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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  NAME
14       pcre - Perl-compatible regular expressions.         PCRE - Perl-compatible regular expressions
15    
16    
17    INTRODUCTION
18    
19  SYNOPSIS         The  PCRE  library is a set of functions that implement regular expres-
20       #include <pcre.h>         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.
49    
50           Some features of PCRE can be included, excluded, or  changed  when  the
51           library  is  built.  The pcre_config() function makes it possible for a
52           client to discover which features are  available.  The  features  them-
53           selves  are described in the pcrebuild page. Documentation about build-
54           ing PCRE for various operating systems can be found in the README  file
55           in the source distribution.
56    
57           The  library  contains  a number of undocumented internal functions and
58           data tables that are used by more than one  of  the  exported  external
59           functions,  but  which  are  not  intended for use by external callers.
60           Their names all begin with "_pcre_", which hopefully will  not  provoke
61           any name clashes. In some environments, it is possible to control which
62           external symbols are exported when a shared library is  built,  and  in
63           these cases the undocumented symbols are not exported.
64    
65    
66    USER DOCUMENTATION
67    
68           The  user  documentation  for PCRE comprises a number of different sec-
69           tions. In the "man" format, each of these is a separate "man page".  In
70           the  HTML  format, each is a separate page, linked from the index page.
71           In the plain text format, all the sections are concatenated,  for  ease
72           of searching. The sections are as follows:
73    
74             pcre              this document
75             pcreapi           details of PCRE's native C API
76             pcrebuild         options for building PCRE
77             pcrecallout       details of the callout feature
78             pcrecompat        discussion of Perl compatibility
79             pcrecpp           details of the C++ wrapper
80             pcregrep          description of the pcregrep command
81             pcrematching      discussion of the two matching algorithms
82             pcrepartial       details of the partial matching facility
83             pcrepattern       syntax and semantics of supported
84                                 regular expressions
85             pcreperform       discussion of performance issues
86             pcreposix         the POSIX-compatible C API
87             pcreprecompile    details of saving and re-using precompiled patterns
88             pcresample        discussion of the sample program
89             pcrestack         discussion of stack usage
90             pcretest          description of the pcretest testing command
91    
92       pcre *pcre_compile(const char *pattern, int options,         In addition, in the "man" and HTML formats, there is a short  page  for
93            const char **errptr, int *erroffset,         each C library function, listing its arguments and results.
           const unsigned char *tableptr);  
94    
      pcre_extra *pcre_study(const pcre *code, int options,  
           const char **errptr);  
95    
96       int pcre_exec(const pcre *code, const pcre_extra *extra,  LIMITATIONS
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
97    
98       int pcre_copy_substring(const char *subject, int *ovector,         There  are some size limitations in PCRE but it is hoped that they will
99            int stringcount, int stringnumber, char *buffer,         never in practice be relevant.
           int buffersize);  
100    
101       int pcre_get_substring(const char *subject, int *ovector,         The maximum length of a compiled pattern is 65539 (sic) bytes  if  PCRE
102            int stringcount, int stringnumber,         is compiled with the default internal linkage size of 2. If you want to
103            const char **stringptr);         process regular expressions that are truly enormous,  you  can  compile
104           PCRE  with  an  internal linkage size of 3 or 4 (see the README file in
105           the source distribution and the pcrebuild documentation  for  details).
106           In  these  cases the limit is substantially larger.  However, the speed
107           of execution is slower.
108    
109           All values in repeating quantifiers must be less than 65536. The  maxi-
110           mum  compiled  length  of  subpattern  with an explicit repeat count is
111           30000 bytes. The maximum number of capturing subpatterns is 65535.
112    
113           There is no limit to the number of parenthesized subpatterns, but there
114           can be no more than 65535 capturing subpatterns.
115    
116           The maximum length of name for a named subpattern is 32 characters, and
117           the maximum number of named subpatterns is 10000.
118    
119           The maximum length of a subject string is the largest  positive  number
120           that  an integer variable can hold. However, when using the traditional
121           matching function, PCRE uses recursion to handle subpatterns and indef-
122           inite  repetition.  This means that the available stack space may limit
123           the size of a subject string that can be processed by certain patterns.
124           For a discussion of stack issues, see the pcrestack documentation.
125    
126    
127    UTF-8 AND UNICODE PROPERTY SUPPORT
128    
129           From  release  3.3,  PCRE  has  had  some support for character strings
130           encoded in the UTF-8 format. For release 4.0 this was greatly  extended
131           to  cover  most common requirements, and in release 5.0 additional sup-
132           port for Unicode general category properties was added.
133    
134           In order process UTF-8 strings, you must build PCRE  to  include  UTF-8
135           support  in  the  code,  and, in addition, you must call pcre_compile()
136           with the PCRE_UTF8 option flag. When you do this, both the pattern  and
137           any  subject  strings  that are matched against it are treated as UTF-8
138           strings instead of just strings of bytes.
139    
140           If you compile PCRE with UTF-8 support, but do not use it at run  time,
141           the  library will be a bit bigger, but the additional run time overhead
142           is limited to testing the PCRE_UTF8 flag occasionally, so should not be
143           very big.
