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1    -----------------------------------------------------------------------------
2    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
4    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
6    separate text files for the pcregrep and pcretest commands.
7    -----------------------------------------------------------------------------
8    
9    
10    PCRE(3)                                                                PCRE(3)
11    
12    
13    NAME
14           PCRE - Perl-compatible regular expressions
15    
16    
17    INTRODUCTION
18    
19           The  PCRE  library is a set of functions that implement regular expres-
20           sion pattern matching using the same syntax and semantics as Perl, with
21           just  a  few differences. (Certain features that appeared in Python and
22           PCRE before they appeared in Perl are also available using  the  Python
23           syntax.)
24    
25           The  current  implementation of PCRE (release 7.x) corresponds approxi-
26           mately with Perl 5.10, including support for UTF-8 encoded strings  and
27           Unicode general category properties. However, UTF-8 and Unicode support
28           has to be explicitly enabled; it is not the default. The Unicode tables
29           correspond to Unicode release 5.0.0.
30    
31           In  addition to the Perl-compatible matching function, PCRE contains an
32           alternative matching function that matches the same  compiled  patterns
33           in  a different way. In certain circumstances, the alternative function
34           has some advantages. For a discussion of the two  matching  algorithms,
35           see the pcrematching page.
36    
37           PCRE  is  written  in C and released as a C library. A number of people
38           have written wrappers and interfaces of various kinds.  In  particular,
39           Google  Inc.   have  provided  a comprehensive C++ wrapper. This is now
40           included as part of the PCRE distribution. The pcrecpp page has details
41           of  this  interface.  Other  people's contributions can be found in the
42           Contrib directory at the primary FTP site, which is:
43    
44           ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45    
46           Details of exactly which Perl regular expression features are  and  are
47           not supported by PCRE are given in separate documents. See the pcrepat-
48           tern and pcrecompat pages. There is a syntax summary in the  pcresyntax
49           page.
50    
51           Some  features  of  PCRE can be included, excluded, or changed when the
52           library is built. The pcre_config() function makes it  possible  for  a
53           client  to  discover  which  features are available. The features them-
54           selves are described in the pcrebuild page. Documentation about  build-
55           ing  PCRE for various operating systems can be found in the README file
56           in the source distribution.
57    
58           The library contains a number of undocumented  internal  functions  and
59           data  tables  that  are  used by more than one of the exported external
60           functions, but which are not intended  for  use  by  external  callers.
61           Their  names  all begin with "_pcre_", which hopefully will not provoke
62           any name clashes. In some environments, it is possible to control which
63           external  symbols  are  exported when a shared library is built, and in
64           these cases the undocumented symbols are not exported.
65    
66    
67    USER DOCUMENTATION
68    
69           The user documentation for PCRE comprises a number  of  different  sec-
70           tions.  In the "man" format, each of these is a separate "man page". In
71           the HTML format, each is a separate page, linked from the  index  page.
72           In  the  plain text format, all the sections are concatenated, for ease
73           of searching. The sections are as follows:
74    
75             pcre              this document
76             pcre-config       show PCRE installation configuration information
77             pcreapi           details of PCRE's native C API
78             pcrebuild         options for building PCRE
79             pcrecallout       details of the callout feature
80             pcrecompat        discussion of Perl compatibility
81             pcrecpp           details of the C++ wrapper
82             pcregrep          description of the pcregrep command
83             pcrematching      discussion of the two matching algorithms
84             pcrepartial       details of the partial matching facility
85             pcrepattern       syntax and semantics of supported
86                                 regular expressions
87             pcresyntax        quick syntax reference
88             pcreperform       discussion of performance issues
89             pcreposix         the POSIX-compatible C API
90             pcreprecompile    details of saving and re-using precompiled patterns
91             pcresample        discussion of the sample program
92             pcrestack         discussion of stack usage
93             pcretest          description of the pcretest testing command
94    
95           In  addition,  in the "man" and HTML formats, there is a short page for
96           each C library function, listing its arguments and results.
97    
98    
99    LIMITATIONS
100    
101           There are some size limitations in PCRE but it is hoped that they  will
102           never in practice be relevant.
103    
104           The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
105           is compiled with the default internal linkage size of 2. If you want to
106           process  regular  expressions  that are truly enormous, you can compile
107           PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
108           the  source  distribution and the pcrebuild documentation for details).
109           In these cases the limit is substantially larger.  However,  the  speed
110           of execution is slower.
111    
112           All values in repeating quantifiers must be less than 65536.
113    
114           There is no limit to the number of parenthesized subpatterns, but there
115           can be no more than 65535 capturing subpatterns.
116    
117           The maximum length of name for a named subpattern is 32 characters, and
118           the maximum number of named subpatterns is 10000.
119    
120           The  maximum  length of a subject string is the largest positive number
121           that an integer variable can hold. However, when using the  traditional
122           matching function, PCRE uses recursion to handle subpatterns and indef-
123           inite repetition.  This means that the available stack space may  limit
124           the size of a subject string that can be processed by certain patterns.
125           For a discussion of stack issues, see the pcrestack documentation.
126    
127    
128    UTF-8 AND UNICODE PROPERTY SUPPORT
129    
130           From release 3.3, PCRE has  had  some  support  for  character  strings
131           encoded  in the UTF-8 format. For release 4.0 this was greatly extended
132           to cover most common requirements, and in release 5.0  additional  sup-
133           port for Unicode general category properties was added.
134    
135           In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
136           support in the code, and, in addition,  you  must  call  pcre_compile()
137           with  the PCRE_UTF8 option flag. When you do this, both the pattern and
138           any subject strings that are matched against it are  treated  as  UTF-8
139           strings instead of just strings of bytes.
140    
141           If  you compile PCRE with UTF-8 support, but do not use it at run time,
142           the library will be a bit bigger, but the additional run time  overhead
143           is limited to testing the PCRE_UTF8 flag occasionally, so should not be
144           very big.
145    
146           If PCRE is built with Unicode character property support (which implies
147           UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
148           ported.  The available properties that can be tested are limited to the
149           general  category  properties such as Lu for an upper case letter or Nd
150           for a decimal number, the Unicode script names such as Arabic  or  Han,
151           and  the  derived  properties  Any  and L&. A full list is given in the
152           pcrepattern documentation. Only the short names for properties are sup-
153           ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
154           ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may
155           optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
156           does not support this.
157    
158       Validity of UTF-8 strings
159    
160           When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and
161           subjects are (by default) checked for validity on entry to the relevant
162           functions. From release 7.3 of PCRE, the check is according  the  rules
163           of  RFC  3629, which are themselves derived from the Unicode specifica-
164           tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which
165           allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current
166           check allows only values in the range U+0 to U+10FFFF, excluding U+D800
167           to U+DFFF.
168    
169           The  excluded  code  points are the "Low Surrogate Area" of Unicode, of
170           which the Unicode Standard says this: "The Low Surrogate Area does  not
171           contain  any  character  assignments,  consequently  no  character code
172           charts or namelists are provided for this area. Surrogates are reserved
173           for  use  with  UTF-16 and then must be used in pairs." The code points
174           that are encoded by UTF-16 pairs  are  available  as  independent  code
175           points  in  the  UTF-8  encoding.  (In other words, the whole surrogate
176           thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
177    
178           If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return
179           (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
180           that your strings are valid, and therefore want to skip these checks in
181           order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
182           compile time or at run time, PCRE assumes that the pattern  or  subject
183           it  is  given  (respectively)  contains only valid UTF-8 codes. In this
184           case, it does not diagnose an invalid UTF-8 string.
185    
186           If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,
187           what  happens  depends on why the string is invalid. If the string con-
188           forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
189           string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,
190           apart from the initial validity test, PCRE (when in UTF-8 mode) handles
191           strings  according  to  the more liberal rules of RFC 2279. However, if
192           the string does not even conform to RFC 2279, the result is  undefined.
193           Your program may crash.
194    
195           If  you  want  to  process  strings  of  values  in the full range 0 to
196           0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
197           set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
198           this situation, you will have to apply your own validity check.
199    
200       General comments about UTF-8 mode
201    
202           1. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
203           two-byte UTF-8 character if the value is greater than 127.
204    
205           2.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
206           characters for values greater than \177.
207    
208           3. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
209           vidual bytes, for example: \x{100}{3}.
210    
211           4.  The dot metacharacter matches one UTF-8 character instead of a sin-
212           gle byte.
213    
214           5. The escape sequence \C can be used to match a single byte  in  UTF-8
215           mode,  but  its  use can lead to some strange effects. This facility is
216           not available in the alternative matching function, pcre_dfa_exec().
217    
218           6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
219           test  characters of any code value, but the characters that PCRE recog-
220           nizes as digits, spaces, or word characters  remain  the  same  set  as
221           before, all with values less than 256. This remains true even when PCRE
222           includes Unicode property support, because to do otherwise  would  slow
223           down  PCRE in many common cases. If you really want to test for a wider
224           sense of, say, "digit", you must use Unicode  property  tests  such  as
225           \p{Nd}.
226    
227           7.  Similarly,  characters that match the POSIX named character classes
228           are all low-valued characters.
229    
230           8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
231           escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
232           acters.
233    
234           9. Case-insensitive matching applies only to  characters  whose  values
235           are  less than 128, unless PCRE is built with Unicode property support.
236           Even when Unicode property support is available, PCRE  still  uses  its
237           own  character  tables when checking the case of low-valued characters,
238           so as not to degrade performance.  The Unicode property information  is
239           used only for characters with higher values. Even when Unicode property
240           support is available, PCRE supports case-insensitive matching only when
241           there  is  a  one-to-one  mapping between a letter's cases. There are a
242           small number of many-to-one mappings in Unicode;  these  are  not  sup-
243           ported by PCRE.
244    
245    
246    AUTHOR
247    
248           Philip Hazel
249           University Computing Service
250           Cambridge CB2 3QH, England.
251    
252           Putting  an actual email address here seems to have been a spam magnet,
253           so I've taken it away. If you want to email me, use  my  two  initials,
254           followed by the two digits 10, at the domain cam.ac.uk.
255    
256    
257    REVISION
258    
259           Last updated: 09 August 2007
260           Copyright (c) 1997-2007 University of Cambridge.
261    ------------------------------------------------------------------------------
262    
263    
264    PCREBUILD(3)                                                      PCREBUILD(3)
265    
266    
267    NAME
268           PCRE - Perl-compatible regular expressions
269    
270    
271    PCRE BUILD-TIME OPTIONS
272    
273           This  document  describes  the  optional  features  of PCRE that can be
274           selected when the library is compiled. It assumes use of the  configure
275           script,  where the optional features are selected or deselected by pro-
276           viding options to configure before running the make  command.  However,
277           the  same  options  can be selected in both Unix-like and non-Unix-like
278           environments using the GUI facility of  CMakeSetup  if  you  are  using
279           CMake instead of configure to build PCRE.
280    
281           The complete list of options for configure (which includes the standard
282           ones such as the  selection  of  the  installation  directory)  can  be
283           obtained by running
284    
285             ./configure --help
286    
287           The  following  sections  include  descriptions  of options whose names
288           begin with --enable or --disable. These settings specify changes to the
289           defaults  for  the configure command. Because of the way that configure
290           works, --enable and --disable always come in pairs, so  the  complemen-
291           tary  option always exists as well, but as it specifies the default, it
292           is not described.
293    
294    
295    C++ SUPPORT
296    
297           By default, the configure script will search for a C++ compiler and C++
298           header files. If it finds them, it automatically builds the C++ wrapper
299           library for PCRE. You can disable this by adding
300    
301             --disable-cpp
302    
303           to the configure command.
304    
305    
306    UTF-8 SUPPORT
307    
308           To build PCRE with support for UTF-8 character strings, add
309    
310             --enable-utf8
311    
312           to the configure command. Of itself, this  does  not  make  PCRE  treat
313           strings  as UTF-8. As well as compiling PCRE with this option, you also
314           have have to set the PCRE_UTF8 option when you call the  pcre_compile()
315           function.
316    
317    
318    UNICODE CHARACTER PROPERTY SUPPORT
319    
320           UTF-8  support allows PCRE to process character values greater than 255
321           in the strings that it handles. On its own, however, it does  not  pro-
322           vide any facilities for accessing the properties of such characters. If
323           you want to be able to use the pattern escapes \P, \p,  and  \X,  which
324           refer to Unicode character properties, you must add
325    
326             --enable-unicode-properties
327    
328           to  the configure command. This implies UTF-8 support, even if you have
329           not explicitly requested it.
330    
331           Including Unicode property support adds around 30K  of  tables  to  the
332           PCRE  library.  Only  the general category properties such as Lu and Nd
333           are supported. Details are given in the pcrepattern documentation.
334    
335    
336    CODE VALUE OF NEWLINE
337    
338           By default, PCRE interprets character 10 (linefeed, LF)  as  indicating
339           the  end  of  a line. This is the normal newline character on Unix-like
340           systems. You can compile PCRE to use character 13 (carriage return, CR)
341           instead, by adding
342    
343             --enable-newline-is-cr
344    
345           to  the  configure  command.  There  is  also  a --enable-newline-is-lf
346           option, which explicitly specifies linefeed as the newline character.
347    
348           Alternatively, you can specify that line endings are to be indicated by
349           the two character sequence CRLF. If you want this, add
350    
351             --enable-newline-is-crlf
352    
353           to the configure command. There is a fourth option, specified by
354    
355             --enable-newline-is-anycrlf
356    
357           which  causes  PCRE  to recognize any of the three sequences CR, LF, or
358           CRLF as indicating a line ending. Finally, a fifth option, specified by
359    
360             --enable-newline-is-any
361    
362           causes PCRE to recognize any Unicode newline sequence.
363    
364           Whatever  line  ending convention is selected when PCRE is built can be
365           overridden when the library functions are called. At build time  it  is
366           conventional to use the standard for your operating system.
367    
368    
369    WHAT \R MATCHES
370    
371           By  default,  the  sequence \R in a pattern matches any Unicode newline
372           sequence, whatever has been selected as the line  ending  sequence.  If
373           you specify
374    
375             --enable-bsr-anycrlf
376    
377           the  default  is changed so that \R matches only CR, LF, or CRLF. What-
378           ever is selected when PCRE is built can be overridden when the  library
379           functions are called.
380    
381    
382    BUILDING SHARED AND STATIC LIBRARIES
383    
384           The  PCRE building process uses libtool to build both shared and static
385           Unix libraries by default. You can suppress one of these by adding  one
386           of
387    
388             --disable-shared
389             --disable-static
390    
391           to the configure command, as required.
392    
393    
394    POSIX MALLOC USAGE
395    
396           When PCRE is called through the POSIX interface (see the pcreposix doc-
397           umentation), additional working storage is  required  for  holding  the
398           pointers  to capturing substrings, because PCRE requires three integers
399           per substring, whereas the POSIX interface provides only  two.  If  the
400           number of expected substrings is small, the wrapper function uses space
401           on the stack, because this is faster than using malloc() for each call.
402           The default threshold above which the stack is no longer used is 10; it
403           can be changed by adding a setting such as
404    
405             --with-posix-malloc-threshold=20
406    
407           to the configure command.
408    
409    
410    HANDLING VERY LARGE PATTERNS
411    
412           Within a compiled pattern, offset values are used  to  point  from  one
413           part  to another (for example, from an opening parenthesis to an alter-
414           nation metacharacter). By default, two-byte values are used  for  these
415           offsets,  leading  to  a  maximum size for a compiled pattern of around
416           64K. This is sufficient to handle all but the most  gigantic  patterns.
417           Nevertheless,  some  people do want to process enormous patterns, so it
418           is possible to compile PCRE to use three-byte or four-byte  offsets  by
419           adding a setting such as
420    
421             --with-link-size=3
422    
423           to  the  configure  command.  The value given must be 2, 3, or 4. Using
424           longer offsets slows down the operation of PCRE because it has to  load
425           additional bytes when handling them.
426    
427    
428    AVOIDING EXCESSIVE STACK USAGE
429    
430           When matching with the pcre_exec() function, PCRE implements backtrack-
431           ing by making recursive calls to an internal function  called  match().
432           In  environments  where  the size of the stack is limited, this can se-
433           verely limit PCRE's operation. (The Unix environment does  not  usually
434           suffer from this problem, but it may sometimes be necessary to increase
435           the maximum stack size.  There is a discussion in the  pcrestack  docu-
436           mentation.)  An alternative approach to recursion that uses memory from
437           the heap to remember data, instead of using recursive  function  calls,
438           has  been  implemented to work round the problem of limited stack size.
439           If you want to build a version of PCRE that works this way, add
440    
441             --disable-stack-for-recursion
442    
443           to the configure command. With this configuration, PCRE  will  use  the
444           pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
445           ment functions. By default these point to malloc() and free(), but  you
446           can replace the pointers so that your own functions are used.
447    
448           Separate  functions  are  provided  rather  than  using pcre_malloc and
449           pcre_free because the  usage  is  very  predictable:  the  block  sizes
450           requested  are  always  the  same,  and  the blocks are always freed in
451           reverse order. A calling program might be able to  implement  optimized
452           functions  that  perform  better  than  malloc()  and free(). PCRE runs
453           noticeably more slowly when built in this way. This option affects only
454           the   pcre_exec()   function;   it   is   not   relevant  for  the  the
455           pcre_dfa_exec() function.
