3649         string if the first or last character matches \w, respectively.         string if the first or last character matches \w, respectively. Neither
3650           PCRE  nor  Perl  has a separte "start of word" or "end of word" metase-
3651           quence. However, whatever follows \b normally determines which  it  is.
3652           For example, the fragment \ba matches "a" at the start of a word.
3653    
3654         The  \A,  \Z,  and \z assertions differ from the traditional circumflex         The  \A,  \Z,  and \z assertions differ from the traditional circumflex
3655         and dollar (described in the next section) in that they only ever match         and dollar (described in the next section) in that they only ever match
# Line 3626  SQUARE BRACKETS AND CHARACTER CLASSES Line 3775  SQUARE BRACKETS AND CHARACTER CLASSES
3775    
3776         An opening square bracket introduces a character class, terminated by a         An opening square bracket introduces a character class, terminated by a
3777         closing square bracket. A closing square bracket on its own is not spe-         closing square bracket. A closing square bracket on its own is not spe-
3778         cial. If a closing square bracket is required as a member of the class,         cial by default.  However, if the PCRE_JAVASCRIPT_COMPAT option is set,
3779         it should be the first data character in the class  (after  an  initial         a lone closing square bracket causes a compile-time error. If a closing
3780         circumflex, if present) or escaped with a backslash.         square  bracket  is required as a member of the class, it should be the
3781           first data character in the class  (after  an  initial  circumflex,  if
3782           present) or escaped with a backslash.
3783    
3784         A  character  class matches a single character in the subject. In UTF-8         A  character  class matches a single character in the subject. In UTF-8
3785         mode, the character may occupy more than one byte. A matched  character         mode, the character may be more than one byte long. A matched character
3786         must be in the set of characters defined by the class, unless the first         must be in the set of characters defined by the class, unless the first
3787         character in the class definition is a circumflex, in  which  case  the         character in the class definition is a circumflex, in  which  case  the
3788         subject  character  must  not  be in the set defined by the class. If a         subject  character  must  not  be in the set defined by the class. If a
# Line 3642  SQUARE BRACKETS AND CHARACTER CLASSES Line 3793  SQUARE BRACKETS AND CHARACTER CLASSES
3793         while [^aeiou] matches any character that is not a  lower  case  vowel.         while [^aeiou] matches any character that is not a  lower  case  vowel.
3794         Note that a circumflex is just a convenient notation for specifying the         Note that a circumflex is just a convenient notation for specifying the
3795         characters that are in the class by enumerating those that are  not.  A         characters that are in the class by enumerating those that are  not.  A
3796         class  that starts with a circumflex is not an assertion: it still con-         class  that starts with a circumflex is not an assertion; it still con-
3797         sumes a character from the subject string, and therefore  it  fails  if         sumes a character from the subject string, and therefore  it  fails  if
3798         the current pointer is at the end of the string.         the current pointer is at the end of the string.
3799    
# Line 3658  SQUARE BRACKETS AND CHARACTER CLASSES Line 3809  SQUARE BRACKETS AND CHARACTER CLASSES
3809         than 128, so caseless matching is always possible. For characters  with         than 128, so caseless matching is always possible. For characters  with
3810         higher  values,  the  concept  of case is supported if PCRE is compiled         higher  values,  the  concept  of case is supported if PCRE is compiled
3811         with Unicode property support, but not otherwise.  If you want  to  use         with Unicode property support, but not otherwise.  If you want  to  use
3812         caseless  matching  for  characters 128 and above, you must ensure that         caseless  matching  in UTF8-mode for characters 128 and above, you must
3813         PCRE is compiled with Unicode property support as well  as  with  UTF-8         ensure that PCRE is compiled with Unicode property support as  well  as
3814         support.         with UTF-8 support.
3815    
3816         Characters  that  might  indicate  line breaks are never treated in any         Characters  that  might  indicate  line breaks are never treated in any
3817         special way  when  matching  character  classes,  whatever  line-ending         special way  when  matching  character  classes,  whatever  line-ending
# Line 3760  POSIX CHARACTER CLASSES Line 3911  POSIX CHARACTER CLASSES
3911    
3912  VERTICAL BAR  VERTICAL BAR
3913    
3914         Vertical  bar characters are used to separate alternative patterns. For         Vertical bar characters are used to separate alternative patterns.  For
3915         example, the pattern         example, the pattern
3916    
3917           gilbert|sullivan           gilbert|sullivan
3918    
3919         matches either "gilbert" or "sullivan". Any number of alternatives  may         matches  either "gilbert" or "sullivan". Any number of alternatives may
3920         appear,  and  an  empty  alternative  is  permitted (matching the empty         appear, and an empty  alternative  is  permitted  (matching  the  empty
3921         string). The matching process tries each alternative in turn, from left         string). The matching process tries each alternative in turn, from left
3922         to  right, and the first one that succeeds is used. If the alternatives         to right, and the first one that succeeds is used. If the  alternatives
3923         are within a subpattern (defined below), "succeeds" means matching  the         are  within a subpattern (defined below), "succeeds" means matching the
3924         rest  of the main pattern as well as the alternative in the subpattern.         rest of the main pattern as well as the alternative in the subpattern.
3925    
3926    
3927  INTERNAL OPTION SETTING  INTERNAL OPTION SETTING
# Line 3796  INTERNAL OPTION SETTING Line 3947  INTERNAL OPTION SETTING
3947         can  be changed in the same way as the Perl-compatible options by using         can  be changed in the same way as the Perl-compatible options by using
3948         the characters J, U and X respectively.         the characters J, U and X respectively.
3949    
3950         When an option change occurs at top level (that is, not inside  subpat-         When one of these option changes occurs at  top  level  (that  is,  not
3951         tern  parentheses),  the change applies to the remainder of the pattern         inside  subpattern parentheses), the change applies to the remainder of
3952         that follows.  If the change is placed right at the start of a pattern,         the pattern that follows. If the change is placed right at the start of
3953         PCRE extracts it into the global options (and it will therefore show up         a pattern, PCRE extracts it into the global options (and it will there-
3954         in data extracted by the pcre_fullinfo() function).         fore show up in data extracted by the pcre_fullinfo() function).
3955    
3956         An option change within a subpattern (see below for  a  description  of         An option change within a subpattern (see below for  a  description  of
3957         subpatterns) affects only that part of the current pattern that follows         subpatterns) affects only that part of the current pattern that follows
# Line 3823  INTERNAL OPTION SETTING Line 3974  INTERNAL OPTION SETTING
3974    
3975         Note:  There  are  other  PCRE-specific  options that can be set by the         Note:  There  are  other  PCRE-specific  options that can be set by the
3976         application when the compile or match functions  are  called.  In  some         application when the compile or match functions  are  called.  In  some
3977         cases  the  pattern  can  contain special leading sequences to override         cases the pattern can contain special leading sequences such as (*CRLF)
3978         what the application has set or what has been  defaulted.  Details  are         to override what the application has set or what  has  been  defaulted.
3979         given in the section entitled "Newline sequences" above.         Details  are  given  in the section entitled "Newline sequences" above.
3980           There is also the (*UTF8) leading sequence that  can  be  used  to  set
3981           UTF-8 mode; this is equivalent to setting the PCRE_UTF8 option.
3982    
3983    
3984  SUBPATTERNS  SUBPATTERNS
# Line 3907  DUPLICATE SUBPATTERN NUMBERS Line 4060  DUPLICATE SUBPATTERN NUMBERS
4060           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
4061           # 1            2         2  3        2     3     4           # 1            2         2  3        2     3     4
4062    
4063         A backreference or a recursive call to  a  numbered  subpattern  always         A back reference to a numbered subpattern uses the  most  recent  value
4064         refers to the first one in the pattern with the given number.         that  is  set  for that number by any subpattern. The following pattern
4065           matches "abcabc" or "defdef":
4066    
4067             /(?|(abc)|(def))\1/
4068    
4069           In contrast, a recursive or "subroutine" call to a numbered  subpattern
4070           always  refers  to  the first one in the pattern with the given number.