144    
145           If PCRE is built with Unicode character property support (which implies
146           UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
147           ported.  The available properties that can be tested are limited to the
148           general category properties such as Lu for an upper case letter  or  Nd
149           for  a  decimal number, the Unicode script names such as Arabic or Han,
150           and the derived properties Any and L&. A full  list  is  given  in  the
151           pcrepattern documentation. Only the short names for properties are sup-
152           ported. For example, \p{L} matches a letter. Its Perl synonym,  \p{Let-
153           ter},  is  not  supported.   Furthermore,  in Perl, many properties may
154           optionally be prefixed by "Is", for compatibility with Perl  5.6.  PCRE
155           does not support this.
156    
157           The following comments apply when PCRE is running in UTF-8 mode:
158    
159           1.  When you set the PCRE_UTF8 flag, the strings passed as patterns and
160           subjects are checked for validity on entry to the  relevant  functions.
161           If an invalid UTF-8 string is passed, an error return is given. In some
162           situations, you may already know  that  your  strings  are  valid,  and
163           therefore want to skip these checks in order to improve performance. If
164           you set the PCRE_NO_UTF8_CHECK flag at compile time  or  at  run  time,
165           PCRE  assumes  that  the  pattern or subject it is given (respectively)
166           contains only valid UTF-8 codes. In this case, it does not diagnose  an
167           invalid  UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
168           PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  may
169           crash.
170    
171           2.  An  unbraced  hexadecimal  escape sequence (such as \xb3) matches a
172           two-byte UTF-8 character if the value is greater than 127.
173    
174           3. Octal numbers up to \777 are recognized, and  match  two-byte  UTF-8
175           characters for values greater than \177.
176    
177           4.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
178           vidual bytes, for example: \x{100}{3}.
179    
180           5. The dot metacharacter matches one UTF-8 character instead of a  sin-
181           gle byte.
182    
183           6.  The  escape sequence \C can be used to match a single byte in UTF-8
184           mode, but its use can lead to some strange effects.  This  facility  is
185           not available in the alternative matching function, pcre_dfa_exec().
186    
187           7.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
188           test characters of any code value, but the characters that PCRE  recog-
189           nizes  as  digits,  spaces,  or  word characters remain the same set as
190           before, all with values less than 256. This remains true even when PCRE
191           includes  Unicode  property support, because to do otherwise would slow
192           down PCRE in many common cases. If you really want to test for a  wider
193           sense  of,  say,  "digit",  you must use Unicode property tests such as
194           \p{Nd}.
195    
196           8. Similarly, characters that match the POSIX named  character  classes
197           are all low-valued characters.
198    
199           9.  Case-insensitive  matching  applies only to characters whose values
200           are less than 128, unless PCRE is built with Unicode property  support.
201           Even  when  Unicode  property support is available, PCRE still uses its
202           own character tables when checking the case of  low-valued  characters,
203           so  as not to degrade performance.  The Unicode property information is
204           used only for characters with higher values. Even when Unicode property
205           support is available, PCRE supports case-insensitive matching only when
206           there is a one-to-one mapping between a letter's  cases.  There  are  a
207           small  number  of  many-to-one  mappings in Unicode; these are not sup-
208           ported by PCRE.
209    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
210    
211       void pcre_free_substring(const char *stringptr);  AUTHOR
212    
213       void pcre_free_substring_list(const char **stringptr);         Philip Hazel
214           University Computing Service,
215           Cambridge CB2 3QH, England.
216    
217       const unsigned char *pcre_maketables(void);         Putting an actual email address here seems to have been a spam  magnet,
218           so I've taken it away. If you want to email me, use my initial and sur-
219           name, separated by a dot, at the domain ucs.cam.ac.uk.
220    
221       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,  Last updated: 23 November 2006
222            int what, void *where);  Copyright (c) 1997-2006 University of Cambridge.
223    ------------------------------------------------------------------------------
224    
      int pcre_info(const pcre *code, int *optptr, *firstcharptr);  
225    
226       char *pcre_version(void);  PCREBUILD(3)                                                      PCREBUILD(3)
227    
      void *(*pcre_malloc)(size_t);  
228    
229       void (*pcre_free)(void *);  NAME
230           PCRE - Perl-compatible regular expressions
231    
232    
233    PCRE BUILD-TIME OPTIONS
234    
235           This  document  describes  the  optional  features  of PCRE that can be
236           selected when the library is compiled. They are all selected, or  dese-
237           lected, by providing options to the configure script that is run before
238           the make command. The complete list of  options  for  configure  (which
239           includes  the  standard  ones such as the selection of the installation
240           directory) can be obtained by running
241    
242  DESCRIPTION           ./configure --help
243       The PCRE library is a set of functions that implement  regu-  
244       lar  expression  pattern  matching using the same syntax and         The following sections describe certain options whose names begin  with
245       semantics as Perl  5,  with  just  a  few  differences  (see         --enable  or  --disable. These settings specify changes to the defaults
246           for the configure command. Because of the  way  that  configure  works,
247       below).  The  current  implementation  corresponds  to  Perl         --enable  and  --disable  always  come  in  pairs, so the complementary
248       5.005, with some additional features  from  later  versions.         option always exists as well, but as it specifies the  default,  it  is
249       This  includes  some  experimental,  incomplete  support for         not described.
250       UTF-8 encoded strings. Details of exactly what is  and  what  
251       is not supported are given below.  