456    
457    
458    LIMITING PCRE RESOURCE USAGE
459    
460           Internally, PCRE has a function called match(), which it calls  repeat-
461           edly   (sometimes   recursively)  when  matching  a  pattern  with  the
462           pcre_exec() function. By controlling the maximum number of  times  this
463           function  may be called during a single matching operation, a limit can
464           be placed on the resources used by a single call  to  pcre_exec().  The
465           limit  can be changed at run time, as described in the pcreapi documen-
466           tation. The default is 10 million, but this can be changed by adding  a
467           setting such as
468    
469             --with-match-limit=500000
470    
471           to   the   configure  command.  This  setting  has  no  effect  on  the
472           pcre_dfa_exec() matching function.
473    
474           In some environments it is desirable to limit the  depth  of  recursive
475           calls of match() more strictly than the total number of calls, in order
476           to restrict the maximum amount of stack (or heap,  if  --disable-stack-
477           for-recursion is specified) that is used. A second limit controls this;
478           it defaults to the value that  is  set  for  --with-match-limit,  which
479           imposes  no  additional constraints. However, you can set a lower limit
480           by adding, for example,
481    
482             --with-match-limit-recursion=10000
483    
484           to the configure command. This value can  also  be  overridden  at  run
485           time.
486    
487    
488    CREATING CHARACTER TABLES AT BUILD TIME
489    
490           PCRE  uses fixed tables for processing characters whose code values are
491           less than 256. By default, PCRE is built with a set of tables that  are
492           distributed  in  the  file pcre_chartables.c.dist. These tables are for
493           ASCII codes only. If you add
494    
495             --enable-rebuild-chartables
496    
497           to the configure command, the distributed tables are  no  longer  used.
498           Instead,  a  program  called dftables is compiled and run. This outputs
499           the source for new set of tables, created in the default locale of your
500           C runtime system. (This method of replacing the tables does not work if
501           you are cross compiling, because dftables is run on the local host.  If
502           you  need  to  create alternative tables when cross compiling, you will
503           have to do so "by hand".)
504    
505    
506    USING EBCDIC CODE
507    
508           PCRE assumes by default that it will run in an  environment  where  the
509           character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
510           This is the case for most computer operating systems.  PCRE  can,  how-
511           ever, be compiled to run in an EBCDIC environment by adding
512    
513             --enable-ebcdic
514    
515           to the configure command. This setting implies --enable-rebuild-charta-
516           bles. You should only use it if you know that  you  are  in  an  EBCDIC
517           environment (for example, an IBM mainframe operating system).
518    
519    
520    PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
521    
522           By default, pcregrep reads all files as plain text. You can build it so
523           that it recognizes files whose names end in .gz or .bz2, and reads them
524           with libz or libbz2, respectively, by adding one or both of
525    
526             --enable-pcregrep-libz
527             --enable-pcregrep-libbz2
528    
529           to the configure command. These options naturally require that the rel-
530           evant libraries are installed on your system. Configuration  will  fail
531           if they are not.
532    
533    
534    PCRETEST OPTION FOR LIBREADLINE SUPPORT
535    
536           If you add
537    
538             --enable-pcretest-libreadline
539    
540           to  the  configure  command,  pcretest  is  linked with the libreadline
541           library, and when its input is from a terminal, it reads it  using  the
542           readline() function. This provides line-editing and history facilities.
543           Note that libreadline is GPL-licenced, so if you distribute a binary of
544           pcretest linked in this way, there may be licensing issues.
545    
546    
547    SEE ALSO
548    
549           pcreapi(3), pcre_config(3).
550    
551    
552    AUTHOR
553    
554           Philip Hazel
555           University Computing Service
556           Cambridge CB2 3QH, England.
557    
558    
559    REVISION
560    
561           Last updated: 18 December 2007
562           Copyright (c) 1997-2007 University of Cambridge.
563    ------------------------------------------------------------------------------
564    
565    
566    PCREMATCHING(3)                                                PCREMATCHING(3)
567    
568    
569    NAME
570           PCRE - Perl-compatible regular expressions
571    
572    
573    PCRE MATCHING ALGORITHMS
574    
575           This document describes the two different algorithms that are available
576           in PCRE for matching a compiled regular expression against a given sub-
577           ject  string.  The  "standard"  algorithm  is  the  one provided by the
578           pcre_exec() function.  This works in the same was  as  Perl's  matching
579           function, and provides a Perl-compatible matching operation.
580    
581           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
582           this operates in a different way, and is not  Perl-compatible.  It  has
583           advantages  and disadvantages compared with the standard algorithm, and
584           these are described below.
585    
586           When there is only one possible way in which a given subject string can
587           match  a pattern, the two algorithms give the same answer. A difference
588           arises, however, when there are multiple possibilities. For example, if
589           the pattern
590    
591             ^<.*>
592    
593           is matched against the string
594    
595             <something> <something else> <something further>
596    
597           there are three possible answers. The standard algorithm finds only one
598           of them, whereas the alternative algorithm finds all three.
599    
600    
601    REGULAR EXPRESSIONS AS TREES
602    
603           The set of strings that are matched by a regular expression can be rep-
604           resented  as  a  tree structure. An unlimited repetition in the pattern
605           makes the tree of infinite size, but it is still a tree.  Matching  the
606           pattern  to a given subject string (from a given starting point) can be
607           thought of as a search of the tree.  There are two  ways  to  search  a
608           tree:  depth-first  and  breadth-first, and these correspond to the two
609           matching algorithms provided by PCRE.
610    
611    
612    THE STANDARD MATCHING ALGORITHM
613    
614           In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
615           sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
616           depth-first search of the pattern tree. That is, it  proceeds  along  a
617           single path through the tree, checking that the subject matches what is
618           required. When there is a mismatch, the algorithm  tries  any  alterna-
619           tives  at  the  current point, and if they all fail, it backs up to the
620           previous branch point in the  tree,  and  tries  the  next  alternative
621           branch  at  that  level.  This often involves backing up (moving to the
622           left) in the subject string as well.  The  order  in  which  repetition
623           branches  are  tried  is controlled by the greedy or ungreedy nature of
624           the quantifier.
625    
626           If a leaf node is reached, a matching string has  been  found,  and  at
627           that  point the algorithm stops. Thus, if there is more than one possi-
628           ble match, this algorithm returns the first one that it finds.  Whether
629           this  is the shortest, the longest, or some intermediate length depends
630           on the way the greedy and ungreedy repetition quantifiers are specified
631           in the pattern.
632    
633           Because  it  ends  up  with a single path through the tree, it is rela-
634           tively straightforward for this algorithm to keep  track  of  the  sub-
635           strings  that  are  matched  by portions of the pattern in parentheses.
636           This provides support for capturing parentheses and back references.
637    
638    
639    THE ALTERNATIVE MATCHING ALGORITHM
640    
641           This algorithm conducts a breadth-first search of  the  tree.  Starting
642           from  the  first  matching  point  in the subject, it scans the subject
643           string from left to right, once, character by character, and as it does
644           this,  it remembers all the paths through the tree that represent valid
645           matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
646           though  it is not implemented as a traditional finite state machine (it
647           keeps multiple states active simultaneously).
648    
649           The scan continues until either the end of the subject is  reached,  or
650           there  are  no more unterminated paths. At this point, terminated paths
651           represent the different matching possibilities (if there are none,  the
652           match  has  failed).   Thus,  if there is more than one possible match,
653           this algorithm finds all of them, and in particular, it finds the long-
654           est.  In PCRE, there is an option to stop the algorithm after the first
655           match (which is necessarily the shortest) has been found.
656    
657           Note that all the matches that are found start at the same point in the
658           subject. If the pattern
659    
660             cat(er(pillar)?)
661    
662           is  matched  against the string "the caterpillar catchment", the result
663           will be the three strings "cat", "cater", and "caterpillar" that  start
664           at the fourth character of the subject. The algorithm does not automat-
665           ically move on to find matches that start at later positions.
666    
667           There are a number of features of PCRE regular expressions that are not
668           supported by the alternative matching algorithm. They are as follows:
669    
670           1.  Because  the  algorithm  finds  all possible matches, the greedy or
671           ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
672           ungreedy quantifiers are treated in exactly the same way. However, pos-
673           sessive quantifiers can make a difference when what follows could  also
674           match what is quantified, for example in a pattern like this:
675    
676             ^a++\w!
677    
678           This  pattern matches "aaab!" but not "aaa!", which would be matched by
679           a non-possessive quantifier. Similarly, if an atomic group is  present,
680           it  is matched as if it were a standalone pattern at the current point,
681           and the longest match is then "locked in" for the rest of  the  overall
682           pattern.
683    
684           2. When dealing with multiple paths through the tree simultaneously, it
685           is not straightforward to keep track of  captured  substrings  for  the
686           different  matching  possibilities,  and  PCRE's implementation of this
687           algorithm does not attempt to do this. This means that no captured sub-
688           strings are available.
689    
690           3.  Because no substrings are captured, back references within the pat-
691           tern are not supported, and cause errors if encountered.
692    
693           4. For the same reason, conditional expressions that use  a  backrefer-
694           ence  as  the  condition or test for a specific group recursion are not
695           supported.
696    
697           5. Because many paths through the tree may be  active,  the  \K  escape
698           sequence, which resets the start of the match when encountered (but may
699           be on some paths and not on others), is not  supported.  It  causes  an
700           error if encountered.
701    
702           6.  Callouts  are  supported, but the value of the capture_top field is
703           always 1, and the value of the capture_last field is always -1.
704    
705           7. The \C escape sequence, which (in the standard algorithm) matches  a
706           single  byte, even in UTF-8 mode, is not supported because the alterna-
707           tive algorithm moves through the subject  string  one  character  at  a
708           time, for all active paths through the tree.
709    
710           8.  None  of  the  backtracking control verbs such as (*PRUNE) are sup-
711           ported.
712    
713    
714    ADVANTAGES OF THE ALTERNATIVE ALGORITHM
715    
716           Using the alternative matching algorithm provides the following  advan-
717           tages:
718    
719           1. All possible matches (at a single point in the subject) are automat-
720           ically found, and in particular, the longest match is  found.  To  find
721           more than one match using the standard algorithm, you have to do kludgy
722           things with callouts.
723    
724           2. There is much better support for partial matching. The  restrictions
725           on  the content of the pattern that apply when using the standard algo-
726           rithm for partial matching do not apply to the  alternative  algorithm.
727           For  non-anchored patterns, the starting position of a partial match is
728           available.
729    
730           3. Because the alternative algorithm  scans  the  subject  string  just
731           once,  and  never  needs to backtrack, it is possible to pass very long
732           subject strings to the matching function in  several  pieces,  checking
733           for partial matching each time.
734    
735    
736    DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
737    
738           The alternative algorithm suffers from a number of disadvantages:
739    
740           1.  It  is  substantially  slower  than the standard algorithm. This is
741           partly because it has to search for all possible matches, but  is  also
742           because it is less susceptible to optimization.
743    
744           2. Capturing parentheses and back references are not supported.
745    
746           3. Although atomic groups are supported, their use does not provide the
747           performance advantage that it does for the standard algorithm.
748    
749    
750    AUTHOR
751    
752           Philip Hazel
753           University Computing Service
754           Cambridge CB2 3QH, England.
755    
756    
757    REVISION
758    
759           Last updated: 08 August 2007
760           Copyright (c) 1997-2007 University of Cambridge.
761    ------------------------------------------------------------------------------
762    
763    
764    PCREAPI(3)                                                          PCREAPI(3)
765    
766    
767    NAME
768           PCRE - Perl-compatible regular expressions
769    
770    
771    PCRE NATIVE API
772    
773           #include <pcre.h>
774    
775           pcre *pcre_compile(const char *pattern, int options,
776                const char **errptr, int *erroffset,
777                const unsigned char *tableptr);
778    
779           pcre *pcre_compile2(const char *pattern, int options,
780                int *errorcodeptr,
781                const char **errptr, int *erroffset,
782                const unsigned char *tableptr);
783    
784           pcre_extra *pcre_study(const pcre *code, int options,
785                const char **errptr);
786    
787           int pcre_exec(const pcre *code, const pcre_extra *extra,
788                const char *subject, int length, int startoffset,
789                int options, int *ovector, int ovecsize);
790    
791           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
792                const char *subject, int length, int startoffset,
793                int options, int *ovector, int ovecsize,
794                int *workspace, int wscount);
795    
796           int pcre_copy_named_substring(const pcre *code,
797                const char *subject, int *ovector,
798                int stringcount, const char *stringname,
799                char *buffer, int buffersize);
800    
801           int pcre_copy_substring(const char *subject, int *ovector,
802                int stringcount, int stringnumber, char *buffer,
803                int buffersize);
804    
805           int pcre_get_named_substring(const pcre *code,
806                const char *subject, int *ovector,
807                int stringcount, const char *stringname,
808                const char **stringptr);
809    
810           int pcre_get_stringnumber(const pcre *code,
811                const char *name);
812    
813           int pcre_get_stringtable_entries(const pcre *code,
814                const char *name, char **first, char **last);
815    
816           int pcre_get_substring(const char *subject, int *ovector,
817                int stringcount, int stringnumber,
818                const char **stringptr);
819    
820           int pcre_get_substring_list(const char *subject,
821                int *ovector, int stringcount, const char ***listptr);
822    
823           void pcre_free_substring(const char *stringptr);
824    
825           void pcre_free_substring_list(const char **stringptr);
826    
827           const unsigned char *pcre_maketables(void);
828    
829           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
830                int what, void *where);
831    
832           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
833    
834           int pcre_refcount(pcre *code, int adjust);
835    
836           int pcre_config(int what, void *where);
837    
838           char *pcre_version(void);
839    
840           void *(*pcre_malloc)(size_t);
841    
842           void (*pcre_free)(void *);
843    
844           void *(*pcre_stack_malloc)(size_t);
845    
846           void (*pcre_stack_free)(void *);
847    
848           int (*pcre_callout)(pcre_callout_block *);
849    
850    
851    PCRE API OVERVIEW
852    
853           PCRE has its own native API, which is described in this document. There
854           are also some wrapper functions that correspond to  the  POSIX  regular
855           expression  API.  These  are  described in the pcreposix documentation.
856           Both of these APIs define a set of C function calls. A C++  wrapper  is
857           distributed with PCRE. It is documented in the pcrecpp page.
858    
859           The  native  API  C  function prototypes are defined in the header file
860           pcre.h, and on Unix systems the library itself is called  libpcre.   It
861           can normally be accessed by adding -lpcre to the command for linking an
862           application  that  uses  PCRE.  The  header  file  defines  the  macros
863           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
864           bers for the library.  Applications can use these  to  include  support
865           for different releases of PCRE.
866    
867           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
868           pcre_exec() are used for compiling and matching regular expressions  in
869           a  Perl-compatible  manner. A sample program that demonstrates the sim-
870           plest way of using them is provided in the file  called  pcredemo.c  in
871           the  source distribution. The pcresample documentation describes how to
872           run it.
873    
874           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
875           ble,  is  also provided. This uses a different algorithm for the match-
876           ing. The alternative algorithm finds all possible matches (at  a  given
877           point  in  the subject), and scans the subject just once. However, this
878           algorithm does not return captured substrings. A description of the two
879           matching  algorithms and their advantages and disadvantages is given in
880           the pcrematching documentation.
881    
882           In addition to the main compiling and  matching  functions,  there  are
883           convenience functions for extracting captured substrings from a subject
884           string that is matched by pcre_exec(). They are:
885    
886             pcre_copy_substring()
887             pcre_copy_named_substring()
888             pcre_get_substring()
889             pcre_get_named_substring()
890             pcre_get_substring_list()
891             pcre_get_stringnumber()
892             pcre_get_stringtable_entries()
893    
894           pcre_free_substring() and pcre_free_substring_list() are also provided,
895           to free the memory used for extracted strings.
896    
897           The  function  pcre_maketables()  is  used  to build a set of character
898           tables  in  the  current  locale   for   passing   to   pcre_compile(),
899           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
900           provided for specialist use.  Most  commonly,  no  special  tables  are
901           passed,  in  which case internal tables that are generated when PCRE is
902           built are used.
903    
904           The function pcre_fullinfo() is used to find out  information  about  a
905           compiled  pattern; pcre_info() is an obsolete version that returns only
906           some of the available information, but is retained for  backwards  com-
907           patibility.   The function pcre_version() returns a pointer to a string
908           containing the version of PCRE and its date of release.
909    
910           The function pcre_refcount() maintains a  reference  count  in  a  data
911           block  containing  a compiled pattern. This is provided for the benefit
912           of object-oriented applications.
913    
914           The global variables pcre_malloc and pcre_free  initially  contain  the
915           entry  points  of  the  standard malloc() and free() functions, respec-
916           tively. PCRE calls the memory management functions via these variables,
917           so  a  calling  program  can replace them if it wishes to intercept the
918           calls. This should be done before calling any PCRE functions.
919    
920           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
921           indirections  to  memory  management functions. These special functions
922           are used only when PCRE is compiled to use  the  heap  for  remembering
923           data, instead of recursive function calls, when running the pcre_exec()
924           function. See the pcrebuild documentation for  details  of  how  to  do
925           this.  It  is  a non-standard way of building PCRE, for use in environ-
926           ments that have limited stacks. Because of the greater  use  of  memory
927           management,  it  runs  more  slowly. Separate functions are provided so
928           that special-purpose external code can be  used  for  this  case.  When
929           used,  these  functions  are always called in a stack-like manner (last
930           obtained, first freed), and always for memory blocks of the same  size.