4071           The following pattern matches "abcabc" or "defabc":
4072    
4073         An  alternative approach to using this "branch reset" feature is to use           /(?|(abc)|(def))(?1)/
4074    
4075           If a condition test for a subpattern's having matched refers to a  non-
4076           unique  number, the test is true if any of the subpatterns of that num-
4077           ber have matched.
4078    
4079           An alternative approach to using this "branch reset" feature is to  use
4080         duplicate named subpatterns, as described in the next section.         duplicate named subpatterns, as described in the next section.
4081    
4082    
4083  NAMED SUBPATTERNS  NAMED SUBPATTERNS
4084    
4085         Identifying capturing parentheses by number is simple, but  it  can  be         Identifying  capturing  parentheses  by number is simple, but it can be
4086         very  hard  to keep track of the numbers in complicated regular expres-         very hard to keep track of the numbers in complicated  regular  expres-
4087         sions. Furthermore, if an  expression  is  modified,  the  numbers  may         sions.  Furthermore,  if  an  expression  is  modified, the numbers may
4088         change.  To help with this difficulty, PCRE supports the naming of sub-         change. To help with this difficulty, PCRE supports the naming of  sub-
4089         patterns. This feature was not added to Perl until release 5.10. Python         patterns. This feature was not added to Perl until release 5.10. Python
4090         had  the  feature earlier, and PCRE introduced it at release 4.0, using         had the feature earlier, and PCRE introduced it at release  4.0,  using
4091         the Python syntax. PCRE now supports both the Perl and the Python  syn-         the  Python syntax. PCRE now supports both the Perl and the Python syn-
4092         tax.         tax. Perl allows identically numbered  subpatterns  to  have  different
4093           names, but PCRE does not.
4094    
4095         In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)         In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)
4096         or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References         or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References
4097         to capturing parentheses from other parts of the pattern, such as back-         to  capturing parentheses from other parts of the pattern, such as back
4098         references, recursion, and conditions, can be made by name as  well  as         references, recursion, and conditions, can be made by name as  well  as
4099         by number.         by number.
4100    
# Line 3940  NAMED SUBPATTERNS Line 4107  NAMED SUBPATTERNS
4107    
4108         By default, a name must be unique within a pattern, but it is  possible         By default, a name must be unique within a pattern, but it is  possible
4109         to relax this constraint by setting the PCRE_DUPNAMES option at compile         to relax this constraint by setting the PCRE_DUPNAMES option at compile
4110         time. This can be useful for patterns where only one  instance  of  the         time. (Duplicate names are also always permitted for  subpatterns  with
4111           the  same  number, set up as described in the previous section.) Dupli-
4112           cate names can be useful for patterns where only one  instance  of  the
4113         named  parentheses  can  match. Suppose you want to match the name of a         named  parentheses  can  match. Suppose you want to match the name of a
4114         weekday, either as a 3-letter abbreviation or as the full name, and  in         weekday, either as a 3-letter abbreviation or as the full name, and  in
4115         both cases you want to extract the abbreviation. This pattern (ignoring         both cases you want to extract the abbreviation. This pattern (ignoring
# Line 3959  NAMED SUBPATTERNS Line 4128  NAMED SUBPATTERNS
4128         The convenience function for extracting the data by  name  returns  the         The convenience function for extracting the data by  name  returns  the
4129         substring  for  the first (and in this example, the only) subpattern of         substring  for  the first (and in this example, the only) subpattern of
4130         that name that matched. This saves searching  to  find  which  numbered         that name that matched. This saves searching  to  find  which  numbered
4131         subpattern  it  was. If you make a reference to a non-unique named sub-         subpattern it was.
4132         pattern from elsewhere in the pattern, the one that corresponds to  the  
4133         lowest  number  is used. For further details of the interfaces for han-         If  you  make  a  back  reference to a non-unique named subpattern from
4134         dling named subpatterns, see the pcreapi documentation.         elsewhere in the pattern, the one that corresponds to the first  occur-
4135           rence of the name is used. In the absence of duplicate numbers (see the
4136           previous section) this is the one with the lowest number. If you use  a
4137           named  reference  in a condition test (see the section about conditions
4138           below), either to check whether a subpattern has matched, or  to  check
4139           for  recursion,  all  subpatterns with the same name are tested. If the
4140           condition is true for any one of them, the overall condition  is  true.
4141           This is the same behaviour as testing by number. For further details of
4142           the interfaces for handling named subpatterns, see the pcreapi documen-
4143           tation.
4144    
4145           Warning: You cannot use different names to distinguish between two sub-
4146           patterns with the same number because PCRE uses only the  numbers  when
4147           matching. For this reason, an error is given at compile time if differ-
4148           ent names are given to subpatterns with the same number.  However,  you
4149           can  give  the same name to subpatterns with the same number, even when
4150           PCRE_DUPNAMES is not set.
4151    
4152    
4153  REPETITION  REPETITION
# Line 3979  REPETITION Line 4164  REPETITION
4164           a character class           a character class
4165           a back reference (see next section)           a back reference (see next section)
4166           a parenthesized subpattern (unless it is an assertion)           a parenthesized subpattern (unless it is an assertion)
4167             a recursive or "subroutine" call to a subpattern
4168    
4169         The  general repetition quantifier specifies a minimum and maximum num-         The  general repetition quantifier specifies a minimum and maximum num-
4170         ber of permitted matches, by giving the two numbers in  curly  brackets         ber of permitted matches, by giving the two numbers in  curly  brackets
# Line 4004  REPETITION Line 4190  REPETITION
4190         the syntax of a quantifier, is taken as a literal character. For  exam-         the syntax of a quantifier, is taken as a literal character. For  exam-
4191         ple, {,6} is not a quantifier, but a literal string of four characters.         ple, {,6} is not a quantifier, but a literal string of four characters.
4192    
4193         In UTF-8 mode, quantifiers apply to UTF-8  characters  rather  than  to         In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
4194         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4195         acters, each of which is represented by a two-byte sequence. Similarly,         acters, each of which is represented by a two-byte sequence. Similarly,
4196         when Unicode property support is available, \X{3} matches three Unicode         when Unicode property support is available, \X{3} matches three Unicode
4197         extended sequences, each of which may be several bytes long  (and  they         extended  sequences,  each of which may be several bytes long (and they
4198         may be of different lengths).         may be of different lengths).
4199    
4200         The quantifier {0} is permitted, causing the expression to behave as if         The quantifier {0} is permitted, causing the expression to behave as if
4201         the previous item and the quantifier were not present. This may be use-         the previous item and the quantifier were not present. This may be use-
4202         ful  for  subpatterns that are referenced as subroutines from elsewhere         ful for subpatterns that are referenced as subroutines  from  elsewhere
4203         in the pattern. Items other than subpatterns that have a {0} quantifier         in the pattern. Items other than subpatterns that have a {0} quantifier
4204         are omitted from the compiled pattern.         are omitted from the compiled pattern.