252    C++ SUPPORT
253       PCRE has its own native API,  which  is  described  in  this  
254       document.  There  is  also  a  set of wrapper functions that         By default, the configure script will search for a C++ compiler and C++
255       correspond to the POSIX regular expression API.   These  are         header files. If it finds them, it automatically builds the C++ wrapper
256       described in the pcreposix documentation.         library for PCRE. You can disable this by adding
257    
258       The native API function prototypes are defined in the header           --disable-cpp
259       file  pcre.h,  and  on  Unix  systems  the library itself is  
260       called libpcre.a, so can be accessed by adding -lpcre to the         to the configure command.
261       command  for  linking  an  application  which  calls it. The  
262       header file defines the macros PCRE_MAJOR and PCRE_MINOR  to  
263       contain the major and minor release numbers for the library.  UTF-8 SUPPORT
264       Applications can use these to include support for  different  
265       releases.         To build PCRE with support for UTF-8 character strings, add
266    
267       The functions pcre_compile(), pcre_study(), and  pcre_exec()           --enable-utf8
268       are  used  for compiling and matching regular expressions. A  
269       sample program that demonstrates the simplest way  of  using         to  the  configure  command.  Of  itself, this does not make PCRE treat
270       them  is  given  in the file pcredemo.c. The last section of         strings as UTF-8. As well as compiling PCRE with this option, you  also
271       this man page describes how to run it.         have  have to set the PCRE_UTF8 option when you call the pcre_compile()
272           function.
273       The functions  pcre_copy_substring(),  pcre_get_substring(),  
274       and  pcre_get_substring_list() are convenience functions for  
275       extracting  captured  substrings  from  a  matched   subject  UNICODE CHARACTER PROPERTY SUPPORT
276       string; pcre_free_substring() and pcre_free_substring_list()  
277       are also provided, to free the  memory  used  for  extracted         UTF-8 support allows PCRE to process character values greater than  255
278       strings.         in  the  strings that it handles. On its own, however, it does not pro-
279           vide any facilities for accessing the properties of such characters. If
280       The function pcre_maketables() is used (optionally) to build         you  want  to  be able to use the pattern escapes \P, \p, and \X, which
281       a  set of character tables in the current locale for passing         refer to Unicode character properties, you must add
282       to pcre_compile().  
283             --enable-unicode-properties
284       The function pcre_fullinfo() is used to find out information  
285       about a compiled pattern; pcre_info() is an obsolete version         to the configure command. This implies UTF-8 support, even if you  have
286       which returns only some of the available information, but is         not explicitly requested it.
287       retained   for   backwards   compatibility.    The  function  
288       pcre_version() returns a pointer to a string containing  the         Including  Unicode  property  support  adds around 90K of tables to the
289       version of PCRE and its date of release.         PCRE library, approximately doubling its size. Only the  general  cate-
290           gory  properties  such as Lu and Nd are supported. Details are given in
291       The global variables  pcre_malloc  and  pcre_free  initially         the pcrepattern documentation.
292       contain the entry points of the standard malloc() and free()  
293       functions respectively. PCRE  calls  the  memory  management  
294       functions  via  these  variables,  so  a calling program can  CODE VALUE OF NEWLINE
295       replace them if it  wishes  to  intercept  the  calls.  This  
296       should be done before calling any PCRE functions.         By default, PCRE interprets character 10 (linefeed, LF)  as  indicating
297           the  end  of  a line. This is the normal newline character on Unix-like
298           systems. You can compile PCRE to use character 13 (carriage return, CR)
299           instead, by adding
300  MULTI-THREADING  
301       The PCRE functions can be used in  multi-threading  applica-           --enable-newline-is-cr
302       tions, with the proviso that the memory management functions  
303       pointed to by pcre_malloc and pcre_free are  shared  by  all         to  the  configure  command.  There  is  also  a --enable-newline-is-lf
304       threads.         option, which explicitly specifies linefeed as the newline character.
305    
306       The compiled form of a regular  expression  is  not  altered         Alternatively, you can specify that line endings are to be indicated by
307       during  matching, so the same compiled pattern can safely be         the two character sequence CRLF. If you want this, add
308       used by several threads at once.  
309             --enable-newline-is-crlf
310    
311           to the configure command. There is a fourth option, specified by
312    
313             --enable-newline-is-any
314    
315           which causes PCRE to recognize any Unicode newline sequence.
316    
317           Whatever  line  ending convention is selected when PCRE is built can be
318           overridden when the library functions are called. At build time  it  is
319           conventional to use the standard for your operating system.