931           There  is  a discussion about PCRE's stack usage in the pcrestack docu-
932           mentation.
933    
934           The global variable pcre_callout initially contains NULL. It can be set
935           by  the  caller  to  a "callout" function, which PCRE will then call at
936           specified points during a matching operation. Details are given in  the
937           pcrecallout documentation.
938    
939    
940    NEWLINES
941    
942           PCRE  supports five different conventions for indicating line breaks in
943           strings: a single CR (carriage return) character, a  single  LF  (line-
944           feed) character, the two-character sequence CRLF, any of the three pre-
945           ceding, or any Unicode newline sequence. The Unicode newline  sequences
946           are  the  three just mentioned, plus the single characters VT (vertical
947           tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
948           separator, U+2028), and PS (paragraph separator, U+2029).
949    
950           Each  of  the first three conventions is used by at least one operating
951           system as its standard newline sequence. When PCRE is built, a  default
952           can  be  specified.  The default default is LF, which is the Unix stan-
953           dard. When PCRE is run, the default can be overridden,  either  when  a
954           pattern is compiled, or when it is matched.
955    
956           At compile time, the newline convention can be specified by the options
957           argument of pcre_compile(), or it can be specified by special  text  at
958           the start of the pattern itself; this overrides any other settings. See
959           the pcrepattern page for details of the special character sequences.
960    
961           In the PCRE documentation the word "newline" is used to mean "the char-
962           acter  or pair of characters that indicate a line break". The choice of
963           newline convention affects the handling of  the  dot,  circumflex,  and
964           dollar metacharacters, the handling of #-comments in /x mode, and, when
965           CRLF is a recognized line ending sequence, the match position  advance-
966           ment for a non-anchored pattern. There is more detail about this in the
967           section on pcre_exec() options below.
968    
969           The choice of newline convention does not affect the interpretation  of
970           the  \n  or  \r  escape  sequences, nor does it affect what \R matches,
971           which is controlled in a similar way, but by separate options.
972    
973    
974    MULTITHREADING
975    
976           The PCRE functions can be used in  multi-threading  applications,  with
977           the  proviso  that  the  memory  management  functions  pointed  to  by
978           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
979           callout function pointed to by pcre_callout, are shared by all threads.
980    
981           The compiled form of a regular expression is not altered during  match-
982           ing, so the same compiled pattern can safely be used by several threads
983           at once.
984    
985    
986    SAVING PRECOMPILED PATTERNS FOR LATER USE
987    
988           The compiled form of a regular expression can be saved and re-used at a
989           later  time,  possibly by a different program, and even on a host other
990           than the one on which  it  was  compiled.  Details  are  given  in  the
991           pcreprecompile  documentation.  However, compiling a regular expression
992           with one version of PCRE for use with a different version is not  guar-
993           anteed to work and may cause crashes.
994    
995    
996    CHECKING BUILD-TIME OPTIONS
997    
998           int pcre_config(int what, void *where);
999    
1000           The  function pcre_config() makes it possible for a PCRE client to dis-
1001           cover which optional features have been compiled into the PCRE library.
1002           The  pcrebuild documentation has more details about these optional fea-
1003           tures.
1004    
1005           The first argument for pcre_config() is an  integer,  specifying  which
1006           information is required; the second argument is a pointer to a variable
1007           into which the information is  placed.  The  following  information  is
1008           available:
1009    
1010             PCRE_CONFIG_UTF8
1011    
1012           The  output is an integer that is set to one if UTF-8 support is avail-
1013           able; otherwise it is set to zero.
1014    
1015             PCRE_CONFIG_UNICODE_PROPERTIES
1016    
1017           The output is an integer that is set to  one  if  support  for  Unicode
1018           character properties is available; otherwise it is set to zero.
1019    
1020             PCRE_CONFIG_NEWLINE
1021    
1022           The  output  is  an integer whose value specifies the default character
1023           sequence that is recognized as meaning "newline". The four values  that
1024           are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1025           and -1 for ANY. The default should normally be  the  standard  sequence
1026           for your operating system.
1027    
1028             PCRE_CONFIG_BSR
1029    
1030           The output is an integer whose value indicates what character sequences
1031           the \R escape sequence matches by default. A value of 0 means  that  \R
1032           matches  any  Unicode  line ending sequence; a value of 1 means that \R
1033           matches only CR, LF, or CRLF. The default can be overridden when a pat-
1034           tern is compiled or matched.
1035    
1036             PCRE_CONFIG_LINK_SIZE
1037    
1038           The  output  is  an  integer that contains the number of bytes used for
1039           internal linkage in compiled regular expressions. The value is 2, 3, or
1040           4.  Larger  values  allow larger regular expressions to be compiled, at
1041           the expense of slower matching. The default value of  2  is  sufficient
1042           for  all  but  the  most massive patterns, since it allows the compiled
1043           pattern to be up to 64K in size.
1044    
1045             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1046    
1047           The output is an integer that contains the threshold  above  which  the
1048           POSIX  interface  uses malloc() for output vectors. Further details are
1049           given in the pcreposix documentation.
1050    
1051             PCRE_CONFIG_MATCH_LIMIT
1052    
1053           The output is an integer that gives the default limit for the number of
1054           internal  matching  function  calls in a pcre_exec() execution. Further
1055           details are given with pcre_exec() below.
1056    
1057             PCRE_CONFIG_MATCH_LIMIT_RECURSION
1058    
1059           The output is an integer that gives the default limit for the depth  of
1060           recursion  when calling the internal matching function in a pcre_exec()
1061           execution. Further details are given with pcre_exec() below.
1062    
1063             PCRE_CONFIG_STACKRECURSE
1064    
1065           The output is an integer that is set to one if internal recursion  when
1066           running pcre_exec() is implemented by recursive function calls that use
1067           the stack to remember their state. This is the usual way that  PCRE  is
1068           compiled. The output is zero if PCRE was compiled to use blocks of data
1069           on the  heap  instead  of  recursive  function  calls.  In  this  case,
1070           pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
1071           blocks on the heap, thus avoiding the use of the stack.
1072    
1073    
1074    COMPILING A PATTERN
1075    
1076           pcre *pcre_compile(const char *pattern, int options,
1077                const char **errptr, int *erroffset,
1078                const unsigned char *tableptr);
1079    
1080           pcre *pcre_compile2(const char *pattern, int options,
1081                int *errorcodeptr,
1082                const char **errptr, int *erroffset,
1083                const unsigned char *tableptr);
1084    
1085           Either of the functions pcre_compile() or pcre_compile2() can be called
1086           to compile a pattern into an internal form. The only difference between
1087           the two interfaces is that pcre_compile2() has an additional  argument,
1088           errorcodeptr, via which a numerical error code can be returned.
1089    
1090           The pattern is a C string terminated by a binary zero, and is passed in
1091           the pattern argument. A pointer to a single block  of  memory  that  is
1092           obtained  via  pcre_malloc is returned. This contains the compiled code
1093           and related data. The pcre type is defined for the returned block; this
1094           is a typedef for a structure whose contents are not externally defined.
1095           It is up to the caller to free the memory (via pcre_free) when it is no
1096           longer required.
1097    
1098           Although  the compiled code of a PCRE regex is relocatable, that is, it
1099           does not depend on memory location, the complete pcre data block is not
1100           fully  relocatable, because it may contain a copy of the tableptr argu-
1101           ment, which is an address (see below).
1102    
1103           The options argument contains various bit settings that affect the com-
1104           pilation.  It  should be zero if no options are required. The available
1105           options are described below. Some of them, in  particular,  those  that
1106           are  compatible  with  Perl,  can also be set and unset from within the
1107           pattern (see the detailed description  in  the  pcrepattern  documenta-
1108           tion).  For  these options, the contents of the options argument speci-
1109           fies their initial settings at the start of compilation and  execution.
1110           The  PCRE_ANCHORED  and PCRE_NEWLINE_xxx options can be set at the time
1111           of matching as well as at compile time.
1112    
1113           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
1114           if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
1115           sets the variable pointed to by errptr to point to a textual error mes-
1116           sage. This is a static string that is part of the library. You must not
1117           try to free it. The offset from the start of the pattern to the charac-
1118           ter where the error was discovered is placed in the variable pointed to
1119           by erroffset, which must not be NULL. If it is, an immediate  error  is
1120           given.
1121    
1122           If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
1123           codeptr argument is not NULL, a non-zero error code number is  returned
1124           via  this argument in the event of an error. This is in addition to the
1125           textual error message. Error codes and messages are listed below.
1126    
1127           If the final argument, tableptr, is NULL, PCRE uses a  default  set  of
1128           character  tables  that  are  built  when  PCRE  is compiled, using the
1129           default C locale. Otherwise, tableptr must be an address  that  is  the
1130           result  of  a  call to pcre_maketables(). This value is stored with the
1131           compiled pattern, and used again by pcre_exec(), unless  another  table
1132           pointer is passed to it. For more discussion, see the section on locale
1133           support below.
1134    
1135           This code fragment shows a typical straightforward  call  to  pcre_com-
1136           pile():
1137    
1138             pcre *re;
1139             const char *error;
1140             int erroffset;
1141             re = pcre_compile(
1142               "^A.*Z",          /* the pattern */
1143               0,                /* default options */
1144               &error,           /* for error message */
1145               &erroffset,       /* for error offset */
1146               NULL);            /* use default character tables */
1147    
1148           The  following  names  for option bits are defined in the pcre.h header
1149           file:
1150    
1151             PCRE_ANCHORED
1152    
1153           If this bit is set, the pattern is forced to be "anchored", that is, it
1154           is  constrained to match only at the first matching point in the string
1155           that is being searched (the "subject string"). This effect can also  be
1156           achieved  by appropriate constructs in the pattern itself, which is the
1157           only way to do it in Perl.
1158    
1159             PCRE_AUTO_CALLOUT
1160    
1161           If this bit is set, pcre_compile() automatically inserts callout items,
1162           all  with  number  255, before each pattern item. For discussion of the
1163           callout facility, see the pcrecallout documentation.
1164    
1165             PCRE_BSR_ANYCRLF
1166             PCRE_BSR_UNICODE
1167    
1168           These options (which are mutually exclusive) control what the \R escape
1169           sequence  matches.  The choice is either to match only CR, LF, or CRLF,
1170           or to match any Unicode newline sequence. The default is specified when
1171           PCRE is built. It can be overridden from within the pattern, or by set-
1172           ting an option when a compiled pattern is matched.
1173    
1174             PCRE_CASELESS
1175    
1176           If this bit is set, letters in the pattern match both upper  and  lower
1177           case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
1178           changed within a pattern by a (?i) option setting. In UTF-8 mode,  PCRE
1179           always  understands the concept of case for characters whose values are
1180           less than 128, so caseless matching is always possible. For  characters
1181           with  higher  values,  the concept of case is supported if PCRE is com-
1182           piled with Unicode property support, but not otherwise. If you want  to
1183           use  caseless  matching  for  characters 128 and above, you must ensure
1184           that PCRE is compiled with Unicode property support  as  well  as  with
1185           UTF-8 support.
1186    
1187             PCRE_DOLLAR_ENDONLY
1188    
1189           If  this bit is set, a dollar metacharacter in the pattern matches only
1190           at the end of the subject string. Without this option,  a  dollar  also
1191           matches  immediately before a newline at the end of the string (but not
1192           before any other newlines). The PCRE_DOLLAR_ENDONLY option  is  ignored
1193           if  PCRE_MULTILINE  is  set.   There is no equivalent to this option in
1194           Perl, and no way to set it within a pattern.
1195    
1196             PCRE_DOTALL
1197    
1198           If this bit is set, a dot metacharater in the pattern matches all char-
1199           acters,  including  those that indicate newline. Without it, a dot does
1200           not match when the current position is at a  newline.  This  option  is
1201           equivalent  to Perl's /s option, and it can be changed within a pattern
1202           by a (?s) option setting. A negative class such as [^a] always  matches
1203           newline characters, independent of the setting of this option.
1204    
1205             PCRE_DUPNAMES
1206    
1207           If  this  bit is set, names used to identify capturing subpatterns need
1208           not be unique. This can be helpful for certain types of pattern when it
1209           is  known  that  only  one instance of the named subpattern can ever be
1210           matched. There are more details of named subpatterns  below;  see  also
1211           the pcrepattern documentation.
1212    
1213             PCRE_EXTENDED
1214    
1215           If  this  bit  is  set,  whitespace  data characters in the pattern are
1216           totally ignored except when escaped or inside a character class. White-
1217           space does not include the VT character (code 11). In addition, charac-
1218           ters between an unescaped # outside a character class and the next new-
1219           line,  inclusive,  are  also  ignored.  This is equivalent to Perl's /x
1220           option, and it can be changed within a pattern by a  (?x)  option  set-
1221           ting.
1222    
1223           This  option  makes  it possible to include comments inside complicated
1224           patterns.  Note, however, that this applies only  to  data  characters.
1225           Whitespace   characters  may  never  appear  within  special  character
1226           sequences in a pattern, for  example  within  the  sequence  (?(  which
1227           introduces a conditional subpattern.
1228    
1229             PCRE_EXTRA
1230    
1231           This  option  was invented in order to turn on additional functionality
1232           of PCRE that is incompatible with Perl, but it  is  currently  of  very
1233           little  use. When set, any backslash in a pattern that is followed by a
1234           letter that has no special meaning  causes  an  error,  thus  reserving
1235           these  combinations  for  future  expansion.  By default, as in Perl, a
1236           backslash followed by a letter with no special meaning is treated as  a
1237           literal.  (Perl can, however, be persuaded to give a warning for this.)
1238           There are at present no other features controlled by  this  option.  It
1239           can also be set by a (?X) option setting within a pattern.
1240    
1241             PCRE_FIRSTLINE
1242    
1243           If  this  option  is  set,  an  unanchored pattern is required to match
1244           before or at the first  newline  in  the  subject  string,  though  the
1245           matched text may continue over the newline.
1246    
1247             PCRE_MULTILINE
1248    
1249           By  default,  PCRE  treats the subject string as consisting of a single
1250           line of characters (even if it actually contains newlines). The  "start
1251           of  line"  metacharacter  (^)  matches only at the start of the string,
1252           while the "end of line" metacharacter ($) matches only at  the  end  of
1253           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1254           is set). This is the same as Perl.
1255    
1256           When PCRE_MULTILINE it is set, the "start of line" and  "end  of  line"
1257           constructs  match  immediately following or immediately before internal
1258           newlines in the subject string, respectively, as well as  at  the  very
1259           start  and  end.  This is equivalent to Perl's /m option, and it can be
1260           changed within a pattern by a (?m) option setting. If there are no new-
1261           lines  in  a  subject string, or no occurrences of ^ or $ in a pattern,
1262           setting PCRE_MULTILINE has no effect.
1263    
1264             PCRE_NEWLINE_CR
1265             PCRE_NEWLINE_LF
1266             PCRE_NEWLINE_CRLF
1267             PCRE_NEWLINE_ANYCRLF
1268             PCRE_NEWLINE_ANY
1269    
1270           These options override the default newline definition that  was  chosen
1271           when  PCRE  was built. Setting the first or the second specifies that a
1272           newline is indicated by a single character (CR  or  LF,  respectively).
1273           Setting  PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1274           two-character CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF  specifies
1275           that any of the three preceding sequences should be recognized. Setting
1276           PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should  be
1277           recognized. The Unicode newline sequences are the three just mentioned,
1278           plus the single characters VT (vertical  tab,  U+000B),  FF  (formfeed,
1279           U+000C),  NEL  (next line, U+0085), LS (line separator, U+2028), and PS
1280           (paragraph separator, U+2029). The last  two  are  recognized  only  in
1281           UTF-8 mode.
1282    
1283           The  newline  setting  in  the  options  word  uses three bits that are
1284           treated as a number, giving eight possibilities. Currently only six are
1285           used  (default  plus the five values above). This means that if you set
1286           more than one newline option, the combination may or may not be  sensi-
1287           ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1288           PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers  and
1289           cause an error.
1290    
1291           The  only time that a line break is specially recognized when compiling
1292           a pattern is if PCRE_EXTENDED is set, and  an  unescaped  #  outside  a
1293           character  class  is  encountered.  This indicates a comment that lasts
1294           until after the next line break sequence. In other circumstances,  line
1295           break   sequences   are   treated  as  literal  data,  except  that  in
1296           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1297           and are therefore ignored.
1298    
1299           The newline option that is set at compile time becomes the default that
1300           is used for pcre_exec() and pcre_dfa_exec(), but it can be  overridden.
1301    
1302             PCRE_NO_AUTO_CAPTURE
1303    
1304           If this option is set, it disables the use of numbered capturing paren-
1305           theses in the pattern. Any opening parenthesis that is not followed  by
1306           ?  behaves as if it were followed by ?: but named parentheses can still
1307           be used for capturing (and they acquire  numbers  in  the  usual  way).
1308           There is no equivalent of this option in Perl.
1309    
1310             PCRE_UNGREEDY
1311    
1312           This  option  inverts  the "greediness" of the quantifiers so that they
1313           are not greedy by default, but become greedy if followed by "?". It  is
1314           not  compatible  with Perl. It can also be set by a (?U) option setting
1315           within the pattern.
1316    
1317             PCRE_UTF8
1318    
1319           This option causes PCRE to regard both the pattern and the  subject  as
1320           strings  of  UTF-8 characters instead of single-byte character strings.