4205    
4206         For  convenience, the three most common quantifiers have single-charac-         For convenience, the three most common quantifiers have  single-charac-
4207         ter abbreviations:         ter abbreviations:
4208    
4209           *    is equivalent to {0,}           *    is equivalent to {0,}
4210           +    is equivalent to {1,}           +    is equivalent to {1,}
4211           ?    is equivalent to {0,1}           ?    is equivalent to {0,1}
4212    
4213         It is possible to construct infinite loops by  following  a  subpattern         It  is  possible  to construct infinite loops by following a subpattern
4214         that can match no characters with a quantifier that has no upper limit,         that can match no characters with a quantifier that has no upper limit,
4215         for example:         for example:
4216    
4217           (a?)*           (a?)*
4218    
4219         Earlier versions of Perl and PCRE used to give an error at compile time         Earlier versions of Perl and PCRE used to give an error at compile time
4220         for  such  patterns. However, because there are cases where this can be         for such patterns. However, because there are cases where this  can  be
4221         useful, such patterns are now accepted, but if any  repetition  of  the         useful,  such  patterns  are now accepted, but if any repetition of the
4222         subpattern  does in fact match no characters, the loop is forcibly bro-         subpattern does in fact match no characters, the loop is forcibly  bro-
4223         ken.         ken.
4224    
4225         By default, the quantifiers are "greedy", that is, they match  as  much         By  default,  the quantifiers are "greedy", that is, they match as much
4226         as  possible  (up  to  the  maximum number of permitted times), without         as possible (up to the maximum  number  of  permitted  times),  without
4227         causing the rest of the pattern to fail. The classic example  of  where         causing  the  rest of the pattern to fail. The classic example of where
4228         this gives problems is in trying to match comments in C programs. These         this gives problems is in trying to match comments in C programs. These
4229         appear between /* and */ and within the comment,  individual  *  and  /         appear  between  /*  and  */ and within the comment, individual * and /
4230         characters  may  appear. An attempt to match C comments by applying the         characters may appear. An attempt to match C comments by  applying  the
4231         pattern         pattern
4232    
4233           /\*.*\*/           /\*.*\*/
# Line 4050  REPETITION Line 4236  REPETITION
4236    
4237           /* first comment */  not comment  /* second comment */           /* first comment */  not comment  /* second comment */
4238    
4239         fails, because it matches the entire string owing to the greediness  of         fails,  because it matches the entire string owing to the greediness of
4240         the .*  item.         the .*  item.
4241    
4242         However,  if  a quantifier is followed by a question mark, it ceases to         However, if a quantifier is followed by a question mark, it  ceases  to
4243         be greedy, and instead matches the minimum number of times possible, so         be greedy, and instead matches the minimum number of times possible, so
4244         the pattern         the pattern
4245    
4246           /\*.*?\*/           /\*.*?\*/
4247    
4248         does  the  right  thing with the C comments. The meaning of the various         does the right thing with the C comments. The meaning  of  the  various
4249         quantifiers is not otherwise changed,  just  the  preferred  number  of         quantifiers  is  not  otherwise  changed,  just the preferred number of
4250         matches.   Do  not  confuse this use of question mark with its use as a         matches.  Do not confuse this use of question mark with its  use  as  a
4251         quantifier in its own right. Because it has two uses, it can  sometimes         quantifier  in its own right. Because it has two uses, it can sometimes
4252         appear doubled, as in         appear doubled, as in
4253    
4254           \d??\d           \d??\d
# Line 4070  REPETITION Line 4256  REPETITION
4256         which matches one digit by preference, but can match two if that is the         which matches one digit by preference, but can match two if that is the
4257         only way the rest of the pattern matches.         only way the rest of the pattern matches.
4258    
4259         If the PCRE_UNGREEDY option is set (an option that is not available  in         If  the PCRE_UNGREEDY option is set (an option that is not available in
4260         Perl),  the  quantifiers are not greedy by default, but individual ones         Perl), the quantifiers are not greedy by default, but  individual  ones
4261         can be made greedy by following them with a  question  mark.  In  other         can  be  made  greedy  by following them with a question mark. In other
4262         words, it inverts the default behaviour.         words, it inverts the default behaviour.
4263    
4264         When  a  parenthesized  subpattern  is quantified with a minimum repeat         When a parenthesized subpattern is quantified  with  a  minimum  repeat
4265         count that is greater than 1 or with a limited maximum, more memory  is         count  that is greater than 1 or with a limited maximum, more memory is
4266         required  for  the  compiled  pattern, in proportion to the size of the         required for the compiled pattern, in proportion to  the  size  of  the
4267         minimum or maximum.         minimum or maximum.
4268    
4269         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4270         alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,         alent to Perl's /s) is set, thus allowing the dot  to  match  newlines,
4271         the pattern is implicitly anchored, because whatever  follows  will  be         the  pattern  is  implicitly anchored, because whatever follows will be
4272         tried  against every character position in the subject string, so there         tried against every character position in the subject string, so  there
4273         is no point in retrying the overall match at  any  position  after  the         is  no  point  in  retrying the overall match at any position after the
4274         first.  PCRE  normally treats such a pattern as though it were preceded         first. PCRE normally treats such a pattern as though it  were  preceded
4275         by \A.         by \A.
4276    
4277         In cases where it is known that the subject  string  contains  no  new-         In  cases  where  it  is known that the subject string contains no new-
4278         lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-         lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
4279         mization, or alternatively using ^ to indicate anchoring explicitly.         mization, or alternatively using ^ to indicate anchoring explicitly.
4280    
4281         However, there is one situation where the optimization cannot be  used.         However,  there is one situation where the optimization cannot be used.
4282         When  .*   is  inside  capturing  parentheses that are the subject of a         When .*  is inside capturing parentheses that are the subject of a back
4283         backreference elsewhere in the pattern, a match at the start  may  fail         reference elsewhere in the pattern, a match at the start may fail where
4284         where a later one succeeds. Consider, for example:         a later one succeeds. Consider, for example:
4285    
4286           (.*)abc\1           (.*)abc\1
4287    
4288         If  the subject is "xyz123abc123" the match point is the fourth charac-         If the subject is "xyz123abc123" the match point is the fourth  charac-
4289         ter. For this reason, such a pattern is not implicitly anchored.         ter. For this reason, such a pattern is not implicitly anchored.
4290    
4291         When a capturing subpattern is repeated, the value captured is the sub-         When a capturing subpattern is repeated, the value captured is the sub-
# Line 4108  REPETITION Line 4294  REPETITION
4294           (tweedle[dume]{3}\s*)+           (tweedle[dume]{3}\s*)+
4295    
4296         has matched "tweedledum tweedledee" the value of the captured substring         has matched "tweedledum tweedledee" the value of the captured substring
4297         is "tweedledee". However, if there are  nested  capturing  subpatterns,         is  "tweedledee".  However,  if there are nested capturing subpatterns,
4298         the  corresponding captured values may have been set in previous itera-         the corresponding captured values may have been set in previous  itera-
4299         tions. For example, after         tions. For example, after
4300    
4301           /(a|(b))+/           /(a|(b))+/
# Line 4119  REPETITION Line 4305  REPETITION
4305    
4306  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4307    
4308         With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")         With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4309         repetition,  failure  of what follows normally causes the repeated item         repetition, failure of what follows normally causes the  repeated  item
4310         to be re-evaluated to see if a different number of repeats  allows  the         to  be  re-evaluated to see if a different number of repeats allows the
4311         rest  of  the pattern to match. Sometimes it is useful to prevent this,         rest of the pattern to match. Sometimes it is useful to  prevent  this,
4312         either to change the nature of the match, or to cause it  fail  earlier         either  to  change the nature of the match, or to cause it fail earlier
4313         than  it otherwise might, when the author of the pattern knows there is         than it otherwise might, when the author of the pattern knows there  is
4314         no point in carrying on.         no point in carrying on.