320    
321    
322    BUILDING SHARED AND STATIC LIBRARIES
323    
324           The  PCRE building process uses libtool to build both shared and static
325           Unix libraries by default. You can suppress one of these by adding  one
326           of
327    
328             --disable-shared
329             --disable-static
330    
331           to the configure command, as required.
332    
333    
334    POSIX MALLOC USAGE
335    
336           When PCRE is called through the POSIX interface (see the pcreposix doc-
337           umentation), additional working storage is  required  for  holding  the
338           pointers  to capturing substrings, because PCRE requires three integers
339           per substring, whereas the POSIX interface provides only  two.  If  the
340           number of expected substrings is small, the wrapper function uses space
341           on the stack, because this is faster than using malloc() for each call.
342           The default threshold above which the stack is no longer used is 10; it
343           can be changed by adding a setting such as
344    
345             --with-posix-malloc-threshold=20
346    
347           to the configure command.
348    
349    
350    HANDLING VERY LARGE PATTERNS
351    
352           Within a compiled pattern, offset values are used  to  point  from  one
353           part  to another (for example, from an opening parenthesis to an alter-
354           nation metacharacter). By default, two-byte values are used  for  these
355           offsets,  leading  to  a  maximum size for a compiled pattern of around
356           64K. This is sufficient to handle all but the most  gigantic  patterns.
357           Nevertheless,  some  people do want to process enormous patterns, so it
358           is possible to compile PCRE to use three-byte or four-byte  offsets  by
359           adding a setting such as
360    
361             --with-link-size=3
362    
363           to  the  configure  command.  The value given must be 2, 3, or 4. Using
364           longer offsets slows down the operation of PCRE because it has to  load
365           additional bytes when handling them.
366    
367           If  you  build  PCRE with an increased link size, test 2 (and test 5 if
368           you are using UTF-8) will fail. Part of the output of these tests is  a
369           representation  of the compiled pattern, and this changes with the link
370           size.
371    
372    
373    AVOIDING EXCESSIVE STACK USAGE
374    
375           When matching with the pcre_exec() function, PCRE implements backtrack-
376           ing  by  making recursive calls to an internal function called match().
377           In environments where the size of the stack is limited,  this  can  se-
378           verely  limit  PCRE's operation. (The Unix environment does not usually
379           suffer from this problem, but it may sometimes be necessary to increase
380           the  maximum  stack size.  There is a discussion in the pcrestack docu-
381           mentation.) An alternative approach to recursion that uses memory  from
382           the  heap  to remember data, instead of using recursive function calls,
383           has been implemented to work round the problem of limited  stack  size.
384           If you want to build a version of PCRE that works this way, add
385    
386             --disable-stack-for-recursion
387    
388           to  the  configure  command. With this configuration, PCRE will use the
389           pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
390           ment  functions.  Separate  functions are provided because the usage is
391           very predictable: the block sizes requested are always  the  same,  and
392           the  blocks  are always freed in reverse order. A calling program might
393           be able to implement optimized functions that perform better  than  the
394           standard  malloc()  and  free()  functions.  PCRE  runs noticeably more
395           slowly when built in this way. This option affects only the pcre_exec()
396           function; it is not relevant for the the pcre_dfa_exec() function.
397    
398    
399    LIMITING PCRE RESOURCE USAGE
400    
401           Internally,  PCRE has a function called match(), which it calls repeat-
402           edly  (sometimes  recursively)  when  matching  a  pattern   with   the
403           pcre_exec()  function.  By controlling the maximum number of times this
404           function may be called during a single matching operation, a limit  can
405           be  placed  on  the resources used by a single call to pcre_exec(). The
406           limit can be changed at run time, as described in the pcreapi  documen-
407           tation.  The default is 10 million, but this can be changed by adding a
408           setting such as
409    
410             --with-match-limit=500000
411    
412           to  the  configure  command.  This  setting  has  no  effect   on   the
413           pcre_dfa_exec() matching function.
414    
415           In  some  environments  it is desirable to limit the depth of recursive
416           calls of match() more strictly than the total number of calls, in order
417           to  restrict  the maximum amount of stack (or heap, if --disable-stack-
418           for-recursion is specified) that is used. A second limit controls this;
419           it  defaults  to  the  value  that is set for --with-match-limit, which
420           imposes no additional constraints. However, you can set a  lower  limit
421           by adding, for example,
422    
423             --with-match-limit-recursion=10000
424    
425           to  the  configure  command.  This  value can also be overridden at run
426           time.
427    
428    
429    USING EBCDIC CODE
430    
431           PCRE assumes by default that it will run in an  environment  where  the
432           character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
433           PCRE can, however, be compiled to  run  in  an  EBCDIC  environment  by
434           adding
435    
436             --enable-ebcdic
437    
438           to the configure command.
439    
440    
441    SEE ALSO
442    
443           pcreapi(3), pcre_config(3).