1321           However, it is available only when PCRE is built to include UTF-8  sup-
1322           port.  If not, the use of this option provokes an error. Details of how
1323           this option changes the behaviour of PCRE are given in the  section  on
1324           UTF-8 support in the main pcre page.
1325    
1326             PCRE_NO_UTF8_CHECK
1327    
1328           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1329           automatically checked. There is a  discussion  about  the  validity  of
1330           UTF-8  strings  in  the main pcre page. If an invalid UTF-8 sequence of
1331           bytes is found, pcre_compile() returns an error. If  you  already  know
1332           that your pattern is valid, and you want to skip this check for perfor-
1333           mance reasons, you can set the PCRE_NO_UTF8_CHECK option.  When  it  is
1334           set,  the  effect  of  passing  an invalid UTF-8 string as a pattern is
1335           undefined. It may cause your program to crash. Note  that  this  option
1336           can  also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1337           UTF-8 validity checking of subject strings.
1338    
1339    
1340    COMPILATION ERROR CODES
1341    
1342           The following table lists the error  codes  than  may  be  returned  by
1343           pcre_compile2(),  along with the error messages that may be returned by
1344           both compiling functions. As PCRE has developed, some error codes  have
1345           fallen out of use. To avoid confusion, they have not been re-used.
1346    
1347              0  no error
1348              1  \ at end of pattern
1349              2  \c at end of pattern
1350              3  unrecognized character follows \
1351              4  numbers out of order in {} quantifier
1352              5  number too big in {} quantifier
1353              6  missing terminating ] for character class
1354              7  invalid escape sequence in character class
1355              8  range out of order in character class
1356              9  nothing to repeat
1357             10  [this code is not in use]
1358             11  internal error: unexpected repeat
1359             12  unrecognized character after (?
1360             13  POSIX named classes are supported only within a class
1361             14  missing )
1362             15  reference to non-existent subpattern
1363             16  erroffset passed as NULL
1364             17  unknown option bit(s) set
1365             18  missing ) after comment
1366             19  [this code is not in use]
1367             20  regular expression too large
1368             21  failed to get memory
1369             22  unmatched parentheses
1370             23  internal error: code overflow
1371             24  unrecognized character after (?<
1372             25  lookbehind assertion is not fixed length
1373             26  malformed number or name after (?(
1374             27  conditional group contains more than two branches
1375             28  assertion expected after (?(
1376             29  (?R or (?[+-]digits must be followed by )
1377             30  unknown POSIX class name
1378             31  POSIX collating elements are not supported
1379             32  this version of PCRE is not compiled with PCRE_UTF8 support
1380             33  [this code is not in use]
1381             34  character value in \x{...} sequence is too large
1382             35  invalid condition (?(0)
1383             36  \C not allowed in lookbehind assertion
1384             37  PCRE does not support \L, \l, \N, \U, or \u
1385             38  number after (?C is > 255
1386             39  closing ) for (?C expected
1387             40  recursive call could loop indefinitely
1388             41  unrecognized character after (?P
1389             42  syntax error in subpattern name (missing terminator)
1390             43  two named subpatterns have the same name
1391             44  invalid UTF-8 string
1392             45  support for \P, \p, and \X has not been compiled
1393             46  malformed \P or \p sequence
1394             47  unknown property name after \P or \p
1395             48  subpattern name is too long (maximum 32 characters)
1396             49  too many named subpatterns (maximum 10,000)
1397             50  [this code is not in use]
1398             51  octal value is greater than \377 (not in UTF-8 mode)
1399             52  internal error: overran compiling workspace
1400             53   internal  error:  previously-checked  referenced  subpattern not
1401           found
1402             54  DEFINE group contains more than one branch
1403             55  repeating a DEFINE group is not allowed
1404             56  inconsistent NEWLINE options
1405             57  \g is not followed by a braced name or an optionally braced
1406                   non-zero number
1407             58  (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
1408    
1409    
1410    STUDYING A PATTERN
1411    
1412           pcre_extra *pcre_study(const pcre *code, int options
1413                const char **errptr);
1414    
1415           If a compiled pattern is going to be used several times,  it  is  worth
1416           spending more time analyzing it in order to speed up the time taken for
1417           matching. The function pcre_study() takes a pointer to a compiled  pat-
1418           tern as its first argument. If studying the pattern produces additional
1419           information that will help speed up matching,  pcre_study()  returns  a
1420           pointer  to a pcre_extra block, in which the study_data field points to
1421           the results of the study.
1422    
1423           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1424           pcre_exec().  However,  a  pcre_extra  block also contains other fields
1425           that can be set by the caller before the block  is  passed;  these  are
1426           described below in the section on matching a pattern.
1427    
1428           If  studying  the  pattern  does not produce any additional information
1429           pcre_study() returns NULL. In that circumstance, if the calling program
1430           wants  to  pass  any of the other fields to pcre_exec(), it must set up
1431           its own pcre_extra block.
1432    
1433           The second argument of pcre_study() contains option bits.  At  present,
1434           no options are defined, and this argument should always be zero.
1435    
1436           The  third argument for pcre_study() is a pointer for an error message.
1437           If studying succeeds (even if no data is  returned),  the  variable  it
1438           points  to  is  set  to NULL. Otherwise it is set to point to a textual
1439           error message. This is a static string that is part of the library. You
1440           must  not  try  to  free it. You should test the error pointer for NULL
1441           after calling pcre_study(), to be sure that it has run successfully.
1442    
1443           This is a typical call to pcre_study():
1444    
1445             pcre_extra *pe;
1446             pe = pcre_study(
1447               re,             /* result of pcre_compile() */
1448               0,              /* no options exist */
1449               &error);        /* set to NULL or points to a message */
1450    
1451           At present, studying a pattern is useful only for non-anchored patterns
1452           that  do not have a single fixed starting character. A bitmap of possi-
1453           ble starting bytes is created.
1454    
1455    
1456    LOCALE SUPPORT
1457    
1458           PCRE handles caseless matching, and determines whether  characters  are
1459           letters,  digits, or whatever, by reference to a set of tables, indexed
1460           by character value. When running in UTF-8 mode, this  applies  only  to
1461           characters  with  codes  less than 128. Higher-valued codes never match
1462           escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
1463           with  Unicode  character property support. The use of locales with Uni-
1464           code is discouraged. If you are handling characters with codes  greater
1465           than  128, you should either use UTF-8 and Unicode, or use locales, but
1466           not try to mix the two.
1467    
1468           PCRE contains an internal set of tables that are used  when  the  final
1469           argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
1470           applications.  Normally, the internal tables recognize only ASCII char-
1471           acters. However, when PCRE is built, it is possible to cause the inter-
1472           nal tables to be rebuilt in the default "C" locale of the local system,
1473           which may cause them to be different.
1474    
1475           The  internal tables can always be overridden by tables supplied by the
1476           application that calls PCRE. These may be created in a different locale
1477           from  the  default.  As more and more applications change to using Uni-
1478           code, the need for this locale support is expected to die away.
1479    
1480           External tables are built by calling  the  pcre_maketables()  function,
1481           which  has no arguments, in the relevant locale. The result can then be
1482           passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
1483           example,  to  build  and use tables that are appropriate for the French
1484           locale (where accented characters with  values  greater  than  128  are
1485           treated as letters), the following code could be used:
1486    
1487             setlocale(LC_CTYPE, "fr_FR");
1488             tables = pcre_maketables();
1489             re = pcre_compile(..., tables);
1490    
1491           The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
1492           if you are using Windows, the name for the French locale is "french".
1493    
1494           When pcre_maketables() runs, the tables are built  in  memory  that  is
1495           obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1496           that the memory containing the tables remains available for as long  as
1497           it is needed.
1498    
1499           The pointer that is passed to pcre_compile() is saved with the compiled
1500           pattern, and the same tables are used via this pointer by  pcre_study()
1501           and normally also by pcre_exec(). Thus, by default, for any single pat-
1502           tern, compilation, studying and matching all happen in the same locale,
1503           but different patterns can be compiled in different locales.
1504    
1505           It  is  possible to pass a table pointer or NULL (indicating the use of
1506           the internal tables) to pcre_exec(). Although  not  intended  for  this
1507           purpose,  this facility could be used to match a pattern in a different
1508           locale from the one in which it was compiled. Passing table pointers at
1509           run time is discussed below in the section on matching a pattern.
1510    
1511    
1512    INFORMATION ABOUT A PATTERN
1513    
1514           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1515                int what, void *where);
1516    
1517           The  pcre_fullinfo() function returns information about a compiled pat-
1518           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1519           less retained for backwards compability (and is documented below).
1520    
1521           The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1522           pattern. The second argument is the result of pcre_study(), or NULL  if
1523           the  pattern  was not studied. The third argument specifies which piece
1524           of information is required, and the fourth argument is a pointer  to  a
1525           variable  to  receive  the  data. The yield of the function is zero for
1526           success, or one of the following negative numbers:
1527    
1528             PCRE_ERROR_NULL       the argument code was NULL
1529                                   the argument where was NULL
1530             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1531             PCRE_ERROR_BADOPTION  the value of what was invalid
1532    
1533           The "magic number" is placed at the start of each compiled  pattern  as
1534           an  simple check against passing an arbitrary memory pointer. Here is a
1535           typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1536           pattern:
1537    
1538             int rc;
1539             size_t length;
1540             rc = pcre_fullinfo(
1541               re,               /* result of pcre_compile() */
1542               pe,               /* result of pcre_study(), or NULL */
1543               PCRE_INFO_SIZE,   /* what is required */
1544               &length);         /* where to put the data */
1545    
1546           The  possible  values for the third argument are defined in pcre.h, and
1547           are as follows:
1548    
1549             PCRE_INFO_BACKREFMAX
1550    
1551           Return the number of the highest back reference  in  the  pattern.  The
1552           fourth  argument  should  point to an int variable. Zero is returned if
1553           there are no back references.
1554    
1555             PCRE_INFO_CAPTURECOUNT
1556    
1557           Return the number of capturing subpatterns in the pattern.  The  fourth
1558           argument should point to an int variable.
1559    
1560             PCRE_INFO_DEFAULT_TABLES
1561    
1562           Return  a pointer to the internal default character tables within PCRE.
1563           The fourth argument should point to an unsigned char *  variable.  This
1564           information call is provided for internal use by the pcre_study() func-
1565           tion. External callers can cause PCRE to use  its  internal  tables  by
1566           passing a NULL table pointer.
1567    
1568             PCRE_INFO_FIRSTBYTE
1569    
1570           Return  information  about  the first byte of any matched string, for a
1571           non-anchored pattern. The fourth argument should point to an int  vari-
1572           able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1573           is still recognized for backwards compatibility.)
1574    
1575           If there is a fixed first byte, for example, from  a  pattern  such  as
1576           (cat|cow|coyote), its value is returned. Otherwise, if either
1577    
1578           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1579           branch starts with "^", or
1580    
1581           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1582           set (if it were set, the pattern would be anchored),
1583    
1584           -1  is  returned, indicating that the pattern matches only at the start
1585           of a subject string or after any newline within the  string.  Otherwise
1586           -2 is returned. For anchored patterns, -2 is returned.
1587    
1588             PCRE_INFO_FIRSTTABLE
1589    
1590           If  the pattern was studied, and this resulted in the construction of a
1591           256-bit table indicating a fixed set of bytes for the first byte in any
1592           matching  string, a pointer to the table is returned. Otherwise NULL is
1593           returned. The fourth argument should point to an unsigned char *  vari-
1594           able.
1595    
1596             PCRE_INFO_HASCRORLF
1597    
1598           Return  1  if  the  pattern  contains any explicit matches for CR or LF
1599           characters, otherwise 0. The fourth argument should  point  to  an  int
1600           variable.  An explicit match is either a literal CR or LF character, or
1601           \r or \n.
1602    
1603             PCRE_INFO_JCHANGED
1604    
1605           Return 1 if the (?J) or (?-J) option setting is used  in  the  pattern,
1606           otherwise  0. The fourth argument should point to an int variable. (?J)
1607           and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1608    
1609             PCRE_INFO_LASTLITERAL
1610    
1611           Return the value of the rightmost literal byte that must exist  in  any
1612           matched  string,  other  than  at  its  start,  if such a byte has been
1613           recorded. The fourth argument should point to an int variable. If there
1614           is  no such byte, -1 is returned. For anchored patterns, a last literal
1615           byte is recorded only if it follows something of variable  length.  For
1616           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1617           /^a\dz\d/ the returned value is -1.
1618    
1619             PCRE_INFO_NAMECOUNT
1620             PCRE_INFO_NAMEENTRYSIZE
1621             PCRE_INFO_NAMETABLE
1622    
1623           PCRE supports the use of named as well as numbered capturing  parenthe-
1624           ses.  The names are just an additional way of identifying the parenthe-
1625           ses, which still acquire numbers. Several convenience functions such as
1626           pcre_get_named_substring()  are  provided  for extracting captured sub-
1627           strings by name. It is also possible to extract the data  directly,  by
1628           first  converting  the  name to a number in order to access the correct
1629           pointers in the output vector (described with pcre_exec() below). To do
1630           the  conversion,  you  need  to  use  the  name-to-number map, which is
1631           described by these three values.
1632    
1633           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1634           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1635           of each entry; both of these  return  an  int  value.  The  entry  size
1636           depends  on the length of the longest name. PCRE_INFO_NAMETABLE returns
1637           a pointer to the first entry of the table  (a  pointer  to  char).  The
1638           first two bytes of each entry are the number of the capturing parenthe-
1639           sis, most significant byte first. The rest of the entry is  the  corre-
1640           sponding  name,  zero  terminated. The names are in alphabetical order.
1641           When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1642           theses  numbers.  For  example,  consider the following pattern (assume
1643           PCRE_EXTENDED is  set,  so  white  space  -  including  newlines  -  is
1644           ignored):
1645    
1646             (?<date> (?<year>(\d\d)?\d\d) -
1647             (?<month>\d\d) - (?<day>\d\d) )
1648    
1649           There  are  four  named subpatterns, so the table has four entries, and
1650           each entry in the table is eight bytes long. The table is  as  follows,
1651           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1652           as ??:
1653    
1654             00 01 d  a  t  e  00 ??
1655             00 05 d  a  y  00 ?? ??
1656             00 04 m  o  n  t  h  00
1657             00 02 y  e  a  r  00 ??
1658    
1659           When writing code to extract data  from  named  subpatterns  using  the
1660           name-to-number  map,  remember that the length of the entries is likely
1661           to be different for each compiled pattern.
1662    
1663             PCRE_INFO_OKPARTIAL
1664    
1665           Return 1 if the pattern can be used for partial matching, otherwise  0.
1666           The  fourth  argument  should point to an int variable. The pcrepartial
1667           documentation lists the restrictions that apply to patterns  when  par-
1668           tial matching is used.
1669    
1670             PCRE_INFO_OPTIONS
1671    
1672           Return  a  copy of the options with which the pattern was compiled. The
1673           fourth argument should point to an unsigned long  int  variable.  These
1674           option bits are those specified in the call to pcre_compile(), modified
1675           by any top-level option settings at the start of the pattern itself. In
1676           other  words,  they are the options that will be in force when matching
1677           starts. For example, if the pattern /(?im)abc(?-i)d/ is  compiled  with
1678           the  PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1679           and PCRE_EXTENDED.
1680    
1681           A pattern is automatically anchored by PCRE if  all  of  its  top-level
1682           alternatives begin with one of the following:
1683    
1684             ^     unless PCRE_MULTILINE is set
1685             \A    always
1686             \G    always
1687             .*    if PCRE_DOTALL is set and there are no back
1688                     references to the subpattern in which .* appears
1689    
1690           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1691           by pcre_fullinfo().
1692    
1693             PCRE_INFO_SIZE
1694    
1695           Return the size of the compiled pattern, that is, the  value  that  was
1696           passed as the argument to pcre_malloc() when PCRE was getting memory in
1697           which to place the compiled data. The fourth argument should point to a
1698           size_t variable.
1699    
1700             PCRE_INFO_STUDYSIZE
1701    
1702           Return the size of the data block pointed to by the study_data field in
1703           a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1704           pcre_malloc() when PCRE was getting memory into which to place the data
1705           created by pcre_study(). The fourth argument should point to  a  size_t
1706           variable.
1707    
1708    
1709    OBSOLETE INFO FUNCTION
1710    
1711           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1712    
1713           The  pcre_info()  function is now obsolete because its interface is too
1714           restrictive to return all the available data about a compiled  pattern.
1715           New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1716           pcre_info() is the number of capturing subpatterns, or one of the  fol-
1717           lowing negative numbers:
1718    
1719             PCRE_ERROR_NULL       the argument code was NULL
1720             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1721    
1722           If  the  optptr  argument is not NULL, a copy of the options with which
1723           the pattern was compiled is placed in the integer  it  points  to  (see
1724           PCRE_INFO_OPTIONS above).
1725    
1726           If  the  pattern  is  not anchored and the firstcharptr argument is not
1727           NULL, it is used to pass back information about the first character  of
1728           any matched string (see PCRE_INFO_FIRSTBYTE above).
1729    
1730    
1731    REFERENCE COUNTS
1732    
1733           int pcre_refcount(pcre *code, int adjust);
1734    
1735           The  pcre_refcount()  function is used to maintain a reference count in
1736           the data block that contains a compiled pattern. It is provided for the
1737           benefit  of  applications  that  operate  in an object-oriented manner,
1738           where different parts of the application may be using the same compiled
1739           pattern, but you want to free the block when they are all done.