4315    
4316         Consider, for example, the pattern \d+foo when applied to  the  subject         Consider,  for  example, the pattern \d+foo when applied to the subject
4317         line         line
4318    
4319           123456bar           123456bar
4320    
4321         After matching all 6 digits and then failing to match "foo", the normal         After matching all 6 digits and then failing to match "foo", the normal
4322         action of the matcher is to try again with only 5 digits  matching  the         action  of  the matcher is to try again with only 5 digits matching the
4323         \d+  item,  and  then  with  4,  and  so on, before ultimately failing.         \d+ item, and then with  4,  and  so  on,  before  ultimately  failing.
4324         "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides         "Atomic  grouping"  (a  term taken from Jeffrey Friedl's book) provides
4325         the  means for specifying that once a subpattern has matched, it is not         the means for specifying that once a subpattern has matched, it is  not
4326         to be re-evaluated in this way.         to be re-evaluated in this way.
4327    
4328         If we use atomic grouping for the previous example, the  matcher  gives         If  we  use atomic grouping for the previous example, the matcher gives
4329         up  immediately  on failing to match "foo" the first time. The notation         up immediately on failing to match "foo" the first time.  The  notation
4330         is a kind of special parenthesis, starting with (?> as in this example:         is a kind of special parenthesis, starting with (?> as in this example:
4331    
4332           (?>\d+)foo           (?>\d+)foo
# Line 4218  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE Line 4404  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE
4404    
4405           ((?>\D+)|<\d+>)*[!?]           ((?>\D+)|<\d+>)*[!?]
4406    
4407         sequences of non-digits cannot be broken, and failure happens  quickly.         sequences of non-digits cannot be broken, and failure happens quickly.
4408    
4409    
4410  BACK REFERENCES  BACK REFERENCES
4411    
4412         Outside a character class, a backslash followed by a digit greater than         Outside a character class, a backslash followed by a digit greater than
4413         0 (and possibly further digits) is a back reference to a capturing sub-         0 (and possibly further digits) is a back reference to a capturing sub-
4414         pattern  earlier  (that is, to its left) in the pattern, provided there         pattern earlier (that is, to its left) in the pattern,  provided  there
4415         have been that many previous capturing left parentheses.         have been that many previous capturing left parentheses.
4416    
4417         However, if the decimal number following the backslash is less than 10,         However, if the decimal number following the backslash is less than 10,
4418         it  is  always  taken  as a back reference, and causes an error only if         it is always taken as a back reference, and causes  an  error  only  if
4419         there are not that many capturing left parentheses in the  entire  pat-         there  are  not that many capturing left parentheses in the entire pat-
4420         tern.  In  other words, the parentheses that are referenced need not be         tern. In other words, the parentheses that are referenced need  not  be
4421         to the left of the reference for numbers less than 10. A "forward  back         to  the left of the reference for numbers less than 10. A "forward back
4422         reference"  of  this  type can make sense when a repetition is involved         reference" of this type can make sense when a  repetition  is  involved
4423         and the subpattern to the right has participated in an  earlier  itera-         and  the  subpattern to the right has participated in an earlier itera-
4424         tion.         tion.
4425    
4426         It  is  not  possible to have a numerical "forward back reference" to a         It is not possible to have a numerical "forward back  reference"  to  a
4427         subpattern whose number is 10 or  more  using  this  syntax  because  a         subpattern  whose  number  is  10  or  more using this syntax because a
4428         sequence  such  as  \50 is interpreted as a character defined in octal.         sequence such as \50 is interpreted as a character  defined  in  octal.
4429         See the subsection entitled "Non-printing characters" above for further         See the subsection entitled "Non-printing characters" above for further
4430         details  of  the  handling of digits following a backslash. There is no         details of the handling of digits following a backslash.  There  is  no
4431         such problem when named parentheses are used. A back reference  to  any         such  problem  when named parentheses are used. A back reference to any
4432         subpattern is possible using named parentheses (see below).         subpattern is possible using named parentheses (see below).
4433    
4434         Another  way  of  avoiding  the ambiguity inherent in the use of digits         Another way of avoiding the ambiguity inherent in  the  use  of  digits
4435         following a backslash is to use the \g escape sequence, which is a fea-         following a backslash is to use the \g escape sequence, which is a fea-
4436         ture  introduced  in  Perl  5.10.  This  escape  must be followed by an         ture introduced in Perl 5.10.  This  escape  must  be  followed  by  an
4437         unsigned number or a negative number, optionally  enclosed  in  braces.         unsigned  number  or  a negative number, optionally enclosed in braces.
4438         These examples are all identical:         These examples are all identical:
4439    
4440           (ring), \1           (ring), \1
4441           (ring), \g1           (ring), \g1
4442           (ring), \g{1}           (ring), \g{1}
4443    
4444         An  unsigned number specifies an absolute reference without the ambigu-         An unsigned number specifies an absolute reference without the  ambigu-
4445         ity that is present in the older syntax. It is also useful when literal         ity that is present in the older syntax. It is also useful when literal
4446         digits follow the reference. A negative number is a relative reference.         digits follow the reference. A negative number is a relative reference.
4447         Consider this example:         Consider this example:
# Line 4263  BACK REFERENCES Line 4449  BACK REFERENCES
4449           (abc(def)ghi)\g{-1}           (abc(def)ghi)\g{-1}
4450    
4451         The sequence \g{-1} is a reference to the most recently started captur-         The sequence \g{-1} is a reference to the most recently started captur-
4452         ing  subpattern  before \g, that is, is it equivalent to \2. Similarly,         ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,
4453         \g{-2} would be equivalent to \1. The use of relative references can be         \g{-2} would be equivalent to \1. The use of relative references can be
4454         helpful  in  long  patterns,  and  also in patterns that are created by         helpful in long patterns, and also in  patterns  that  are  created  by
4455         joining together fragments that contain references within themselves.         joining together fragments that contain references within themselves.
4456    
4457         A back reference matches whatever actually matched the  capturing  sub-         A  back  reference matches whatever actually matched the capturing sub-
4458         pattern  in  the  current subject string, rather than anything matching         pattern in the current subject string, rather  than  anything  matching
4459         the subpattern itself (see "Subpatterns as subroutines" below for a way         the subpattern itself (see "Subpatterns as subroutines" below for a way
4460         of doing that). So the pattern         of doing that). So the pattern
4461    
4462           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4463    
4464         matches  "sense and sensibility" and "response and responsibility", but         matches "sense and sensibility" and "response and responsibility",  but
4465         not "sense and responsibility". If caseful matching is in force at  the         not  "sense and responsibility". If caseful matching is in force at the
4466         time  of the back reference, the case of letters is relevant. For exam-         time of the back reference, the case of letters is relevant. For  exam-
4467         ple,         ple,
4468    
4469           ((?i)rah)\s+\1           ((?i)rah)\s+\1
4470    
4471         matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the         matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
4472         original capturing subpattern is matched caselessly.         original capturing subpattern is matched caselessly.