444    
445    Last updated: 30 November 2006
446    Copyright (c) 1997-2006 University of Cambridge.
447    ------------------------------------------------------------------------------
448    
449    
450    PCREMATCHING(3)                                                PCREMATCHING(3)
451    
452    
453    NAME
454           PCRE - Perl-compatible regular expressions
455    
456    
457    PCRE MATCHING ALGORITHMS
458    
459           This document describes the two different algorithms that are available
460           in PCRE for matching a compiled regular expression against a given sub-
461           ject  string.  The  "standard"  algorithm  is  the  one provided by the
462           pcre_exec() function.  This works in the same was  as  Perl's  matching
463           function, and provides a Perl-compatible matching operation.
464    
465           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
466           this operates in a different way, and is not  Perl-compatible.  It  has
467           advantages  and disadvantages compared with the standard algorithm, and
468           these are described below.
469    
470           When there is only one possible way in which a given subject string can
471           match  a pattern, the two algorithms give the same answer. A difference
472           arises, however, when there are multiple possibilities. For example, if
473           the pattern
474    
475             ^<.*>
476    
477           is matched against the string
478    
479             <something> <something else> <something further>
480    
481           there are three possible answers. The standard algorithm finds only one
482           of them, whereas the alternative algorithm finds all three.
483    
484    
485    REGULAR EXPRESSIONS AS TREES
486    
487           The set of strings that are matched by a regular expression can be rep-
488           resented  as  a  tree structure. An unlimited repetition in the pattern
489           makes the tree of infinite size, but it is still a tree.  Matching  the
490           pattern  to a given subject string (from a given starting point) can be
491           thought of as a search of the tree.  There are two  ways  to  search  a
492           tree:  depth-first  and  breadth-first, and these correspond to the two
493           matching algorithms provided by PCRE.
494    
495    
496    THE STANDARD MATCHING ALGORITHM
497    
498           In the terminology of Jeffrey Friedl's book Mastering  Regular  Expres-
499           sions,  the  standard  algorithm  is  an "NFA algorithm". It conducts a
500           depth-first search of the pattern tree. That is, it  proceeds  along  a
501           single path through the tree, checking that the subject matches what is
502           required. When there is a mismatch, the algorithm  tries  any  alterna-
503           tives  at  the  current point, and if they all fail, it backs up to the
504           previous branch point in the  tree,  and  tries  the  next  alternative
505           branch  at  that  level.  This often involves backing up (moving to the
506           left) in the subject string as well.  The  order  in  which  repetition
507           branches  are  tried  is controlled by the greedy or ungreedy nature of
508           the quantifier.
509    
510           If a leaf node is reached, a matching string has  been  found,  and  at
511           that  point the algorithm stops. Thus, if there is more than one possi-
512           ble match, this algorithm returns the first one that it finds.  Whether
513           this  is the shortest, the longest, or some intermediate length depends
514           on the way the greedy and ungreedy repetition quantifiers are specified
515           in the pattern.
516    
517           Because  it  ends  up  with a single path through the tree, it is rela-
518           tively straightforward for this algorithm to keep  track  of  the  sub-
519           strings  that  are  matched  by portions of the pattern in parentheses.
520           This provides support for capturing parentheses and back references.
521    
522    
523    THE ALTERNATIVE MATCHING ALGORITHM
524    
525           This algorithm conducts a breadth-first search of  the  tree.  Starting
526           from  the  first  matching  point  in the subject, it scans the subject
527           string from left to right, once, character by character, and as it does
528           this,  it remembers all the paths through the tree that represent valid
529           matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
530           though  it is not implemented as a traditional finite state machine (it
531           keeps multiple states active simultaneously).
532    
533           The scan continues until either the end of the subject is  reached,  or
534           there  are  no more unterminated paths. At this point, terminated paths
535           represent the different matching possibilities (if there are none,  the
536           match  has  failed).   Thus,  if there is more than one possible match,
537           this algorithm finds all of them, and in particular, it finds the long-
538           est.  In PCRE, there is an option to stop the algorithm after the first
539           match (which is necessarily the shortest) has been found.
540    
541           Note that all the matches that are found start at the same point in the
542           subject. If the pattern
543    
544             cat(er(pillar)?)
545    
546           is  matched  against the string "the caterpillar catchment", the result
547           will be the three strings "cat", "cater", and "caterpillar" that  start
548           at the fourth character of the subject. The algorithm does not automat-
549           ically move on to find matches that start at later positions.
550    
551           There are a number of features of PCRE regular expressions that are not
552           supported by the alternative matching algorithm. They are as follows:
553    
554           1.  Because  the  algorithm  finds  all possible matches, the greedy or
555           ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
556           ungreedy quantifiers are treated in exactly the same way. However, pos-
557           sessive quantifiers can make a difference when what follows could  also
558           match what is quantified, for example in a pattern like this:
559    
560             ^a++\w!
561    
562           This  pattern matches "aaab!" but not "aaa!", which would be matched by
563           a non-possessive quantifier. Similarly, if an atomic group is  present,
564           it  is matched as if it were a standalone pattern at the current point,
565           and the longest match is then "locked in" for the rest of  the  overall
566           pattern.
567    
568           2. When dealing with multiple paths through the tree simultaneously, it
569           is not straightforward to keep track of  captured  substrings  for  the
570           different  matching  possibilities,  and  PCRE's implementation of this
571           algorithm does not attempt to do this. This means that no captured sub-
572           strings are available.