1740    
1741           When a pattern is compiled, the reference count field is initialized to
1742           zero.  It is changed only by calling this function, whose action is  to
1743           add  the  adjust  value  (which may be positive or negative) to it. The
1744           yield of the function is the new value. However, the value of the count
1745           is  constrained to lie between 0 and 65535, inclusive. If the new value
1746           is outside these limits, it is forced to the appropriate limit value.
1747    
1748           Except when it is zero, the reference count is not correctly  preserved
1749           if  a  pattern  is  compiled on one host and then transferred to a host
1750           whose byte-order is different. (This seems a highly unlikely scenario.)
1751    
1752    
1753    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1754    
1755           int pcre_exec(const pcre *code, const pcre_extra *extra,
1756                const char *subject, int length, int startoffset,
1757                int options, int *ovector, int ovecsize);
1758    
1759           The  function pcre_exec() is called to match a subject string against a
1760           compiled pattern, which is passed in the code argument. If the  pattern
1761           has been studied, the result of the study should be passed in the extra
1762           argument. This function is the main matching facility of  the  library,
1763           and it operates in a Perl-like manner. For specialist use there is also
1764           an alternative matching function, which is described below in the  sec-
1765           tion about the pcre_dfa_exec() function.
1766    
1767           In  most applications, the pattern will have been compiled (and option-
1768           ally studied) in the same process that calls pcre_exec().  However,  it
1769           is possible to save compiled patterns and study data, and then use them
1770           later in different processes, possibly even on different hosts.  For  a
1771           discussion about this, see the pcreprecompile documentation.
1772    
1773           Here is an example of a simple call to pcre_exec():
1774    
1775             int rc;
1776             int ovector[30];
1777             rc = pcre_exec(
1778               re,             /* result of pcre_compile() */
1779               NULL,           /* we didn't study the pattern */
1780               "some string",  /* the subject string */
1781               11,             /* the length of the subject string */
1782               0,              /* start at offset 0 in the subject */
1783               0,              /* default options */
1784               ovector,        /* vector of integers for substring information */
1785               30);            /* number of elements (NOT size in bytes) */
1786    
1787       Extra data for pcre_exec()
1788    
1789           If  the  extra argument is not NULL, it must point to a pcre_extra data
1790           block. The pcre_study() function returns such a block (when it  doesn't
1791           return  NULL), but you can also create one for yourself, and pass addi-
1792           tional information in it. The pcre_extra block contains  the  following
1793           fields (not necessarily in this order):
1794    
1795             unsigned long int flags;
1796             void *study_data;
1797             unsigned long int match_limit;
1798             unsigned long int match_limit_recursion;
1799             void *callout_data;
1800             const unsigned char *tables;
1801    
1802           The  flags  field  is a bitmap that specifies which of the other fields
1803           are set. The flag bits are:
1804    
1805             PCRE_EXTRA_STUDY_DATA
1806             PCRE_EXTRA_MATCH_LIMIT
1807             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1808             PCRE_EXTRA_CALLOUT_DATA
1809             PCRE_EXTRA_TABLES
1810    
1811           Other flag bits should be set to zero. The study_data field is  set  in
1812           the  pcre_extra  block  that is returned by pcre_study(), together with
1813           the appropriate flag bit. You should not set this yourself, but you may
1814           add  to  the  block by setting the other fields and their corresponding
1815           flag bits.
1816    
1817           The match_limit field provides a means of preventing PCRE from using up
1818           a  vast amount of resources when running patterns that are not going to
1819           match, but which have a very large number  of  possibilities  in  their
1820           search  trees.  The  classic  example  is  the  use of nested unlimited
1821           repeats.
1822    
1823           Internally, PCRE uses a function called match() which it calls  repeat-
1824           edly  (sometimes  recursively). The limit set by match_limit is imposed
1825           on the number of times this function is called during  a  match,  which
1826           has  the  effect  of  limiting the amount of backtracking that can take
1827           place. For patterns that are not anchored, the count restarts from zero
1828           for each position in the subject string.
1829    
1830           The  default  value  for  the  limit can be set when PCRE is built; the
1831           default default is 10 million, which handles all but the  most  extreme
1832           cases.  You  can  override  the  default by suppling pcre_exec() with a
1833           pcre_extra    block    in    which    match_limit    is    set,     and
1834           PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1835           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1836    
1837           The match_limit_recursion field is similar to match_limit, but  instead
1838           of limiting the total number of times that match() is called, it limits
1839           the depth of recursion. The recursion depth is a  smaller  number  than
1840           the  total number of calls, because not all calls to match() are recur-
1841           sive.  This limit is of use only if it is set smaller than match_limit.
1842    
1843           Limiting  the  recursion  depth  limits the amount of stack that can be
1844           used, or, when PCRE has been compiled to use memory on the heap instead
1845           of the stack, the amount of heap memory that can be used.
1846    
1847           The  default  value  for  match_limit_recursion can be set when PCRE is
1848           built; the default default  is  the  same  value  as  the  default  for
1849           match_limit.  You can override the default by suppling pcre_exec() with
1850           a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
1851           PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
1852           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1853    
1854           The pcre_callout field is used in conjunction with the  "callout"  fea-
1855           ture, which is described in the pcrecallout documentation.
1856    
1857           The  tables  field  is  used  to  pass  a  character  tables pointer to
1858           pcre_exec(); this overrides the value that is stored with the  compiled
1859           pattern.  A  non-NULL value is stored with the compiled pattern only if
1860           custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1861           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1862           PCRE's internal tables to be used. This facility is  helpful  when  re-
1863           using  patterns  that  have been saved after compiling with an external
1864           set of tables, because the external tables  might  be  at  a  different
1865           address  when  pcre_exec() is called. See the pcreprecompile documenta-
1866           tion for a discussion of saving compiled patterns for later use.
1867    
1868       Option bits for pcre_exec()
1869    
1870           The unused bits of the options argument for pcre_exec() must  be  zero.
1871           The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1872           PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1873           PCRE_PARTIAL.
1874    
1875             PCRE_ANCHORED
1876    
1877           The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
1878           matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
1879           turned  out to be anchored by virtue of its contents, it cannot be made
1880           unachored at matching time.
1881    
1882             PCRE_BSR_ANYCRLF
1883             PCRE_BSR_UNICODE
1884    
1885           These options (which are mutually exclusive) control what the \R escape
1886           sequence  matches.  The choice is either to match only CR, LF, or CRLF,
1887           or to match any Unicode newline sequence. These  options  override  the
1888           choice that was made or defaulted when the pattern was compiled.
1889    
1890             PCRE_NEWLINE_CR
1891             PCRE_NEWLINE_LF
1892             PCRE_NEWLINE_CRLF
1893             PCRE_NEWLINE_ANYCRLF
1894             PCRE_NEWLINE_ANY
1895    
1896           These  options  override  the  newline  definition  that  was chosen or
1897           defaulted when the pattern was compiled. For details, see the  descrip-
1898           tion  of  pcre_compile()  above.  During  matching,  the newline choice
1899           affects the behaviour of the dot, circumflex,  and  dollar  metacharac-
1900           ters.  It may also alter the way the match position is advanced after a
1901           match failure for an unanchored pattern.
1902    
1903           When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY  is
1904           set,  and a match attempt for an unanchored pattern fails when the cur-
1905           rent position is at a  CRLF  sequence,  and  the  pattern  contains  no
1906           explicit  matches  for  CR  or  LF  characters,  the  match position is
1907           advanced by two characters instead of one, in other words, to after the
1908           CRLF.
1909    
1910           The above rule is a compromise that makes the most common cases work as
1911           expected. For example, if the  pattern  is  .+A  (and  the  PCRE_DOTALL
1912           option is not set), it does not match the string "\r\nA" because, after
1913           failing at the start, it skips both the CR and the LF before  retrying.
1914           However,  the  pattern  [\r\n]A does match that string, because it con-
1915           tains an explicit CR or LF reference, and so advances only by one char-
1916           acter after the first failure.
1917    
1918           An explicit match for CR of LF is either a literal appearance of one of
1919           those characters, or one of the \r or  \n  escape  sequences.  Implicit
1920           matches  such  as [^X] do not count, nor does \s (which includes CR and
1921           LF in the characters that it matches).
1922    
1923           Notwithstanding the above, anomalous effects may still occur when  CRLF
1924           is a valid newline sequence and explicit \r or \n escapes appear in the
1925           pattern.
1926    
1927             PCRE_NOTBOL
1928    
1929           This option specifies that first character of the subject string is not
1930           the  beginning  of  a  line, so the circumflex metacharacter should not
1931           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1932           causes  circumflex  never to match. This option affects only the behav-
1933           iour of the circumflex metacharacter. It does not affect \A.
1934    
1935             PCRE_NOTEOL
1936    
1937           This option specifies that the end of the subject string is not the end
1938           of  a line, so the dollar metacharacter should not match it nor (except
1939           in multiline mode) a newline immediately before it. Setting this  with-
1940           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1941           option affects only the behaviour of the dollar metacharacter. It  does
1942           not affect \Z or \z.
1943    
1944             PCRE_NOTEMPTY
1945    
1946           An empty string is not considered to be a valid match if this option is
1947           set. If there are alternatives in the pattern, they are tried.  If  all
1948           the  alternatives  match  the empty string, the entire match fails. For
1949           example, if the pattern
1950    
1951             a?b?
1952    
1953           is applied to a string not beginning with "a" or "b",  it  matches  the
1954           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1955           match is not valid, so PCRE searches further into the string for occur-
1956           rences of "a" or "b".
1957    
1958           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1959           cial case of a pattern match of the empty  string  within  its  split()
1960           function,  and  when  using  the /g modifier. It is possible to emulate
1961           Perl's behaviour after matching a null string by first trying the match
1962           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1963           if that fails by advancing the starting offset (see below)  and  trying
1964           an ordinary match again. There is some code that demonstrates how to do
1965           this in the pcredemo.c sample program.
1966    
1967             PCRE_NO_UTF8_CHECK
1968    
1969           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1970           UTF-8  string is automatically checked when pcre_exec() is subsequently
1971           called.  The value of startoffset is also checked  to  ensure  that  it
1972           points  to  the start of a UTF-8 character. There is a discussion about
1973           the validity of UTF-8 strings in the section on UTF-8  support  in  the
1974           main  pcre  page.  If  an  invalid  UTF-8  sequence  of bytes is found,
1975           pcre_exec() returns the error PCRE_ERROR_BADUTF8. If  startoffset  con-
1976           tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1977    
1978           If  you  already  know that your subject is valid, and you want to skip
1979           these   checks   for   performance   reasons,   you   can    set    the
1980           PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
1981           do this for the second and subsequent calls to pcre_exec() if  you  are
1982           making  repeated  calls  to  find  all  the matches in a single subject
1983           string. However, you should be  sure  that  the  value  of  startoffset
1984           points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1985           set, the effect of passing an invalid UTF-8 string as a subject,  or  a
1986           value  of startoffset that does not point to the start of a UTF-8 char-
1987           acter, is undefined. Your program may crash.
1988    
1989             PCRE_PARTIAL
1990    
1991           This option turns on the  partial  matching  feature.  If  the  subject
1992           string  fails to match the pattern, but at some point during the match-
1993           ing process the end of the subject was reached (that  is,  the  subject
1994           partially  matches  the  pattern and the failure to match occurred only
1995           because there were not enough subject characters), pcre_exec()  returns
1996           PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1997           used, there are restrictions on what may appear in the  pattern.  These
1998           are discussed in the pcrepartial documentation.
1999    
2000       The string to be matched by pcre_exec()
2001    
2002           The  subject string is passed to pcre_exec() as a pointer in subject, a
2003           length in length, and a starting byte offset in startoffset.  In  UTF-8
2004           mode,  the  byte  offset  must point to the start of a UTF-8 character.
2005           Unlike the pattern string, the subject may contain binary  zero  bytes.
2006           When  the starting offset is zero, the search for a match starts at the
2007           beginning of the subject, and this is by far the most common case.
2008    
2009           A non-zero starting offset is useful when searching for  another  match
2010           in  the same subject by calling pcre_exec() again after a previous suc-
2011           cess.  Setting startoffset differs from just passing over  a  shortened
2012           string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
2013           with any kind of lookbehind. For example, consider the pattern
2014    
2015             \Biss\B
2016    
2017           which finds occurrences of "iss" in the middle of  words.  (\B  matches
2018           only  if  the  current position in the subject is not a word boundary.)
2019           When applied to the string "Mississipi" the first call  to  pcre_exec()
2020           finds  the  first  occurrence. If pcre_exec() is called again with just
2021           the remainder of the subject,  namely  "issipi",  it  does  not  match,
2022           because \B is always false at the start of the subject, which is deemed
2023           to be a word boundary. However, if pcre_exec()  is  passed  the  entire
2024           string again, but with startoffset set to 4, it finds the second occur-
2025           rence of "iss" because it is able to look behind the starting point  to
2026           discover that it is preceded by a letter.
2027    
2028           If  a  non-zero starting offset is passed when the pattern is anchored,
2029           one attempt to match at the given offset is made. This can only succeed
2030           if  the  pattern  does  not require the match to be at the start of the
2031           subject.
2032    
2033       How pcre_exec() returns captured substrings
2034    
2035           In general, a pattern matches a certain portion of the subject, and  in
2036           addition,  further  substrings  from  the  subject may be picked out by
2037           parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
2038           this  is  called "capturing" in what follows, and the phrase "capturing
2039           subpattern" is used for a fragment of a pattern that picks out  a  sub-
2040           string.  PCRE  supports several other kinds of parenthesized subpattern
2041           that do not cause substrings to be captured.
2042    
2043           Captured substrings are returned to the caller via a vector of  integer
2044           offsets  whose  address is passed in ovector. The number of elements in
2045           the vector is passed in ovecsize, which must be a non-negative  number.
2046           Note: this argument is NOT the size of ovector in bytes.
2047    
2048           The  first  two-thirds of the vector is used to pass back captured sub-
2049           strings, each substring using a pair of integers. The  remaining  third
2050           of  the  vector is used as workspace by pcre_exec() while matching cap-
2051           turing subpatterns, and is not available for passing back  information.
2052           The  length passed in ovecsize should always be a multiple of three. If
2053           it is not, it is rounded down.
2054    
2055           When a match is successful, information about  captured  substrings  is
2056           returned  in  pairs  of integers, starting at the beginning of ovector,
2057           and continuing up to two-thirds of its length at the  most.  The  first
2058           element of a pair is set to the offset of the first character in a sub-
2059           string, and the second is set to the  offset  of  the  first  character
2060           after  the  end  of  a  substring. The first pair, ovector[0] and ovec-
2061           tor[1], identify the portion of  the  subject  string  matched  by  the
2062           entire  pattern.  The next pair is used for the first capturing subpat-
2063           tern, and so on. The value returned by pcre_exec() is one more than the
2064           highest numbered pair that has been set. For example, if two substrings
2065           have been captured, the returned value is 3. If there are no  capturing
2066           subpatterns,  the return value from a successful match is 1, indicating
2067           that just the first pair of offsets has been set.
2068    
2069           If a capturing subpattern is matched repeatedly, it is the last portion
2070           of the string that it matched that is returned.
2071    
2072           If  the vector is too small to hold all the captured substring offsets,
2073           it is used as far as possible (up to two-thirds of its length), and the
2074           function  returns a value of zero. In particular, if the substring off-
2075           sets are not of interest, pcre_exec() may be called with ovector passed
2076           as  NULL  and  ovecsize  as zero. However, if the pattern contains back
2077           references and the ovector is not big enough to  remember  the  related
2078           substrings,  PCRE has to get additional memory for use during matching.
2079           Thus it is usually advisable to supply an ovector.
2080    
2081           The pcre_info() function can be used to find  out  how  many  capturing
2082           subpatterns  there  are  in  a  compiled pattern. The smallest size for
2083           ovector that will allow for n captured substrings, in addition  to  the
2084           offsets of the substring matched by the whole pattern, is (n+1)*3.
2085    
2086           It  is  possible for capturing subpattern number n+1 to match some part
2087           of the subject when subpattern n has not been used at all. For example,
2088           if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
2089           return from the function is 4, and subpatterns 1 and 3 are matched, but
2090           2  is  not.  When  this happens, both values in the offset pairs corre-
2091           sponding to unused subpatterns are set to -1.
2092    
2093           Offset values that correspond to unused subpatterns at the end  of  the
2094           expression  are  also  set  to  -1. For example, if the string "abc" is
2095           matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not
2096           matched.  The  return  from the function is 2, because the highest used
2097           capturing subpattern number is 1. However, you can refer to the offsets
2098           for  the  second  and third capturing subpatterns if you wish (assuming
2099           the vector is large enough, of course).
2100    
2101           Some convenience functions are provided  for  extracting  the  captured
2102           substrings as separate strings. These are described below.
2103    
2104       Error return values from pcre_exec()
2105    
2106           If  pcre_exec()  fails, it returns a negative number. The following are
2107           defined in the header file:
2108    
2109             PCRE_ERROR_NOMATCH        (-1)
2110    
2111           The subject string did not match the pattern.
2112    
2113             PCRE_ERROR_NULL           (-2)
2114    
2115           Either code or subject was passed as NULL,  or  ovector  was  NULL  and
2116           ovecsize was not zero.