4473    
4474         There  are  several  different ways of writing back references to named         There are several different ways of writing back  references  to  named
4475         subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or         subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
4476         \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's         \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's
4477         unified back reference syntax, in which \g can be used for both numeric         unified back reference syntax, in which \g can be used for both numeric
4478         and  named  references,  is  also supported. We could rewrite the above         and named references, is also supported. We  could  rewrite  the  above
4479         example in any of the following ways:         example in any of the following ways:
4480    
4481           (?<p1>(?i)rah)\s+\k<p1>           (?<p1>(?i)rah)\s+\k<p1>
# Line 4297  BACK REFERENCES Line 4483  BACK REFERENCES
4483           (?P<p1>(?i)rah)\s+(?P=p1)           (?P<p1>(?i)rah)\s+(?P=p1)
4484           (?<p1>(?i)rah)\s+\g{p1}           (?<p1>(?i)rah)\s+\g{p1}
4485    
4486         A subpattern that is referenced by  name  may  appear  in  the  pattern         A  subpattern  that  is  referenced  by  name may appear in the pattern
4487         before or after the reference.         before or after the reference.
4488    
4489         There  may be more than one back reference to the same subpattern. If a         There may be more than one back reference to the same subpattern. If  a
4490         subpattern has not actually been used in a particular match,  any  back         subpattern  has  not actually been used in a particular match, any back
4491         references to it always fail. For example, the pattern         references to it always fail by default. For example, the pattern
4492    
4493           (a|(bc))\2           (a|(bc))\2
4494    
4495         always  fails if it starts to match "a" rather than "bc". Because there         always fails if it starts to match "a" rather than  "bc".  However,  if
4496         may be many capturing parentheses in a pattern,  all  digits  following         the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
4497         the  backslash  are taken as part of a potential back reference number.         ence to an unset value matches an empty string.
4498         If the pattern continues with a digit character, some delimiter must be  
4499         used  to  terminate  the back reference. If the PCRE_EXTENDED option is         Because there may be many capturing parentheses in a pattern, all  dig-
4500         set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-         its  following a backslash are taken as part of a potential back refer-
4501         ments" below) can be used.         ence number.  If the pattern continues with  a  digit  character,  some
4502           delimiter  must  be  used  to  terminate  the  back  reference.  If the
4503           PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
4504           syntax or an empty comment (see "Comments" below) can be used.
4505    
4506       Recursive back references
4507    
4508         A  back reference that occurs inside the parentheses to which it refers         A  back reference that occurs inside the parentheses to which it refers
4509         fails when the subpattern is first used, so, for example,  (a\1)  never         fails when the subpattern is first used, so, for example,  (a\1)  never
# Line 4328  BACK REFERENCES Line 4519  BACK REFERENCES
4519         to match the back reference. This can be done using alternation, as  in         to match the back reference. This can be done using alternation, as  in
4520         the example above, or by a quantifier with a minimum of zero.         the example above, or by a quantifier with a minimum of zero.
4521    
4522           Back  references of this type cause the group that they reference to be
4523           treated as an atomic group.  Once the whole group has been  matched,  a
4524           subsequent  matching  failure cannot cause backtracking into the middle
4525           of the group.
4526    
4527    
4528  ASSERTIONS  ASSERTIONS
4529    
4530         An  assertion  is  a  test on the characters following or preceding the         An assertion is a test on the characters  following  or  preceding  the
4531         current matching point that does not actually consume  any  characters.         current  matching  point that does not actually consume any characters.
4532         The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are         The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
4533         described above.         described above.
4534    
4535         More complicated assertions are coded as  subpatterns.  There  are  two         More  complicated  assertions  are  coded as subpatterns. There are two
4536         kinds:  those  that  look  ahead of the current position in the subject         kinds: those that look ahead of the current  position  in  the  subject
4537         string, and those that look  behind  it.  An  assertion  subpattern  is         string,  and  those  that  look  behind  it. An assertion subpattern is
4538         matched  in  the  normal way, except that it does not cause the current         matched in the normal way, except that it does not  cause  the  current
4539         matching position to be changed.         matching position to be changed.
4540    
4541         Assertion subpatterns are not capturing subpatterns,  and  may  not  be         Assertion  subpatterns  are  not  capturing subpatterns, and may not be
4542         repeated,  because  it  makes no sense to assert the same thing several         repeated, because it makes no sense to assert the  same  thing  several
4543         times. If any kind of assertion contains capturing  subpatterns  within         times.  If  any kind of assertion contains capturing subpatterns within
4544         it,  these are counted for the purposes of numbering the capturing sub-         it, these are counted for the purposes of numbering the capturing  sub-
4545         patterns in the whole pattern.  However, substring capturing is carried         patterns in the whole pattern.  However, substring capturing is carried
4546         out  only  for  positive assertions, because it does not make sense for         out only for positive assertions, because it does not  make  sense  for
4547         negative assertions.         negative assertions.
4548    
4549     Lookahead assertions     Lookahead assertions
# Line 4357  ASSERTIONS Line 4553  ASSERTIONS
4553    
4554           \w+(?=;)           \w+(?=;)
4555    
4556         matches  a word followed by a semicolon, but does not include the semi-         matches a word followed by a semicolon, but does not include the  semi-
4557         colon in the match, and         colon in the match, and
4558    
4559           foo(?!bar)           foo(?!bar)
4560    
4561         matches any occurrence of "foo" that is not  followed  by  "bar".  Note         matches  any  occurrence  of  "foo" that is not followed by "bar". Note
4562         that the apparently similar pattern         that the apparently similar pattern
4563    
4564           (?!foo)bar           (?!foo)bar
4565    
4566         does  not  find  an  occurrence  of "bar" that is preceded by something         does not find an occurrence of "bar"  that  is  preceded  by  something
4567         other than "foo"; it finds any occurrence of "bar" whatsoever,  because         other  than "foo"; it finds any occurrence of "bar" whatsoever, because
4568         the assertion (?!foo) is always true when the next three characters are         the assertion (?!foo) is always true when the next three characters are
4569         "bar". A lookbehind assertion is needed to achieve the other effect.         "bar". A lookbehind assertion is needed to achieve the other effect.
4570    
4571         If you want to force a matching failure at some point in a pattern, the         If you want to force a matching failure at some point in a pattern, the
4572         most  convenient  way  to  do  it  is with (?!) because an empty string         most convenient way to do it is  with  (?!)  because  an  empty  string
4573         always matches, so an assertion that requires there not to be an  empty         always  matches, so an assertion that requires there not to be an empty
4574         string must always fail.         string must always fail.   The  Perl  5.10  backtracking  control  verb
4575           (*FAIL) or (*F) is essentially a synonym for (?!).
4576    
4577     Lookbehind assertions     Lookbehind assertions
4578    
# Line 4398  ASSERTIONS Line 4595  ASSERTIONS
4595    
4596         causes an error at compile time. Branches that match  different  length         causes an error at compile time. Branches that match  different  length
4597         strings  are permitted only at the top level of a lookbehind assertion.         strings  are permitted only at the top level of a lookbehind assertion.
4598         This is an extension compared with  Perl  (at  least  for  5.8),  which         This is an extension compared with Perl (5.8 and 5.10), which  requires
4599         requires  all branches to match the same length of string. An assertion         all branches to match the same length of string. An assertion such as
        such as  
4600    
4601           (?<=ab(c|de))           (?<=ab(c|de))
4602    
4603         is not permitted, because its single top-level  branch  can  match  two         is  not  permitted,  because  its single top-level branch can match two
4604         different  lengths,  but  it is acceptable if rewritten to use two top-         different lengths, but it is acceptable to PCRE if rewritten to use two
4605         level branches:         top-level branches:
4606    
4607           (?<=abc|abde)           (?<=abc|abde)
4608    
4609         In some cases, the Perl 5.10 escape sequence \K (see above) can be used         In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4610         instead  of  a lookbehind assertion; this is not restricted to a fixed-         instead of  a  lookbehind  assertion  to  get  round  the  fixed-length
4611         length.         restriction.