573    
574           3.  Because no substrings are captured, back references within the pat-
575           tern are not supported, and cause errors if encountered.
576    
577           4. For the same reason, conditional expressions that use  a  backrefer-
578           ence  as  the  condition or test for a specific group recursion are not
579           supported.
580    
581           5. Callouts are supported, but the value of the  capture_top  field  is
582           always 1, and the value of the capture_last field is always -1.
583    
584           6.  The \C escape sequence, which (in the standard algorithm) matches a
585           single byte, even in UTF-8 mode, is not supported because the  alterna-
586           tive  algorithm  moves  through  the  subject string one character at a
587           time, for all active paths through the tree.
588    
589    
590    ADVANTAGES OF THE ALTERNATIVE ALGORITHM
591    
592           Using the alternative matching algorithm provides the following  advan-
593           tages:
594    
595           1. All possible matches (at a single point in the subject) are automat-
596           ically found, and in particular, the longest match is  found.  To  find
597           more than one match using the standard algorithm, you have to do kludgy
598           things with callouts.
599    
600           2. There is much better support for partial matching. The  restrictions
601           on  the content of the pattern that apply when using the standard algo-
602           rithm for partial matching do not apply to the  alternative  algorithm.
603           For  non-anchored patterns, the starting position of a partial match is
604           available.
605    
606           3. Because the alternative algorithm  scans  the  subject  string  just
607           once,  and  never  needs to backtrack, it is possible to pass very long
608           subject strings to the matching function in  several  pieces,  checking
609           for partial matching each time.
610    
611    
612    DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
613    
614           The alternative algorithm suffers from a number of disadvantages:
615    
616           1.  It  is  substantially  slower  than the standard algorithm. This is
617           partly because it has to search for all possible matches, but  is  also
618           because it is less susceptible to optimization.
619    
620           2. Capturing parentheses and back references are not supported.
621    
622           3. Although atomic groups are supported, their use does not provide the
623           performance advantage that it does for the standard algorithm.
624    
625    Last updated: 24 November 2006
626    Copyright (c) 1997-2006 University of Cambridge.
627    ------------------------------------------------------------------------------
628    
629    
630    PCREAPI(3)                                                          PCREAPI(3)
631    
632    
633    NAME
634           PCRE - Perl-compatible regular expressions
635    
636    
637    PCRE NATIVE API
638    
639           #include <pcre.h>
640    
641           pcre *pcre_compile(const char *pattern, int options,
642                const char **errptr, int *erroffset,
643                const unsigned char *tableptr);
644    
645           pcre *pcre_compile2(const char *pattern, int options,
646                int *errorcodeptr,
647                const char **errptr, int *erroffset,
648                const unsigned char *tableptr);
649    
650           pcre_extra *pcre_study(const pcre *code, int options,
651                const char **errptr);
652    
653           int pcre_exec(const pcre *code, const pcre_extra *extra,
654                const char *subject, int length, int startoffset,
655                int options, int *ovector, int ovecsize);
656    
657           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
658                const char *subject, int length, int startoffset,
659                int options, int *ovector, int ovecsize,
660                int *workspace, int wscount);
661    
662           int pcre_copy_named_substring(const pcre *code,
663                const char *subject, int *ovector,
664                int stringcount, const char *stringname,
665                char *buffer, int buffersize);
666    
667           int pcre_copy_substring(const char *subject, int *ovector,
668                int stringcount, int stringnumber, char *buffer,
669                int buffersize);
670    
671           int pcre_get_named_substring(const pcre *code,
672                const char *subject, int *ovector,
673                int stringcount, const char *stringname,
674                const char **stringptr);
675    
676           int pcre_get_stringnumber(const pcre *code,
677                const char *name);
678    
679           int pcre_get_stringtable_entries(const pcre *code,
680                const char *name, char **first, char **last);
681    
682           int pcre_get_substring(const char *subject, int *ovector,
683                int stringcount, int stringnumber,
684                const char **stringptr);
685    
686           int pcre_get_substring_list(const char *subject,
687                int *ovector, int stringcount, const char ***listptr);
688    
689           void pcre_free_substring(const char *stringptr);
690    
691           void pcre_free_substring_list(const char **stringptr);
692    
693           const unsigned char *pcre_maketables(void);
694    
695           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
696                int what, void *where);
697    
698           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
699    
700           int pcre_refcount(pcre *code, int adjust);
701    
702           int pcre_config(int what, void *where);
703    
704           char *pcre_version(void);
705    
706           void *(*pcre_malloc)(size_t);
707    
708           void (*pcre_free)(void *);
709    
710           void *(*pcre_stack_malloc)(size_t);
711    
712           void (*pcre_stack_free)(void *);
713    
714           int (*pcre_callout)(pcre_callout_block *);
715    
716    
717    PCRE API OVERVIEW
718    
719           PCRE has its own native API, which is described in this document. There
720           are also some wrapper functions that correspond to  the  POSIX  regular
721           expression  API.  These  are  described in the pcreposix documentation.