2117    
2118             PCRE_ERROR_BADOPTION      (-3)
2119    
2120           An unrecognized bit was set in the options argument.
2121    
2122             PCRE_ERROR_BADMAGIC       (-4)
2123    
2124           PCRE  stores a 4-byte "magic number" at the start of the compiled code,
2125           to catch the case when it is passed a junk pointer and to detect when a
2126           pattern that was compiled in an environment of one endianness is run in
2127           an environment with the other endianness. This is the error  that  PCRE
2128           gives when the magic number is not present.
2129    
2130             PCRE_ERROR_UNKNOWN_OPCODE (-5)
2131    
2132           While running the pattern match, an unknown item was encountered in the
2133           compiled pattern. This error could be caused by a bug  in  PCRE  or  by
2134           overwriting of the compiled pattern.
2135    
2136             PCRE_ERROR_NOMEMORY       (-6)
2137    
2138           If  a  pattern contains back references, but the ovector that is passed
2139           to pcre_exec() is not big enough to remember the referenced substrings,
2140           PCRE  gets  a  block of memory at the start of matching to use for this
2141           purpose. If the call via pcre_malloc() fails, this error is given.  The
2142           memory is automatically freed at the end of matching.
2143    
2144             PCRE_ERROR_NOSUBSTRING    (-7)
2145    
2146           This  error is used by the pcre_copy_substring(), pcre_get_substring(),
2147           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
2148           returned by pcre_exec().
2149    
2150             PCRE_ERROR_MATCHLIMIT     (-8)
2151    
2152           The  backtracking  limit,  as  specified  by the match_limit field in a
2153           pcre_extra structure (or defaulted) was reached.  See  the  description
2154           above.
2155    
2156             PCRE_ERROR_CALLOUT        (-9)
2157    
2158           This error is never generated by pcre_exec() itself. It is provided for
2159           use by callout functions that want to yield a distinctive  error  code.
2160           See the pcrecallout documentation for details.
2161    
2162             PCRE_ERROR_BADUTF8        (-10)
2163    
2164           A  string  that contains an invalid UTF-8 byte sequence was passed as a
2165           subject.
2166    
2167             PCRE_ERROR_BADUTF8_OFFSET (-11)
2168    
2169           The UTF-8 byte sequence that was passed as a subject was valid, but the
2170           value  of startoffset did not point to the beginning of a UTF-8 charac-
2171           ter.
2172    
2173             PCRE_ERROR_PARTIAL        (-12)
2174    
2175           The subject string did not match, but it did match partially.  See  the
2176           pcrepartial documentation for details of partial matching.
2177    
2178             PCRE_ERROR_BADPARTIAL     (-13)
2179    
2180           The  PCRE_PARTIAL  option  was  used with a compiled pattern containing
2181           items that are not supported for partial matching. See the  pcrepartial
2182           documentation for details of partial matching.
2183    
2184             PCRE_ERROR_INTERNAL       (-14)
2185    
2186           An  unexpected  internal error has occurred. This error could be caused
2187           by a bug in PCRE or by overwriting of the compiled pattern.
2188    
2189             PCRE_ERROR_BADCOUNT       (-15)
2190    
2191           This error is given if the value of the ovecsize argument is  negative.
2192    
2193             PCRE_ERROR_RECURSIONLIMIT (-21)
2194    
2195           The internal recursion limit, as specified by the match_limit_recursion
2196           field in a pcre_extra structure (or defaulted)  was  reached.  See  the
2197           description above.
2198    
2199             PCRE_ERROR_BADNEWLINE     (-23)
2200    
2201           An invalid combination of PCRE_NEWLINE_xxx options was given.
2202    
2203           Error numbers -16 to -20 and -22 are not used by pcre_exec().
2204    
2205    
2206    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2207    
2208           int pcre_copy_substring(const char *subject, int *ovector,
2209                int stringcount, int stringnumber, char *buffer,
2210                int buffersize);
2211    
2212           int pcre_get_substring(const char *subject, int *ovector,
2213                int stringcount, int stringnumber,
2214                const char **stringptr);
2215    
2216           int pcre_get_substring_list(const char *subject,
2217                int *ovector, int stringcount, const char ***listptr);
2218    
2219           Captured  substrings  can  be  accessed  directly  by using the offsets
2220           returned by pcre_exec() in  ovector.  For  convenience,  the  functions
2221           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2222           string_list() are provided for extracting captured substrings  as  new,
2223           separate,  zero-terminated strings. These functions identify substrings
2224           by number. The next section describes functions  for  extracting  named
2225           substrings.
2226    
2227           A  substring that contains a binary zero is correctly extracted and has
2228           a further zero added on the end, but the result is not, of course, a  C
2229           string.   However,  you  can  process such a string by referring to the
2230           length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
2231           string().  Unfortunately, the interface to pcre_get_substring_list() is
2232           not adequate for handling strings containing binary zeros, because  the
2233           end of the final string is not independently indicated.
2234    
2235           The  first  three  arguments  are the same for all three of these func-
2236           tions: subject is the subject string that has  just  been  successfully
2237           matched, ovector is a pointer to the vector of integer offsets that was
2238           passed to pcre_exec(), and stringcount is the number of substrings that
2239           were  captured  by  the match, including the substring that matched the
2240           entire regular expression. This is the value returned by pcre_exec() if
2241           it  is greater than zero. If pcre_exec() returned zero, indicating that
2242           it ran out of space in ovector, the value passed as stringcount  should
2243           be the number of elements in the vector divided by three.
2244    
2245           The  functions pcre_copy_substring() and pcre_get_substring() extract a
2246           single substring, whose number is given as  stringnumber.  A  value  of
2247           zero  extracts  the  substring that matched the entire pattern, whereas
2248           higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
2249           string(),  the  string  is  placed  in buffer, whose length is given by
2250           buffersize, while for pcre_get_substring() a new  block  of  memory  is
2251           obtained  via  pcre_malloc,  and its address is returned via stringptr.
2252           The yield of the function is the length of the  string,  not  including
2253           the terminating zero, or one of these error codes:
2254    
2255             PCRE_ERROR_NOMEMORY       (-6)
2256    
2257           The  buffer  was too small for pcre_copy_substring(), or the attempt to
2258           get memory failed for pcre_get_substring().
2259    
2260             PCRE_ERROR_NOSUBSTRING    (-7)
2261    
2262           There is no substring whose number is stringnumber.
2263    
2264           The pcre_get_substring_list()  function  extracts  all  available  sub-
2265           strings  and  builds  a list of pointers to them. All this is done in a
2266           single block of memory that is obtained via pcre_malloc. The address of
2267           the  memory  block  is returned via listptr, which is also the start of
2268           the list of string pointers. The end of the list is marked  by  a  NULL
2269           pointer.  The  yield  of  the function is zero if all went well, or the
2270           error code
2271    
2272             PCRE_ERROR_NOMEMORY       (-6)
2273    
2274           if the attempt to get the memory block failed.
2275    
2276           When any of these functions encounter a substring that is unset,  which
2277           can  happen  when  capturing subpattern number n+1 matches some part of
2278           the subject, but subpattern n has not been used at all, they return  an
2279           empty string. This can be distinguished from a genuine zero-length sub-
2280           string by inspecting the appropriate offset in ovector, which is  nega-
2281           tive for unset substrings.
2282    
2283           The  two convenience functions pcre_free_substring() and pcre_free_sub-
2284           string_list() can be used to free the memory  returned  by  a  previous
2285           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2286           tively. They do nothing more than  call  the  function  pointed  to  by
2287           pcre_free,  which  of course could be called directly from a C program.
2288           However, PCRE is used in some situations where it is linked via a  spe-
2289           cial   interface  to  another  programming  language  that  cannot  use
2290           pcre_free directly; it is for these cases that the functions  are  pro-
2291           vided.
2292    
2293    
2294    EXTRACTING CAPTURED SUBSTRINGS BY NAME
2295    
2296           int pcre_get_stringnumber(const pcre *code,
2297                const char *name);
2298    
2299           int pcre_copy_named_substring(const pcre *code,
2300                const char *subject, int *ovector,
2301                int stringcount, const char *stringname,
2302                char *buffer, int buffersize);
2303    
2304           int pcre_get_named_substring(const pcre *code,
2305                const char *subject, int *ovector,
2306                int stringcount, const char *stringname,
2307                const char **stringptr);
2308    
2309           To  extract a substring by name, you first have to find associated num-
2310           ber.  For example, for this pattern
2311    
2312             (a+)b(?<xxx>\d+)...
2313    
2314           the number of the subpattern called "xxx" is 2. If the name is known to
2315           be unique (PCRE_DUPNAMES was not set), you can find the number from the
2316           name by calling pcre_get_stringnumber(). The first argument is the com-
2317           piled pattern, and the second is the name. The yield of the function is
2318           the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no
2319           subpattern of that name.
2320    
2321           Given the number, you can extract the substring directly, or use one of
2322           the functions described in the previous section. For convenience, there
2323           are also two functions that do the whole job.
2324    
2325           Most    of    the    arguments   of   pcre_copy_named_substring()   and
2326           pcre_get_named_substring() are the same  as  those  for  the  similarly
2327           named  functions  that extract by number. As these are described in the
2328           previous section, they are not re-described here. There  are  just  two
2329           differences:
2330    
2331           First,  instead  of a substring number, a substring name is given. Sec-
2332           ond, there is an extra argument, given at the start, which is a pointer
2333           to  the compiled pattern. This is needed in order to gain access to the
2334           name-to-number translation table.
2335    
2336           These functions call pcre_get_stringnumber(), and if it succeeds,  they
2337           then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
2338           ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
2339           behaviour may not be what you want (see the next section).
2340    
2341    
2342    DUPLICATE SUBPATTERN NAMES
2343    
2344           int pcre_get_stringtable_entries(const pcre *code,
2345                const char *name, char **first, char **last);
2346    
2347           When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
2348           subpatterns are not required to  be  unique.  Normally,  patterns  with
2349           duplicate  names  are such that in any one match, only one of the named
2350           subpatterns participates. An example is shown in the pcrepattern  docu-
2351           mentation.
2352    
2353           When    duplicates   are   present,   pcre_copy_named_substring()   and
2354           pcre_get_named_substring() return the first substring corresponding  to
2355           the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING
2356           (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
2357           function  returns one of the numbers that are associated with the name,
2358           but it is not defined which it is.
2359    
2360           If you want to get full details of all captured substrings for a  given
2361           name,  you  must  use  the pcre_get_stringtable_entries() function. The
2362           first argument is the compiled pattern, and the second is the name. The
2363           third  and  fourth  are  pointers to variables which are updated by the
2364           function. After it has run, they point to the first and last entries in
2365           the  name-to-number  table  for  the  given  name.  The function itself
2366           returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
2367           there  are none. The format of the table is described above in the sec-
2368           tion entitled Information about a  pattern.   Given  all  the  relevant
2369           entries  for the name, you can extract each of their numbers, and hence
2370           the captured data, if any.
2371    
2372    
2373    FINDING ALL POSSIBLE MATCHES
2374    
2375           The traditional matching function uses a  similar  algorithm  to  Perl,
2376           which stops when it finds the first match, starting at a given point in
2377           the subject. If you want to find all possible matches, or  the  longest
2378           possible  match,  consider using the alternative matching function (see
2379           below) instead. If you cannot use the alternative function,  but  still
2380           need  to  find all possible matches, you can kludge it up by making use
2381           of the callout facility, which is described in the pcrecallout documen-
2382           tation.
2383    
2384           What you have to do is to insert a callout right at the end of the pat-
2385           tern.  When your callout function is called, extract and save the  cur-
2386           rent  matched  substring.  Then  return  1, which forces pcre_exec() to
2387           backtrack and try other alternatives. Ultimately, when it runs  out  of
2388           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2389    
2390    
2391    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2392    
2393           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2394                const char *subject, int length, int startoffset,
2395                int options, int *ovector, int ovecsize,
2396                int *workspace, int wscount);
2397    
2398           The  function  pcre_dfa_exec()  is  called  to  match  a subject string
2399           against a compiled pattern, using a matching algorithm that  scans  the
2400           subject  string  just  once, and does not backtrack. This has different
2401           characteristics to the normal algorithm, and  is  not  compatible  with
2402           Perl.  Some  of the features of PCRE patterns are not supported. Never-
2403           theless, there are times when this kind of matching can be useful.  For
2404           a discussion of the two matching algorithms, see the pcrematching docu-
2405           mentation.
2406    
2407           The arguments for the pcre_dfa_exec() function  are  the  same  as  for
2408           pcre_exec(), plus two extras. The ovector argument is used in a differ-
2409           ent way, and this is described below. The other  common  arguments  are
2410           used  in  the  same way as for pcre_exec(), so their description is not
2411           repeated here.
2412    
2413           The two additional arguments provide workspace for  the  function.  The
2414           workspace  vector  should  contain at least 20 elements. It is used for
2415           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2416           workspace  will  be  needed for patterns and subjects where there are a
2417           lot of potential matches.
2418    
2419           Here is an example of a simple call to pcre_dfa_exec():
2420    
2421             int rc;
2422             int ovector[10];
2423             int wspace[20];
2424             rc = pcre_dfa_exec(
2425               re,             /* result of pcre_compile() */
2426               NULL,           /* we didn't study the pattern */
2427               "some string",  /* the subject string */
2428               11,             /* the length of the subject string */
2429               0,              /* start at offset 0 in the subject */
2430               0,              /* default options */
2431               ovector,        /* vector of integers for substring information */
2432               10,             /* number of elements (NOT size in bytes) */
2433               wspace,         /* working space vector */
2434               20);            /* number of elements (NOT size in bytes) */
2435    
2436       Option bits for pcre_dfa_exec()
2437    
2438           The unused bits of the options argument  for  pcre_dfa_exec()  must  be
2439           zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
2440           LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,
2441           PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2442           three of these are the same as for pcre_exec(), so their description is
2443           not repeated here.
2444    
2445             PCRE_PARTIAL
2446    
2447           This  has  the  same general effect as it does for pcre_exec(), but the
2448           details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
2449           pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
2450           PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
2451           been no complete matches, but there is still at least one matching pos-
2452           sibility. The portion of the string that provided the partial match  is
2453           set as the first matching string.
2454    
2455             PCRE_DFA_SHORTEST
2456    
2457           Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
2458           stop as soon as it has found one match. Because of the way the alterna-
2459           tive  algorithm  works, this is necessarily the shortest possible match
2460           at the first possible matching point in the subject string.
2461    
2462             PCRE_DFA_RESTART
2463    
2464           When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
2465           returns  a  partial  match, it is possible to call it again, with addi-
2466           tional subject characters, and have it continue with  the  same  match.
2467           The  PCRE_DFA_RESTART  option requests this action; when it is set, the
2468           workspace and wscount options must reference the same vector as  before
2469           because  data  about  the  match so far is left in them after a partial
2470           match. There is more discussion of this  facility  in  the  pcrepartial
2471           documentation.
2472    
2473       Successful returns from pcre_dfa_exec()
2474    
2475           When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
2476           string in the subject. Note, however, that all the matches from one run
2477           of  the  function  start  at the same point in the subject. The shorter
2478           matches are all initial substrings of the longer matches. For  example,
2479           if the pattern
2480    
2481             <.*>
2482    
2483           is matched against the string
2484    
2485             This is <something> <something else> <something further> no more
2486    
2487           the three matched strings are
2488    
2489             <something>
2490             <something> <something else>
2491             <something> <something else> <something further>
2492    
2493           On  success,  the  yield of the function is a number greater than zero,
2494           which is the number of matched substrings.  The  substrings  themselves
2495           are  returned  in  ovector. Each string uses two elements; the first is
2496           the offset to the start, and the second is the offset to  the  end.  In
2497           fact,  all  the  strings  have the same start offset. (Space could have
2498           been saved by giving this only once, but it was decided to retain  some
2499           compatibility  with  the  way pcre_exec() returns data, even though the
2500           meaning of the strings is different.)
2501    
2502           The strings are returned in reverse order of length; that is, the long-
2503           est  matching  string is given first. If there were too many matches to
2504           fit into ovector, the yield of the function is zero, and the vector  is
2505           filled with the longest matches.
2506    
2507       Error returns from pcre_dfa_exec()
2508    
2509           The  pcre_dfa_exec()  function returns a negative number when it fails.
2510           Many of the errors are the same  as  for  pcre_exec(),  and  these  are
2511           described  above.   There are in addition the following errors that are
2512           specific to pcre_dfa_exec():
2513    
2514             PCRE_ERROR_DFA_UITEM      (-16)
2515    
2516           This return is given if pcre_dfa_exec() encounters an item in the  pat-
2517           tern  that  it  does not support, for instance, the use of \C or a back
2518           reference.
2519    
2520             PCRE_ERROR_DFA_UCOND      (-17)
2521    
2522           This return is given if pcre_dfa_exec()  encounters  a  condition  item
2523           that  uses  a back reference for the condition, or a test for recursion
2524           in a specific group. These are not supported.
2525    
2526             PCRE_ERROR_DFA_UMLIMIT    (-18)
2527    
2528           This return is given if pcre_dfa_exec() is called with an  extra  block
2529           that contains a setting of the match_limit field. This is not supported
2530           (it is meaningless).
2531    
2532             PCRE_ERROR_DFA_WSSIZE     (-19)
2533    
2534           This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
2535           workspace vector.