4612    
4613         The implementation of lookbehind assertions is, for  each  alternative,         The  implementation  of lookbehind assertions is, for each alternative,
4614         to  temporarily  move the current position back by the fixed length and         to temporarily move the current position back by the fixed  length  and
4615         then try to match. If there are insufficient characters before the cur-         then try to match. If there are insufficient characters before the cur-
4616         rent position, the assertion fails.         rent position, the assertion fails.
4617    
4618         PCRE does not allow the \C escape (which matches a single byte in UTF-8         PCRE does not allow the \C escape (which matches a single byte in UTF-8
4619         mode) to appear in lookbehind assertions, because it makes it  impossi-         mode)  to appear in lookbehind assertions, because it makes it impossi-
4620         ble  to  calculate the length of the lookbehind. The \X and \R escapes,         ble to calculate the length of the lookbehind. The \X and  \R  escapes,
4621         which can match different numbers of bytes, are also not permitted.         which can match different numbers of bytes, are also not permitted.
4622    
4623         Possessive quantifiers can  be  used  in  conjunction  with  lookbehind         "Subroutine"  calls  (see below) such as (?2) or (?&X) are permitted in
4624         assertions  to  specify  efficient  matching  at the end of the subject         lookbehinds, as long as the subpattern matches a  fixed-length  string.
4625         string. Consider a simple pattern such as         Recursion, however, is not supported.
4626    
4627           Possessive  quantifiers  can  be  used  in  conjunction with lookbehind
4628           assertions to specify efficient matching of fixed-length strings at the
4629           end of subject strings. Consider a simple pattern such as
4630    
4631           abcd$           abcd$
4632    
4633         when applied to a long string that does  not  match.  Because  matching         when  applied  to  a  long string that does not match. Because matching
4634         proceeds from left to right, PCRE will look for each "a" in the subject         proceeds from left to right, PCRE will look for each "a" in the subject
4635         and then see if what follows matches the rest of the  pattern.  If  the         and  then  see  if what follows matches the rest of the pattern. If the
4636         pattern is specified as         pattern is specified as
4637    
4638           ^.*abcd$           ^.*abcd$
4639    
4640         the  initial .* matches the entire string at first, but when this fails         the initial .* matches the entire string at first, but when this  fails
4641         (because there is no following "a"), it backtracks to match all but the         (because there is no following "a"), it backtracks to match all but the
4642         last  character,  then all but the last two characters, and so on. Once         last character, then all but the last two characters, and so  on.  Once
4643         again the search for "a" covers the entire string, from right to  left,         again  the search for "a" covers the entire string, from right to left,
4644         so we are no better off. However, if the pattern is written as         so we are no better off. However, if the pattern is written as
4645    
4646           ^.*+(?<=abcd)           ^.*+(?<=abcd)
4647    
4648         there  can  be  no backtracking for the .*+ item; it can match only the         there can be no backtracking for the .*+ item; it can  match  only  the
4649         entire string. The subsequent lookbehind assertion does a  single  test         entire  string.  The subsequent lookbehind assertion does a single test
4650         on  the last four characters. If it fails, the match fails immediately.         on the last four characters. If it fails, the match fails  immediately.
4651         For long strings, this approach makes a significant difference  to  the         For  long  strings, this approach makes a significant difference to the
4652         processing time.         processing time.
4653    
4654     Using multiple assertions     Using multiple assertions
# Line 4457  ASSERTIONS Line 4657  ASSERTIONS
4657    
4658           (?<=\d{3})(?<!999)foo           (?<=\d{3})(?<!999)foo
4659    
4660         matches  "foo" preceded by three digits that are not "999". Notice that         matches "foo" preceded by three digits that are not "999". Notice  that
4661         each of the assertions is applied independently at the  same  point  in         each  of  the  assertions is applied independently at the same point in
4662         the  subject  string.  First  there  is a check that the previous three         the subject string. First there is a  check  that  the  previous  three
4663         characters are all digits, and then there is  a  check  that  the  same         characters  are  all  digits,  and  then there is a check that the same
4664         three characters are not "999".  This pattern does not match "foo" pre-         three characters are not "999".  This pattern does not match "foo" pre-
4665         ceded by six characters, the first of which are  digits  and  the  last         ceded  by  six  characters,  the first of which are digits and the last
4666         three  of  which  are not "999". For example, it doesn't match "123abc-         three of which are not "999". For example, it  doesn't  match  "123abc-
4667         foo". A pattern to do that is         foo". A pattern to do that is
4668    
4669           (?<=\d{3}...)(?<!999)foo           (?<=\d{3}...)(?<!999)foo
4670    
4671         This time the first assertion looks at the  preceding  six  characters,         This  time  the  first assertion looks at the preceding six characters,
4672         checking that the first three are digits, and then the second assertion         checking that the first three are digits, and then the second assertion
4673         checks that the preceding three characters are not "999".         checks that the preceding three characters are not "999".
4674    
# Line 4476  ASSERTIONS Line 4676  ASSERTIONS
4676    
4677           (?<=(?<!foo)bar)baz           (?<=(?<!foo)bar)baz
4678    
4679         matches an occurrence of "baz" that is preceded by "bar" which in  turn         matches  an occurrence of "baz" that is preceded by "bar" which in turn
4680         is not preceded by "foo", while         is not preceded by "foo", while
4681    
4682           (?<=\d{3}(?!999)...)foo           (?<=\d{3}(?!999)...)foo
4683    
4684         is  another pattern that matches "foo" preceded by three digits and any         is another pattern that matches "foo" preceded by three digits and  any
4685         three characters that are not "999".         three characters that are not "999".
4686    
4687    
4688  CONDITIONAL SUBPATTERNS  CONDITIONAL SUBPATTERNS
4689    
4690         It is possible to cause the matching process to obey a subpattern  con-         It  is possible to cause the matching process to obey a subpattern con-
4691         ditionally  or to choose between two alternative subpatterns, depending         ditionally or to choose between two alternative subpatterns,  depending
4692         on the result of an assertion, or whether a previous capturing  subpat-         on  the result of an assertion, or whether a specific capturing subpat-
4693         tern  matched  or not. The two possible forms of conditional subpattern         tern has already been matched. The two possible  forms  of  conditional
4694         are         subpattern are:
4695    
4696           (?(condition)yes-pattern)           (?(condition)yes-pattern)
4697           (?(condition)yes-pattern|no-pattern)           (?(condition)yes-pattern|no-pattern)
4698    
4699         If the condition is satisfied, the yes-pattern is used;  otherwise  the         If  the  condition is satisfied, the yes-pattern is used; otherwise the
4700         no-pattern  (if  present)  is used. If there are more than two alterna-         no-pattern (if present) is used. If there are more  than  two  alterna-
4701         tives in the subpattern, a compile-time error occurs.         tives in the subpattern, a compile-time error occurs.
4702    
4703         There are four kinds of condition: references  to  subpatterns,  refer-         There  are  four  kinds of condition: references to subpatterns, refer-
4704         ences to recursion, a pseudo-condition called DEFINE, and assertions.         ences to recursion, a pseudo-condition called DEFINE, and assertions.