722           Both of these APIs define a set of C function calls. A C++  wrapper  is
723           distributed with PCRE. It is documented in the pcrecpp page.
724    
725           The  native  API  C  function prototypes are defined in the header file
726           pcre.h, and on Unix systems the library itself is called  libpcre.   It
727           can normally be accessed by adding -lpcre to the command for linking an
728           application  that  uses  PCRE.  The  header  file  defines  the  macros
729           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
730           bers for the library.  Applications can use these  to  include  support
731           for different releases of PCRE.
732    
733           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
734           pcre_exec() are used for compiling and matching regular expressions  in
735           a  Perl-compatible  manner. A sample program that demonstrates the sim-
736           plest way of using them is provided in the file  called  pcredemo.c  in
737           the  source distribution. The pcresample documentation describes how to
738           run it.
739    
740           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
741           ble,  is  also provided. This uses a different algorithm for the match-
742           ing. The alternative algorithm finds all possible matches (at  a  given
743           point  in  the subject), and scans the subject just once. However, this
744           algorithm does not return captured substrings. A description of the two
745           matching  algorithms and their advantages and disadvantages is given in
746           the pcrematching documentation.
747    
748           In addition to the main compiling and  matching  functions,  there  are
749           convenience functions for extracting captured substrings from a subject
750           string that is matched by pcre_exec(). They are:
751    
752             pcre_copy_substring()
753             pcre_copy_named_substring()
754             pcre_get_substring()
755             pcre_get_named_substring()
756             pcre_get_substring_list()
757             pcre_get_stringnumber()
758             pcre_get_stringtable_entries()
759    
760           pcre_free_substring() and pcre_free_substring_list() are also provided,
761           to free the memory used for extracted strings.
762    
763           The  function  pcre_maketables()  is  used  to build a set of character
764           tables  in  the  current  locale   for   passing   to   pcre_compile(),
765           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
766           provided for specialist use.  Most  commonly,  no  special  tables  are
767           passed,  in  which case internal tables that are generated when PCRE is
768           built are used.
769    
770           The function pcre_fullinfo() is used to find out  information  about  a
771           compiled  pattern; pcre_info() is an obsolete version that returns only
772           some of the available information, but is retained for  backwards  com-
773           patibility.   The function pcre_version() returns a pointer to a string
774           containing the version of PCRE and its date of release.
775    
776           The function pcre_refcount() maintains a  reference  count  in  a  data
777           block  containing  a compiled pattern. This is provided for the benefit
778           of object-oriented applications.
779    
780           The global variables pcre_malloc and pcre_free  initially  contain  the
781           entry  points  of  the  standard malloc() and free() functions, respec-
782           tively. PCRE calls the memory management functions via these variables,
783           so  a  calling  program  can replace them if it wishes to intercept the
784           calls. This should be done before calling any PCRE functions.
785    
786           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
787           indirections  to  memory  management functions. These special functions
788           are used only when PCRE is compiled to use  the  heap  for  remembering
789           data, instead of recursive function calls, when running the pcre_exec()
790           function. See the pcrebuild documentation for  details  of  how  to  do
791           this.  It  is  a non-standard way of building PCRE, for use in environ-
792           ments that have limited stacks. Because of the greater  use  of  memory
793           management,  it  runs  more  slowly. Separate functions are provided so
794           that special-purpose external code can be  used  for  this  case.  When
795           used,  these  functions  are always called in a stack-like manner (last
796           obtained, first freed), and always for memory blocks of the same  size.
797           There  is  a discussion about PCRE's stack usage in the pcrestack docu-
798           mentation.
799    
800           The global variable pcre_callout initially contains NULL. It can be set
801           by  the  caller  to  a "callout" function, which PCRE will then call at
802           specified points during a matching operation. Details are given in  the
803           pcrecallout documentation.
804    
805    
806    NEWLINES
807    
808           PCRE  supports four different conventions for indicating line breaks in
809           strings: a single CR (carriage return) character, a  single  LF  (line-
810           feed)  character,  the two-character sequence CRLF, or any Unicode new-
811           line sequence.  The Unicode newline sequences are the three  just  men-
812           tioned, plus the single characters VT (vertical tab, U+000B), FF (form-
813           feed, U+000C), NEL (next line, U+0085), LS  (line  separator,  U+2028),
814           and PS (paragraph separator, U+2029).
815    
816           Each  of  the first three conventions is used by at least one operating
817           system as its standard newline sequence. When PCRE is built, a  default
818           can  be  specified.  The default default is LF, which is the Unix stan-
819           dard. When PCRE is run, the default can be overridden,  either  when  a
820           pattern is compiled, or when it is matched.
821    
822           In the PCRE documentation the word "newline" is used to mean "the char-
823           acter or pair of characters that indicate a line break". The choice  of
824           newline  convention  affects  the  handling of the dot, circumflex, and
825           dollar metacharacters, the handling of #-comments in /x mode, and, when
826           CRLF  is a recognized line ending sequence, the match position advance-
827           ment for a non-anchored pattern. The choice of newline convention  does
828           not affect the interpretation of the \n or \r escape sequences.