2536    
2537             PCRE_ERROR_DFA_RECURSE    (-20)
2538    
2539           When  a  recursive subpattern is processed, the matching function calls
2540           itself recursively, using private vectors for  ovector  and  workspace.
2541           This  error  is  given  if  the output vector is not large enough. This
2542           should be extremely rare, as a vector of size 1000 is used.
2543    
2544    
2545    SEE ALSO
2546    
2547           pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
2548           tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2549    
2550    
2551    AUTHOR
2552    
2553           Philip Hazel
2554           University Computing Service
2555           Cambridge CB2 3QH, England.
2556    
2557    
2558    REVISION
2559    
2560           Last updated: 27 November 2007
2561           Copyright (c) 1997-2007 University of Cambridge.
2562    ------------------------------------------------------------------------------
2563    
2564    
2565    PCRECALLOUT(3)                                                  PCRECALLOUT(3)
2566    
2567    
2568    NAME
2569           PCRE - Perl-compatible regular expressions
2570    
2571    
2572    PCRE CALLOUTS
2573    
2574           int (*pcre_callout)(pcre_callout_block *);
2575    
2576           PCRE provides a feature called "callout", which is a means of temporar-
2577           ily passing control to the caller of PCRE  in  the  middle  of  pattern
2578           matching.  The  caller of PCRE provides an external function by putting
2579           its entry point in the global variable pcre_callout. By  default,  this
2580           variable contains NULL, which disables all calling out.
2581    
2582           Within  a  regular  expression,  (?C) indicates the points at which the
2583           external function is to be called.  Different  callout  points  can  be
2584           identified  by  putting  a number less than 256 after the letter C. The
2585           default value is zero.  For  example,  this  pattern  has  two  callout
2586           points:
2587    
2588             (?C1)abc(?C2)def
2589    
2590           If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2591           called, PCRE automatically  inserts  callouts,  all  with  number  255,
2592           before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2593           used with the pattern
2594    
2595             A(\d{2}|--)
2596    
2597           it is processed as if it were
2598    
2599           (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2600    
2601           Notice that there is a callout before and after  each  parenthesis  and
2602           alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2603           progress of pattern matching. The pcretest command has an  option  that
2604           sets  automatic callouts; when it is used, the output indicates how the
2605           pattern is matched. This is useful information when you are  trying  to
2606           optimize the performance of a particular pattern.
2607    
2608    
2609    MISSING CALLOUTS
2610    
2611           You  should  be  aware  that,  because of optimizations in the way PCRE
2612           matches patterns, callouts sometimes do not happen. For example, if the
2613           pattern is
2614    
2615             ab(?C4)cd
2616    
2617           PCRE knows that any matching string must contain the letter "d". If the
2618           subject string is "abyz", the lack of "d" means that  matching  doesn't
2619           ever  start,  and  the  callout is never reached. However, with "abyd",
2620           though the result is still no match, the callout is obeyed.
2621    
2622    
2623    THE CALLOUT INTERFACE
2624    
2625           During matching, when PCRE reaches a callout point, the external  func-
2626           tion  defined by pcre_callout is called (if it is set). This applies to
2627           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2628           only  argument  to  the callout function is a pointer to a pcre_callout
2629           block. This structure contains the following fields:
2630    
2631             int          version;
2632             int          callout_number;
2633             int         *offset_vector;
2634             const char  *subject;
2635             int          subject_length;
2636             int          start_match;
2637             int          current_position;
2638             int          capture_top;
2639             int          capture_last;
2640             void        *callout_data;
2641             int          pattern_position;
2642             int          next_item_length;
2643    
2644           The version field is an integer containing the version  number  of  the
2645           block  format. The initial version was 0; the current version is 1. The
2646           version number will change again in future  if  additional  fields  are
2647           added, but the intention is never to remove any of the existing fields.
2648    
2649           The callout_number field contains the number of the  callout,  as  com-
2650           piled  into  the pattern (that is, the number after ?C for manual call-
2651           outs, and 255 for automatically generated callouts).
2652    
2653           The offset_vector field is a pointer to the vector of offsets that  was
2654           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2655           pcre_exec() is used, the contents can be inspected in order to  extract
2656           substrings  that  have  been  matched  so  far,  in the same way as for
2657           extracting substrings after a match has completed. For  pcre_dfa_exec()
2658           this field is not useful.
2659    
2660           The subject and subject_length fields contain copies of the values that
2661           were passed to pcre_exec().
2662    
2663           The start_match field normally contains the offset within  the  subject
2664           at  which  the  current  match  attempt started. However, if the escape
2665           sequence \K has been encountered, this value is changed to reflect  the
2666           modified  starting  point.  If the pattern is not anchored, the callout
2667           function may be called several times from the same point in the pattern
2668           for different starting points in the subject.
2669    
2670           The  current_position  field  contains the offset within the subject of
2671           the current match pointer.
2672    
2673           When the pcre_exec() function is used, the capture_top  field  contains
2674           one  more than the number of the highest numbered captured substring so
2675           far. If no substrings have been captured, the value of  capture_top  is
2676           one.  This  is always the case when pcre_dfa_exec() is used, because it
2677           does not support captured substrings.
2678    
2679           The capture_last field contains the number of the  most  recently  cap-
2680           tured  substring. If no substrings have been captured, its value is -1.
2681           This is always the case when pcre_dfa_exec() is used.
2682    
2683           The callout_data field contains a value that is passed  to  pcre_exec()
2684           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2685           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2686           structure.  If  no such data was passed, the value of callout_data in a
2687           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2688           structure in the pcreapi documentation.
2689    
2690           The  pattern_position field is present from version 1 of the pcre_call-
2691           out structure. It contains the offset to the next item to be matched in
2692           the pattern string.
2693    
2694           The  next_item_length field is present from version 1 of the pcre_call-
2695           out structure. It contains the length of the next item to be matched in
2696           the  pattern  string. When the callout immediately precedes an alterna-
2697           tion bar, a closing parenthesis, or the end of the pattern, the  length
2698           is  zero.  When the callout precedes an opening parenthesis, the length
2699           is that of the entire subpattern.
2700    
2701           The pattern_position and next_item_length fields are intended  to  help
2702           in  distinguishing between different automatic callouts, which all have
2703           the same callout number. However, they are set for all callouts.
2704    
2705    
2706    RETURN VALUES
2707    
2708           The external callout function returns an integer to PCRE. If the  value
2709           is  zero,  matching  proceeds  as  normal. If the value is greater than
2710           zero, matching fails at the current point, but  the  testing  of  other
2711           matching possibilities goes ahead, just as if a lookahead assertion had
2712           failed. If the value is less than zero, the  match  is  abandoned,  and
2713           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2714    
2715           Negative   values   should   normally   be   chosen  from  the  set  of
2716           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2717           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2718           reserved for use by callout functions; it will never be  used  by  PCRE
2719           itself.
2720    
2721    
2722    AUTHOR
2723    
2724           Philip Hazel
2725           University Computing Service
2726           Cambridge CB2 3QH, England.
2727    
2728    
2729    REVISION
2730    
2731           Last updated: 29 May 2007
2732           Copyright (c) 1997-2007 University of Cambridge.
2733    ------------------------------------------------------------------------------
2734    
2735    
2736    PCRECOMPAT(3)                                                    PCRECOMPAT(3)
2737    
2738    
2739    NAME
2740           PCRE - Perl-compatible regular expressions
2741    
2742    
2743    DIFFERENCES BETWEEN PCRE AND PERL
2744    
2745           This  document describes the differences in the ways that PCRE and Perl
2746           handle regular expressions. The differences described here  are  mainly
2747           with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
2748           some features that are expected to be in the forthcoming Perl 5.10.
2749    
2750           1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2751           of  what  it does have are given in the section on UTF-8 support in the
2752           main pcre page.
2753    
2754           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2755           permits  them,  but they do not mean what you might think. For example,
2756           (?!a){3} does not assert that the next three characters are not "a". It
2757           just asserts that the next character is not "a" three times.
2758    
2759           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2760           tions are counted, but their entries in the offsets  vector  are  never
2761           set.  Perl sets its numerical variables from any such patterns that are
2762           matched before the assertion fails to match something (thereby succeed-
2763           ing),  but  only  if the negative lookahead assertion contains just one
2764           branch.
2765    
2766           4. Though binary zero characters are supported in the  subject  string,
2767           they are not allowed in a pattern string because it is passed as a nor-
2768           mal C string, terminated by zero. The escape sequence \0 can be used in
2769           the pattern to represent a binary zero.
2770    
2771           5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2772           \U, and \N. In fact these are implemented by Perl's general string-han-
2773           dling  and are not part of its pattern matching engine. If any of these
2774           are encountered by PCRE, an error is generated.
2775    
2776           6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2777           is  built  with Unicode character property support. The properties that
2778           can be tested with \p and \P are limited to the general category  prop-
2779           erties  such  as  Lu and Nd, script names such as Greek or Han, and the
2780           derived properties Any and L&.
2781    
2782           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2783           ters  in  between  are  treated as literals. This is slightly different
2784           from Perl in that $ and @ are  also  handled  as  literals  inside  the
2785           quotes.  In Perl, they cause variable interpolation (but of course PCRE
2786           does not have variables). Note the following examples:
2787    
2788               Pattern            PCRE matches      Perl matches
2789    
2790               \Qabc$xyz\E        abc$xyz           abc followed by the
2791                                                      contents of $xyz
2792               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2793               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2794    
2795           The \Q...\E sequence is recognized both inside  and  outside  character
2796           classes.
2797    
2798           8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2799           constructions. However, there is support for recursive  patterns.  This
2800           is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2801           "callout" feature allows an external function to be called during  pat-
2802           tern matching. See the pcrecallout documentation for details.
2803    
2804           9.  Subpatterns  that  are  called  recursively or as "subroutines" are
2805           always treated as atomic groups in  PCRE.  This  is  like  Python,  but
2806           unlike Perl.
2807    
2808           10.  There are some differences that are concerned with the settings of
2809           captured strings when part of  a  pattern  is  repeated.  For  example,
2810           matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
2811           unset, but in PCRE it is set to "b".
2812    
2813           11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
2814           (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2815           the forms without an  argument.  PCRE  does  not  support  (*MARK).  If
2816           (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-
2817           ture group; this is different to Perl.
2818    
2819           12. PCRE provides some extensions to the Perl regular expression facil-
2820           ities.   Perl  5.10  will  include new features that are not in earlier
2821           versions, some of which (such as named parentheses) have been  in  PCRE
2822           for some time. This list is with respect to Perl 5.10:
2823    
2824           (a)  Although  lookbehind  assertions  must match fixed length strings,
2825           each alternative branch of a lookbehind assertion can match a different
2826           length of string. Perl requires them all to have the same length.
2827    
2828           (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2829           meta-character matches only at the very end of the string.
2830    
2831           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2832           cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
2833           ignored.  (Perl can be made to issue a warning.)
2834    
2835           (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2836           fiers is inverted, that is, by default they are not greedy, but if fol-
2837           lowed by a question mark they are.
2838    
2839           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2840           tried only at the first matching position in the subject string.
2841    
2842           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2843           TURE options for pcre_exec() have no Perl equivalents.
2844    
2845           (g) The \R escape sequence can be restricted to match only CR,  LF,  or
2846           CRLF by the PCRE_BSR_ANYCRLF option.
2847    
2848           (h) The callout facility is PCRE-specific.
2849    
2850           (i) The partial matching facility is PCRE-specific.
2851    
2852           (j) Patterns compiled by PCRE can be saved and re-used at a later time,
2853           even on different hosts that have the other endianness.
2854    
2855           (k) The alternative matching function (pcre_dfa_exec())  matches  in  a
2856           different way and is not Perl-compatible.
2857    
2858           (l)  PCRE  recognizes some special sequences such as (*CR) at the start
2859           of a pattern that set overall options that cannot be changed within the
2860           pattern.
2861    
2862    
2863    AUTHOR
2864    
2865           Philip Hazel
2866           University Computing Service
2867           Cambridge CB2 3QH, England.
2868    
2869    
2870    REVISION
2871    
2872           Last updated: 11 September 2007
2873           Copyright (c) 1997-2007 University of Cambridge.
2874    ------------------------------------------------------------------------------
2875    
2876    
2877    PCREPATTERN(3)                                                  PCREPATTERN(3)
2878    
2879    
2880    NAME
2881           PCRE - Perl-compatible regular expressions
2882    
2883    
2884    PCRE REGULAR EXPRESSION DETAILS
2885    
2886           The  syntax and semantics of the regular expressions that are supported
2887           by PCRE are described in detail below. There is a quick-reference  syn-
2888           tax  summary  in  the  pcresyntax  page. Perl's regular expressions are
2889           described in its own documentation, and regular expressions in  general
2890           are  covered in a number of books, some of which have copious examples.
2891           Jeffrey  Friedl's  "Mastering  Regular   Expressions",   published   by
2892           O'Reilly,  covers regular expressions in great detail. This description
2893           of PCRE's regular expressions is intended as reference material.
2894    
2895           The original operation of PCRE was on strings of  one-byte  characters.
2896           However,  there is now also support for UTF-8 character strings. To use
2897           this, you must build PCRE to  include  UTF-8  support,  and  then  call
2898           pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
2899           matching is mentioned in several places below. There is also a  summary
2900           of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
2901           page.
2902    
2903           The remainder of this document discusses the  patterns  that  are  sup-
2904           ported  by  PCRE when its main matching function, pcre_exec(), is used.
2905           From  release  6.0,   PCRE   offers   a   second   matching   function,
2906           pcre_dfa_exec(),  which matches using a different algorithm that is not
2907           Perl-compatible. Some of the features discussed below are not available
2908           when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
2909           alternative function, and how it differs from the normal function,  are
2910           discussed in the pcrematching page.
2911    
2912    
2913    NEWLINE CONVENTIONS
2914    
2915           PCRE  supports five different conventions for indicating line breaks in
2916           strings: a single CR (carriage return) character, a  single  LF  (line-
2917           feed) character, the two-character sequence CRLF, any of the three pre-
2918           ceding, or any Unicode newline sequence. The pcreapi page  has  further
2919           discussion  about newlines, and shows how to set the newline convention
2920           in the options arguments for the compiling and matching functions.
2921    
2922           It is also possible to specify a newline convention by starting a  pat-
2923           tern string with one of the following five sequences:
2924    
2925             (*CR)        carriage return
2926             (*LF)        linefeed
2927             (*CRLF)      carriage return, followed by linefeed
2928             (*ANYCRLF)   any of the three above
2929             (*ANY)       all Unicode newline sequences
2930    
2931           These override the default and the options given to pcre_compile(). For
2932           example, on a Unix system where LF is the default newline sequence, the
2933           pattern
2934    
2935             (*CR)a.b
2936    
2937           changes the convention to CR. That pattern matches "a\nb" because LF is
2938           no longer a newline. Note that these special settings,  which  are  not
2939           Perl-compatible,  are  recognized  only at the very start of a pattern,
2940           and that they must be in upper case.  If  more  than  one  of  them  is
2941           present, the last one is used.
2942    
2943           The  newline  convention  does  not  affect what the \R escape sequence
2944           matches. By default, this is any Unicode  newline  sequence,  for  Perl
2945           compatibility.  However, this can be changed; see the description of \R
2946           in the section entitled "Newline sequences" below. A change of \R  set-
2947           ting can be combined with a change of newline convention.
2948    
2949    
2950    CHARACTERS AND METACHARACTERS
2951    
2952           A  regular  expression  is  a pattern that is matched against a subject
2953           string from left to right. Most characters stand for  themselves  in  a
2954           pattern,  and  match  the corresponding characters in the subject. As a
2955           trivial example, the pattern
2956    
2957             The quick brown fox
2958    
2959           matches a portion of a subject string that is identical to itself. When
2960           caseless  matching is specified (the PCRE_CASELESS option), letters are
2961           matched independently of case. In UTF-8 mode, PCRE  always  understands
2962           the  concept  of case for characters whose values are less than 128, so
2963           caseless matching is always possible. For characters with  higher  val-
2964           ues,  the concept of case is supported if PCRE is compiled with Unicode
2965           property support, but not otherwise.   If  you  want  to  use  caseless
2966           matching  for  characters  128  and above, you must ensure that PCRE is
2967           compiled with Unicode property support as well as with UTF-8 support.
2968    
2969           The power of regular expressions comes  from  the  ability  to  include
2970           alternatives  and  repetitions in the pattern. These are encoded in the
2971           pattern by the use of metacharacters, which do not stand for themselves
2972           but instead are interpreted in some special way.
2973    
2974           There  are  two different sets of metacharacters: those that are recog-
2975           nized anywhere in the pattern except within square brackets, and  those
2976           that  are  recognized  within square brackets. Outside square brackets,
2977           the metacharacters are as follows:
2978    
2979             \      general escape character with several uses
2980             ^      assert start of string (or line, in multiline mode)
2981             $      assert end of string (or line, in multiline mode)
2982             .      match any character except newline (by default)
2983             [      start character class definition
2984             |      start of alternative branch
2985             (      start subpattern
2986             )      end subpattern
2987             ?      extends the meaning of (
2988                    also 0 or 1 quantifier
2989                    also quantifier minimizer
2990             *      0 or more quantifier
2991             +      1 or more quantifier
2992                    also "possessive quantifier"
2993             {      start min/max quantifier
2994    
2995           Part of a pattern that is in square brackets  is  called  a  "character
2996           class". In a character class the only metacharacters are:
2997    
2998             \      general escape character
2999             ^      negate the class, but only if the first character
3000             -      indicates character range
3001             [      POSIX character class (only if followed by POSIX
3002                      syntax)
3003             ]      terminates the character class
3004    
3005           The  following sections describe the use of each of the metacharacters.
3006    
3007    
3008    BACKSLASH
3009    
3010           The backslash character has several uses. Firstly, if it is followed by
3011           a  non-alphanumeric  character,  it takes away any special meaning that
3012           character may have. This  use  of  backslash  as  an  escape  character
3013           applies both inside and outside character classes.