4705    
4706     Checking for a used subpattern by number     Checking for a used subpattern by number
4707    
4708         If  the  text between the parentheses consists of a sequence of digits,         If the text between the parentheses consists of a sequence  of  digits,
4709         the condition is true if the capturing subpattern of  that  number  has         the condition is true if a capturing subpattern of that number has pre-
4710         previously  matched.  An  alternative notation is to precede the digits         viously matched. If there is more than one  capturing  subpattern  with
4711         with a plus or minus sign. In this case, the subpattern number is rela-         the  same  number  (see  the earlier section about duplicate subpattern
4712         tive rather than absolute.  The most recently opened parentheses can be         numbers), the condition is true if any of them have been set. An alter-
4713         referenced by (?(-1), the next most recent by (?(-2),  and  so  on.  In         native  notation is to precede the digits with a plus or minus sign. In
4714         looping constructs it can also make sense to refer to subsequent groups         this case, the subpattern number is relative rather than absolute.  The
4715         with constructs such as (?(+2).         most  recently opened parentheses can be referenced by (?(-1), the next
4716           most recent by (?(-2), and so on. In looping  constructs  it  can  also
4717           make  sense  to  refer  to  subsequent  groups  with constructs such as
4718           (?(+2).
4719    
4720         Consider the following pattern, which  contains  non-significant  white         Consider the following pattern, which  contains  non-significant  white
4721         space to make it more readable (assume the PCRE_EXTENDED option) and to         space to make it more readable (assume the PCRE_EXTENDED option) and to
# Line 4555  CONDITIONAL SUBPATTERNS Line 4758  CONDITIONAL SUBPATTERNS
4758    
4759           (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )           (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )
4760    
4761           If  the  name used in a condition of this kind is a duplicate, the test
4762           is applied to all subpatterns of the same name, and is true if any  one
4763           of them has matched.
4764    
4765     Checking for pattern recursion     Checking for pattern recursion
4766    
# Line 4565  CONDITIONAL SUBPATTERNS Line 4771  CONDITIONAL SUBPATTERNS
4771    
4772           (?(R3)...) or (?(R&name)...)           (?(R3)...) or (?(R&name)...)
4773    
4774         the condition is true if the most recent recursion is into the  subpat-         the condition is true if the most recent recursion is into a subpattern
4775         tern  whose  number or name is given. This condition does not check the         whose number or name is given. This condition does not check the entire
4776         entire recursion stack.         recursion stack. If the name used in a condition  of  this  kind  is  a
4777           duplicate, the test is applied to all subpatterns of the same name, and
4778           is true if any one of them is the most recent recursion.
4779    
4780         At "top level", all these recursion test conditions are  false.  Recur-         At "top level", all these recursion test  conditions  are  false.   The
4781         sive patterns are described below.         syntax for recursive patterns is described below.
4782    
4783     Defining subpatterns for use by reference only     Defining subpatterns for use by reference only
4784    
# Line 4590  CONDITIONAL SUBPATTERNS Line 4798  CONDITIONAL SUBPATTERNS
4798         group  named "byte" is defined. This matches an individual component of         group  named "byte" is defined. This matches an individual component of
4799         an IPv4 address (a number less than 256). When  matching  takes  place,         an IPv4 address (a number less than 256). When  matching  takes  place,
4800         this  part  of  the pattern is skipped because DEFINE acts like a false         this  part  of  the pattern is skipped because DEFINE acts like a false
4801         condition.         condition. The rest of the pattern uses references to the  named  group
4802           to  match the four dot-separated components of an IPv4 address, insist-
4803         The rest of the pattern uses references to the named group to match the         ing on a word boundary at each end.
        four  dot-separated  components of an IPv4 address, insisting on a word  
        boundary at each end.  
4804    
4805     Assertion conditions     Assertion conditions
4806    
# Line 4649  RECURSIVE PATTERNS Line 4855  RECURSIVE PATTERNS
4855         Obviously, PCRE cannot support the interpolation of Perl code. Instead,         Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4856         it supports special syntax for recursion of  the  entire  pattern,  and         it supports special syntax for recursion of  the  entire  pattern,  and
4857         also  for  individual  subpattern  recursion. After its introduction in         also  for  individual  subpattern  recursion. After its introduction in
4858         PCRE and Python, this kind of recursion was  introduced  into  Perl  at         PCRE and Python, this kind of  recursion  was  subsequently  introduced
4859         release 5.10.         into Perl at release 5.10.
4860    
4861         A  special  item  that consists of (? followed by a number greater than         A  special  item  that consists of (? followed by a number greater than
4862         zero and a closing parenthesis is a recursive call of the subpattern of         zero and a closing parenthesis is a recursive call of the subpattern of
# Line 4659  RECURSIVE PATTERNS Line 4865  RECURSIVE PATTERNS
4865         tion.)  The special item (?R) or (?0) is a recursive call of the entire         tion.)  The special item (?R) or (?0) is a recursive call of the entire
4866         regular expression.         regular expression.
4867    
4868         In PCRE (like Python, but unlike Perl), a recursive subpattern call  is         This PCRE pattern solves the nested  parentheses  problem  (assume  the
        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  
4869         PCRE_EXTENDED option is set so that white space is ignored):         PCRE_EXTENDED option is set so that white space is ignored):
4870    
4871           \( ( (?>[^()]+) | (?R) )* \)           \( ( [^()]++ | (?R) )* \)
4872    
4873         First it matches an opening parenthesis. Then it matches any number  of         First  it matches an opening parenthesis. Then it matches any number of
4874         substrings  which  can  either  be  a sequence of non-parentheses, or a         substrings which can either be a  sequence  of  non-parentheses,  or  a
4875         recursive match of the pattern itself (that is, a  correctly  parenthe-         recursive  match  of the pattern itself (that is, a correctly parenthe-
4876         sized substring).  Finally there is a closing parenthesis.         sized substring).  Finally there is a closing parenthesis. Note the use
4877           of a possessive quantifier to avoid backtracking into sequences of non-
4878           parentheses.
4879    
4880         If  this  were  part of a larger pattern, you would not want to recurse         If this were part of a larger pattern, you would not  want  to  recurse
4881         the entire pattern, so instead you could use this:         the entire pattern, so instead you could use this:
4882    
4883           ( \( ( (?>[^()]+) | (?1) )* \) )           ( \( ( [^()]++ | (?1) )* \) )
4884    
4885         We have put the pattern into parentheses, and caused the  recursion  to         We  have  put the pattern into parentheses, and caused the recursion to
4886         refer to them instead of the whole pattern.         refer to them instead of the whole pattern.
4887    
4888         In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be         In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
4889         tricky. This is made easier by the use of relative references. (A  Perl         tricky.  This  is made easier by the use of relative references (a Perl
4890         5.10  feature.)   Instead  of  (?1)  in the pattern above you can write         5.10 feature).  Instead of (?1) in the  pattern  above  you  can  write
4891         (?-2) to refer to the second most recently opened parentheses preceding         (?-2) to refer to the second most recently opened parentheses preceding
4892         the  recursion.  In  other  words,  a  negative number counts capturing         the recursion. In other  words,  a  negative  number  counts  capturing
4893         parentheses leftwards from the point at which it is encountered.         parentheses leftwards from the point at which it is encountered.
4894    
4895         It is also possible to refer to  subsequently  opened  parentheses,  by         It  is  also  possible  to refer to subsequently opened parentheses, by
4896         writing  references  such  as (?+2). However, these cannot be recursive         writing references such as (?+2). However, these  cannot  be  recursive
4897         because the reference is not inside the  parentheses  that  are  refer-         because  the  reference  is  not inside the parentheses that are refer-
4898         enced.  They  are  always  "subroutine" calls, as described in the next         enced. They are always "subroutine" calls, as  described  in  the  next
4899         section.         section.
4900    
4901         An alternative approach is to use named parentheses instead.  The  Perl         An  alternative  approach is to use named parentheses instead. The Perl
4902         syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also         syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also
4903         supported. We could rewrite the above example as follows:         supported. We could rewrite the above example as follows:
4904    
4905           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )           (?<pn> \( ( [^()]++ | (?&pn) )* \) )
4906    
4907         If there is more than one subpattern with the same name,  the  earliest         If  there  is more than one subpattern with the same name, the earliest
4908         one is used.         one is used.