3014    
3015           For  example,  if  you want to match a * character, you write \* in the
3016           pattern.  This escaping action applies whether  or  not  the  following
3017           character  would  otherwise be interpreted as a metacharacter, so it is
3018           always safe to precede a non-alphanumeric  with  backslash  to  specify
3019           that  it stands for itself. In particular, if you want to match a back-
3020           slash, you write \\.
3021    
3022           If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
3023           the  pattern (other than in a character class) and characters between a
3024           # outside a character class and the next newline are ignored. An escap-
3025           ing  backslash  can  be  used to include a whitespace or # character as
3026           part of the pattern.
3027    
3028           If you want to remove the special meaning from a  sequence  of  charac-
3029           ters,  you can do so by putting them between \Q and \E. This is differ-
3030           ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
3031           sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
3032           tion. Note the following examples:
3033    
3034             Pattern            PCRE matches   Perl matches
3035    
3036             \Qabc$xyz\E        abc$xyz        abc followed by the
3037                                                 contents of $xyz
3038             \Qabc\$xyz\E       abc\$xyz       abc\$xyz
3039             \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
3040    
3041           The \Q...\E sequence is recognized both inside  and  outside  character
3042           classes.
3043    
3044       Non-printing characters
3045    
3046           A second use of backslash provides a way of encoding non-printing char-
3047           acters in patterns in a visible manner. There is no restriction on  the
3048           appearance  of non-printing characters, apart from the binary zero that
3049           terminates a pattern, but when a pattern  is  being  prepared  by  text
3050           editing,  it  is  usually  easier  to  use  one of the following escape
3051           sequences than the binary character it represents:
3052    
3053             \a        alarm, that is, the BEL character (hex 07)
3054             \cx       "control-x", where x is any character
3055             \e        escape (hex 1B)
3056             \f        formfeed (hex 0C)
3057             \n        linefeed (hex 0A)
3058             \r        carriage return (hex 0D)
3059             \t        tab (hex 09)
3060             \ddd      character with octal code ddd, or backreference
3061             \xhh      character with hex code hh
3062             \x{hhh..} character with hex code hhh..
3063    
3064           The precise effect of \cx is as follows: if x is a lower  case  letter,
3065           it  is converted to upper case. Then bit 6 of the character (hex 40) is
3066           inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;
3067           becomes hex 7B.
3068    
3069           After  \x, from zero to two hexadecimal digits are read (letters can be
3070           in upper or lower case). Any number of hexadecimal  digits  may  appear
3071           between  \x{  and  },  but the value of the character code must be less
3072           than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3073           the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3074           than the largest Unicode code point, which is 10FFFF.
3075    
3076           If characters other than hexadecimal digits appear between \x{  and  },
3077           or if there is no terminating }, this form of escape is not recognized.
3078           Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal
3079           escape,  with  no  following  digits, giving a character whose value is
3080           zero.
3081    
3082           Characters whose value is less than 256 can be defined by either of the
3083           two  syntaxes  for  \x. There is no difference in the way they are han-
3084           dled. For example, \xdc is exactly the same as \x{dc}.
3085    
3086           After \0 up to two further octal digits are read. If  there  are  fewer
3087           than  two  digits,  just  those  that  are  present  are used. Thus the
3088           sequence \0\x\07 specifies two binary zeros followed by a BEL character
3089           (code  value 7). Make sure you supply two digits after the initial zero
3090           if the pattern character that follows is itself an octal digit.
3091    
3092           The handling of a backslash followed by a digit other than 0 is compli-
3093           cated.  Outside a character class, PCRE reads it and any following dig-
3094           its as a decimal number. If the number is less than  10,  or  if  there
3095           have been at least that many previous capturing left parentheses in the
3096           expression, the entire  sequence  is  taken  as  a  back  reference.  A
3097           description  of how this works is given later, following the discussion
3098           of parenthesized subpatterns.
3099    
3100           Inside a character class, or if the decimal number is  greater  than  9
3101           and  there have not been that many capturing subpatterns, PCRE re-reads
3102           up to three octal digits following the backslash, and uses them to gen-
3103           erate  a data character. Any subsequent digits stand for themselves. In
3104           non-UTF-8 mode, the value of a character specified  in  octal  must  be
3105           less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
3106           example:
3107    
3108             \040   is another way of writing a space
3109             \40    is the same, provided there are fewer than 40
3110                       previous capturing subpatterns
3111             \7     is always a back reference
3112             \11    might be a back reference, or another way of
3113                       writing a tab
3114             \011   is always a tab
3115             \0113  is a tab followed by the character "3"
3116             \113   might be a back reference, otherwise the
3117                       character with octal code 113
3118             \377   might be a back reference, otherwise
3119                       the byte consisting entirely of 1 bits
3120             \81    is either a back reference, or a binary zero
3121                       followed by the two characters "8" and "1"
3122    
3123           Note that octal values of 100 or greater must not be  introduced  by  a
3124           leading zero, because no more than three octal digits are ever read.
3125    
3126           All the sequences that define a single character value can be used both
3127           inside and outside character classes. In addition, inside  a  character
3128           class,  the  sequence \b is interpreted as the backspace character (hex
3129           08), and the sequences \R and \X are interpreted as the characters  "R"
3130           and  "X", respectively. Outside a character class, these sequences have
3131           different meanings (see below).
3132    
3133       Absolute and relative back references
3134    
3135           The sequence \g followed by an unsigned or a negative  number,  option-
3136           ally  enclosed  in braces, is an absolute or relative back reference. A
3137           named back reference can be coded as \g{name}. Back references are dis-
3138           cussed later, following the discussion of parenthesized subpatterns.
3139    
3140       Generic character types
3141    
3142           Another use of backslash is for specifying generic character types. The
3143           following are always recognized:
3144    
3145             \d     any decimal digit
3146             \D     any character that is not a decimal digit
3147             \h     any horizontal whitespace character
3148             \H     any character that is not a horizontal whitespace character
3149             \s     any whitespace character
3150             \S     any character that is not a whitespace character
3151             \v     any vertical whitespace character
3152             \V     any character that is not a vertical whitespace character
3153             \w     any "word" character
3154             \W     any "non-word" character
3155    
3156           Each pair of escape sequences partitions the complete set of characters
3157           into  two disjoint sets. Any given character matches one, and only one,
3158           of each pair.
3159    
3160           These character type sequences can appear both inside and outside char-
3161           acter  classes.  They each match one character of the appropriate type.
3162           If the current matching point is at the end of the subject string,  all
3163           of them fail, since there is no character to match.
3164    
3165           For  compatibility  with Perl, \s does not match the VT character (code
3166           11).  This makes it different from the the POSIX "space" class. The  \s
3167           characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If
3168           "use locale;" is included in a Perl script, \s may match the VT charac-
3169           ter. In PCRE, it never does.
3170    
3171           In  UTF-8 mode, characters with values greater than 128 never match \d,
3172           \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3173           code  character  property  support is available. These sequences retain
3174           their original meanings from before UTF-8 support was available, mainly
3175           for efficiency reasons.
3176    
3177           The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3178           the other sequences, these do match certain high-valued  codepoints  in
3179           UTF-8 mode.  The horizontal space characters are:
3180    
3181             U+0009     Horizontal tab
3182             U+0020     Space
3183             U+00A0     Non-break space
3184             U+1680     Ogham space mark
3185             U+180E     Mongolian vowel separator
3186             U+2000     En quad
3187             U+2001     Em quad
3188             U+2002     En space
3189             U+2003     Em space
3190             U+2004     Three-per-em space
3191             U+2005     Four-per-em space
3192             U+2006     Six-per-em space
3193             U+2007     Figure space
3194             U+2008     Punctuation space
3195             U+2009     Thin space
3196             U+200A     Hair space
3197             U+202F     Narrow no-break space
3198             U+205F     Medium mathematical space
3199             U+3000     Ideographic space
3200    
3201           The vertical space characters are:
3202    
3203             U+000A     Linefeed
3204             U+000B     Vertical tab
3205             U+000C     Formfeed
3206             U+000D     Carriage return
3207             U+0085     Next line
3208             U+2028     Line separator
3209             U+2029     Paragraph separator
3210    
3211           A "word" character is an underscore or any character less than 256 that
3212           is a letter or digit. The definition of  letters  and  digits  is  con-
3213           trolled  by PCRE's low-valued character tables, and may vary if locale-
3214           specific matching is taking place (see "Locale support" in the  pcreapi
3215           page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
3216           systems, or "french" in Windows, some character codes greater than  128
3217           are  used for accented letters, and these are matched by \w. The use of
3218           locales with Unicode is discouraged.
3219    
3220       Newline sequences
3221    
3222           Outside a character class, by default, the escape sequence  \R  matches
3223           any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3224           mode \R is equivalent to the following:
3225    
3226             (?>\r\n|\n|\x0b|\f|\r|\x85)
3227    
3228           This is an example of an "atomic group", details  of  which  are  given
3229           below.  This particular group matches either the two-character sequence
3230           CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
3231           U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3232           return, U+000D), or NEL (next line, U+0085). The two-character sequence
3233           is treated as a single unit that cannot be split.
3234    
3235           In  UTF-8  mode, two additional characters whose codepoints are greater
3236           than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3237           rator,  U+2029).   Unicode character property support is not needed for
3238           these characters to be recognized.
3239    
3240           It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3241           the  complete  set  of  Unicode  line  endings)  by  setting the option
3242           PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3243           (BSR is an abbrevation for "backslash R".) This can be made the default
3244           when PCRE is built; if this is the case, the  other  behaviour  can  be
3245           requested  via  the  PCRE_BSR_UNICODE  option.   It is also possible to
3246           specify these settings by starting a pattern string  with  one  of  the
3247           following sequences:
3248    
3249             (*BSR_ANYCRLF)   CR, LF, or CRLF only
3250             (*BSR_UNICODE)   any Unicode newline sequence
3251    
3252           These override the default and the options given to pcre_compile(), but
3253           they can be overridden by options given to pcre_exec(). Note that these
3254           special settings, which are not Perl-compatible, are recognized only at
3255           the very start of a pattern, and that they must be in  upper  case.  If
3256           more  than  one  of  them is present, the last one is used. They can be
3257           combined with a change of newline convention, for  example,  a  pattern
3258           can start with:
3259    
3260             (*ANY)(*BSR_ANYCRLF)
3261    
3262           Inside a character class, \R matches the letter "R".
3263    
3264       Unicode character properties
3265    
3266           When PCRE is built with Unicode character property support, three addi-
3267           tional escape sequences that match characters with specific  properties
3268           are  available.   When not in UTF-8 mode, these sequences are of course
3269           limited to testing characters whose codepoints are less than  256,  but
3270           they do work in this mode.  The extra escape sequences are:
3271    
3272             \p{xx}   a character with the xx property
3273             \P{xx}   a character without the xx property
3274             \X       an extended Unicode sequence
3275    
3276           The  property  names represented by xx above are limited to the Unicode
3277           script names, the general category properties, and "Any", which matches
3278           any character (including newline). Other properties such as "InMusical-
3279           Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does
3280           not match any characters, so always causes a match failure.
3281    
3282           Sets of Unicode characters are defined as belonging to certain scripts.
3283           A character from one of these sets can be matched using a script  name.
3284           For example:
3285    
3286             \p{Greek}
3287             \P{Han}
3288    
3289           Those  that are not part of an identified script are lumped together as
3290           "Common". The current list of scripts is:
3291    
3292           Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
3293           Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,
3294           Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3295           Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3296           gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,
3297           Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
3298           Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,
3299           Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
3300           Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3301    
3302           Each character has exactly one general category property, specified  by
3303           a two-letter abbreviation. For compatibility with Perl, negation can be
3304           specified by including a circumflex between the opening brace  and  the
3305           property name. For example, \p{^Lu} is the same as \P{Lu}.
3306    
3307           If only one letter is specified with \p or \P, it includes all the gen-
3308           eral category properties that start with that letter. In this case,  in
3309           the  absence of negation, the curly brackets in the escape sequence are
3310           optional; these two examples have the same effect:
3311    
3312             \p{L}
3313             \pL
3314    
3315           The following general category property codes are supported:
3316    
3317             C     Other
3318             Cc    Control
3319             Cf    Format
3320             Cn    Unassigned
3321             Co    Private use
3322             Cs    Surrogate
3323    
3324             L     Letter
3325             Ll    Lower case letter
3326             Lm    Modifier letter
3327             Lo    Other letter
3328             Lt    Title case letter
3329             Lu    Upper case letter
3330    
3331             M     Mark
3332             Mc    Spacing mark
3333             Me    Enclosing mark
3334             Mn    Non-spacing mark
3335    
3336             N     Number
3337             Nd    Decimal number
3338             Nl    Letter number
3339             No    Other number
3340    
3341             P     Punctuation
3342             Pc    Connector punctuation
3343             Pd    Dash punctuation
3344             Pe    Close punctuation
3345             Pf    Final punctuation
3346             Pi    Initial punctuation
3347             Po    Other punctuation
3348             Ps    Open punctuation
3349    
3350             S     Symbol
3351             Sc    Currency symbol
3352             Sk    Modifier symbol
3353             Sm    Mathematical symbol
3354             So    Other symbol
3355    
3356             Z     Separator
3357             Zl    Line separator
3358             Zp    Paragraph separator
3359             Zs    Space separator
3360    
3361           The special property L& is also supported: it matches a character  that
3362           has  the  Lu,  Ll, or Lt property, in other words, a letter that is not
3363           classified as a modifier or "other".
3364    
3365           The Cs (Surrogate) property applies only to  characters  in  the  range
3366           U+D800  to  U+DFFF. Such characters are not valid in UTF-8 strings (see
3367           RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3368           ing  has  been  turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3369           the pcreapi page).
3370    
3371           The long synonyms for these properties  that  Perl  supports  (such  as
3372           \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
3373           any of these properties with "Is".
3374    
3375           No character that is in the Unicode table has the Cn (unassigned) prop-
3376           erty.  Instead, this property is assumed for any code point that is not
3377           in the Unicode table.
3378    
3379           Specifying caseless matching does not affect  these  escape  sequences.
3380           For example, \p{Lu} always matches only upper case letters.
3381    
3382           The  \X  escape  matches  any number of Unicode characters that form an
3383           extended Unicode sequence. \X is equivalent to
3384    
3385             (?>\PM\pM*)
3386    
3387           That is, it matches a character without the "mark"  property,  followed
3388           by  zero  or  more  characters with the "mark" property, and treats the
3389           sequence as an atomic group (see below).  Characters  with  the  "mark"
3390           property  are  typically  accents  that affect the preceding character.
3391           None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
3392           matches any one character.
3393    
3394           Matching  characters  by Unicode property is not fast, because PCRE has
3395           to search a structure that contains  data  for  over  fifteen  thousand
3396           characters. That is why the traditional escape sequences such as \d and
3397           \w do not use Unicode properties in PCRE.
3398    
3399       Resetting the match start
3400    
3401           The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3402           ously  matched  characters  not  to  be  included  in the final matched
3403           sequence. For example, the pattern:
3404    
3405             foo\Kbar
3406    
3407           matches "foobar", but reports that it has matched "bar".  This  feature
3408           is  similar  to  a lookbehind assertion (described below).  However, in
3409           this case, the part of the subject before the real match does not  have
3410           to  be of fixed length, as lookbehind assertions do. The use of \K does
3411           not interfere with the setting of captured  substrings.   For  example,
3412           when the pattern
3413    
3414             (foo)\Kbar
3415    
3416           matches "foobar", the first substring is still set to "foo".
3417    
3418       Simple assertions
3419    
3420           The  final use of backslash is for certain simple assertions. An asser-
3421           tion specifies a condition that has to be met at a particular point  in
3422           a  match, without consuming any characters from the subject string. The
3423           use of subpatterns for more complicated assertions is described  below.
3424           The backslashed assertions are:
3425    
3426             \b     matches at a word boundary
3427             \B     matches when not at a word boundary
3428             \A     matches at the start of the subject
3429             \Z     matches at the end of the subject
3430                     also matches before a newline at the end of the subject
3431             \z     matches only at the end of the subject
3432             \G     matches at the first matching position in the subject
3433    
3434           These  assertions may not appear in character classes (but note that \b
3435           has a different meaning, namely the backspace character, inside a char-
3436           acter class).
3437    
3438           A  word  boundary is a position in the subject string where the current
3439           character and the previous character do not both match \w or  \W  (i.e.
3440           one  matches  \w  and the other matches \W), or the start or end of the
3441           string if the first or last character matches \w, respectively.
3442    
3443           The \A, \Z, and \z assertions differ from  the  traditional  circumflex
3444           and dollar (described in the next section) in that they only ever match
3445           at the very start and end of the subject string, whatever  options  are
3446           set.  Thus,  they are independent of multiline mode. These three asser-
3447           tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3448           affect  only the behaviour of the circumflex and dollar metacharacters.
3449