4909    
4910         This  particular  example pattern that we have been looking at contains         This particular example pattern that we have been looking  at  contains
4911         nested unlimited repeats, and so the use of atomic grouping for  match-         nested unlimited repeats, and so the use of a possessive quantifier for
4912         ing  strings  of non-parentheses is important when applying the pattern         matching strings of non-parentheses is important when applying the pat-
4913         to strings that do not match. For example, when this pattern is applied         tern  to  strings  that do not match. For example, when this pattern is
4914         to         applied to
4915    
4916           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4917    
4918         it  yields "no match" quickly. However, if atomic grouping is not used,         it yields "no match" quickly. However, if a  possessive  quantifier  is
4919         the match runs for a very long time indeed because there  are  so  many         not  used, the match runs for a very long time indeed because there are
4920         different  ways  the  + and * repeats can carve up the subject, and all         so many different ways the + and * repeats can carve  up  the  subject,
4921         have to be tested before failure can be reported.         and all have to be tested before failure can be reported.
4922    
4923         At the end of a match, the values set for any capturing subpatterns are         At  the  end  of a match, the values of capturing parentheses are those
4924         those from the outermost level of the recursion at which the subpattern         from the outermost level. If you want to obtain intermediate values,  a
4925         value is set.  If you want to obtain  intermediate  values,  a  callout         callout  function can be used (see below and the pcrecallout documenta-
4926         function  can be used (see below and the pcrecallout documentation). If         tion). If the pattern above is matched against
        the pattern above is matched against  
4927    
4928           (ab(cd)ef)           (ab(cd)ef)
4929    
4930         the value for the capturing parentheses is  "ef",  which  is  the  last         the value for the inner capturing parentheses  (numbered  2)  is  "ef",
4931         value  taken  on at the top level. If additional parentheses are added,         which  is the last value taken on at the top level. If a capturing sub-
4932         giving         pattern is not matched at the top level, its final value is unset, even
4933           if it is (temporarily) set at a deeper level.
4934           \( ( ( (?>[^()]+) | (?R) )* ) \)  
4935              ^                        ^         If  there are more than 15 capturing parentheses in a pattern, PCRE has
4936              ^                        ^         to obtain extra memory to store data during a recursion, which it  does
4937           by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
4938         the string they capture is "ab(cd)ef", the contents of  the  top  level         can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
4939         parentheses.  If there are more than 15 capturing parentheses in a pat-  
4940         tern, PCRE has to obtain extra memory to store data during a recursion,         Do not confuse the (?R) item with the condition (R),  which  tests  for
4941         which  it  does  by  using pcre_malloc, freeing it via pcre_free after-         recursion.   Consider  this pattern, which matches text in angle brack-
4942         wards. If  no  memory  can  be  obtained,  the  match  fails  with  the         ets, allowing for arbitrary nesting. Only digits are allowed in  nested
4943         PCRE_ERROR_NOMEMORY error.         brackets  (that is, when recursing), whereas any characters are permit-
   
        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-  
4944         ted at the outer level.         ted at the outer level.
4945    
4946           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
4947    
4948         In  this  pattern, (?(R) is the start of a conditional subpattern, with         In this pattern, (?(R) is the start of a conditional  subpattern,  with
4949         two different alternatives for the recursive and  non-recursive  cases.         two  different  alternatives for the recursive and non-recursive cases.
4950         The (?R) item is the actual recursive call.         The (?R) item is the actual recursive call.
4951    
4952       Recursion difference from Perl
4953    
4954           In PCRE (like Python, but unlike Perl), a recursive subpattern call  is
4955           always treated as an atomic group. That is, once it has matched some of
4956           the subject string, it is never re-entered, even if it contains untried
4957           alternatives  and  there  is a subsequent matching failure. This can be
4958           illustrated by the following pattern, which purports to match a  palin-
4959           dromic  string  that contains an odd number of characters (for example,
4960           "a", "aba", "abcba", "abcdcba"):
4961    
4962             ^(.|(.)(?1)\2)$
4963    
4964           The idea is that it either matches a single character, or two identical
4965           characters  surrounding  a sub-palindrome. In Perl, this pattern works;
4966           in PCRE it does not if the pattern is  longer  than  three  characters.
4967           Consider the subject string "abcba":
4968    
4969           At  the  top level, the first character is matched, but as it is not at
4970           the end of the string, the first alternative fails; the second alterna-
4971           tive is taken and the recursion kicks in. The recursive call to subpat-
4972           tern 1 successfully matches the next character ("b").  (Note  that  the
4973           beginning and end of line tests are not part of the recursion).
4974    
4975           Back  at  the top level, the next character ("c") is compared with what
4976           subpattern 2 matched, which was "a". This fails. Because the  recursion
4977           is  treated  as  an atomic group, there are now no backtracking points,
4978           and so the entire match fails. (Perl is able, at  this  point,  to  re-
4979           enter  the  recursion  and try the second alternative.) However, if the
4980           pattern is written with the alternatives in the other order, things are
4981           different:
4982    
4983             ^((.)(?1)\2|.)$
4984    
4985           This  time,  the recursing alternative is tried first, and continues to
4986           recurse until it runs out of characters, at which point  the  recursion
4987           fails.  But  this  time  we  do  have another alternative to try at the
4988           higher level. That is the big difference:  in  the  previous  case  the
4989           remaining alternative is at a deeper recursion level, which PCRE cannot
4990           use.
4991    
4992           To change the pattern so that matches all palindromic strings, not just
4993           those  with  an  odd number of characters, it is tempting to change the
4994           pattern to this:
4995    
4996             ^((.)(?1)\2|.?)$
4997    
4998           Again, this works in Perl, but not in PCRE, and for  the  same  reason.
4999           When  a  deeper  recursion has matched a single character, it cannot be
5000           entered again in order to match an empty string.  The  solution  is  to
5001           separate  the two cases, and write out the odd and even cases as alter-
5002           natives at the higher level:
5003    
5004             ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
5005    
5006           If you want to match typical palindromic phrases, the  pattern  has  to
5007           ignore all non-word characters, which can be done like this:
5008    
5009             ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
5010    
5011           If run with the PCRE_CASELESS option, this pattern matches phrases such
5012           as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
5013           Perl.  Note the use of the possessive quantifier *+ to avoid backtrack-
5014           ing into sequences of non-word characters. Without this, PCRE  takes  a
5015           great  deal  longer  (ten  times or more) to match typical phrases, and
5016           Perl takes so long that you think it has gone into a loop.
5017    
5018           WARNING: The palindrome-matching patterns above work only if  the  sub-
5019           ject  string  does not start with a palindrome that is shorter than the
5020           entire string.  For example, although "abcba" is correctly matched,  if
5021           the  subject  is "ababa", PCRE finds the palindrome "aba" at the start,
5022           then fails at top level because the end of the string does not  follow.
5023           Once  again, it cannot jump back into the recursion to try other alter-
5024           natives, so the entire match fails.
5025    
5026    
5027  SUBPATTERNS AS SUBROUTINES  SUBPATTERNS AS SUBROUTINES
5028    
5029         If the syntax for a recursive subpattern reference (either by number or         If the syntax for a recursive subpattern reference (either by number or
5030         by name) is used outside the parentheses to which it refers,  it  oper-         by  name)  is used outside the parentheses to which it refers, it oper-
5031         ates  like a subroutine in a programming language. The "called" subpat-