829    
830    
831    MULTITHREADING
832    
833           The  PCRE  functions  can be used in multi-threading applications, with
834           the  proviso  that  the  memory  management  functions  pointed  to  by
835           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
836           callout function pointed to by pcre_callout, are shared by all threads.
837    
838           The  compiled form of a regular expression is not altered during match-
839           ing, so the same compiled pattern can safely be used by several threads
840           at once.
841    
842    
843    SAVING PRECOMPILED PATTERNS FOR LATER USE
844    
845           The compiled form of a regular expression can be saved and re-used at a
846           later time, possibly by a different program, and even on a  host  other
847           than  the  one  on  which  it  was  compiled.  Details are given in the
848           pcreprecompile documentation.
849    
850    
851    CHECKING BUILD-TIME OPTIONS
852    
853           int pcre_config(int what, void *where);
854    
855           The function pcre_config() makes it possible for a PCRE client to  dis-
856           cover which optional features have been compiled into the PCRE library.
857           The pcrebuild documentation has more details about these optional  fea-
858           tures.
859    
860           The  first  argument  for pcre_config() is an integer, specifying which
861           information is required; the second argument is a pointer to a variable
862           into  which  the  information  is  placed. The following information is
863           available:
864    
865             PCRE_CONFIG_UTF8
866    
867           The output is an integer that is set to one if UTF-8 support is  avail-
868           able; otherwise it is set to zero.
869    
870             PCRE_CONFIG_UNICODE_PROPERTIES
871    
872           The  output  is  an  integer  that is set to one if support for Unicode
873           character properties is available; otherwise it is set to zero.
874    
875             PCRE_CONFIG_NEWLINE
876    
877           The output is an integer whose value specifies  the  default  character
878           sequence  that is recognized as meaning "newline". The four values that
879           are supported are: 10 for LF, 13 for CR, 3338 for CRLF, and -1 for ANY.
880           The default should normally be the standard sequence for your operating
881           system.
882    
883             PCRE_CONFIG_LINK_SIZE
884    
885           The output is an integer that contains the number  of  bytes  used  for
886           internal linkage in compiled regular expressions. The value is 2, 3, or
887           4. Larger values allow larger regular expressions to  be  compiled,  at
888           the  expense  of  slower matching. The default value of 2 is sufficient
889           for all but the most massive patterns, since  it  allows  the  compiled
890           pattern to be up to 64K in size.
891    
892             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
893    
894           The  output  is  an integer that contains the threshold above which the
895           POSIX interface uses malloc() for output vectors. Further  details  are
896           given in the pcreposix documentation.
897    
898             PCRE_CONFIG_MATCH_LIMIT
899    
900           The output is an integer that gives the default limit for the number of
901           internal matching function calls in a  pcre_exec()  execution.  Further
902           details are given with pcre_exec() below.
903    
904             PCRE_CONFIG_MATCH_LIMIT_RECURSION
905    
906           The  output is an integer that gives the default limit for the depth of
907           recursion when calling the internal matching function in a  pcre_exec()
908           execution. Further details are given with pcre_exec() below.
909    
910             PCRE_CONFIG_STACKRECURSE
911    
912           The  output is an integer that is set to one if internal recursion when
913           running pcre_exec() is implemented by recursive function calls that use
914           the  stack  to remember their state. This is the usual way that PCRE is
915           compiled. The output is zero if PCRE was compiled to use blocks of data
916           on  the  heap  instead  of  recursive  function  calls.  In  this case,
917           pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
918           blocks on the heap, thus avoiding the use of the stack.
919    
920    
921  COMPILING A PATTERN  COMPILING A PATTERN
      The function pcre_compile() is called to compile  a  pattern  
      into  an internal form. The pattern is a C string terminated  
      by a binary zero, and is passed in the argument  pattern.  A  
      pointer  to  a  single  block of memory that is obtained via  
      pcre_malloc is returned. This contains the compiled code and  
      related  data.  The  pcre  type  is defined for the returned  
      block; this is a typedef for a structure whose contents  are  
      not  externally  defined. It is up to the caller to free the  
      memory when it is no longer required.  
   
      Although the compiled code of a PCRE regex  is  relocatable,  
      that is, it does not depend on memory location, the complete  
      pcre data block is not fully relocatable,  because  it  con-  
      tains  a  copy of the tableptr argument, which is an address  
      (see below).  
   
      The size of a compiled pattern is  roughly  proportional  to  
      the length of the pattern string, except that each character  
      class (other than those containing just a single  character,  
      negated  or  not)  requires 33 bytes, and repeat quantifiers  
      with a minimum greater than one or a bounded  maximum  cause  
      the  relevant  portions of the compiled pattern to be repli-  
      cated.  
   
      The options argument contains independent bits  that  affect  
      the  compilation.  It  should  be  zero  if  no  options are  
      required. Some of the options, in particular, those that are  
      compatible  with Perl, can also be set and unset from within  
      the pattern (see the detailed description of regular expres-  
      sions below). For these options, the contents of the options  
      argument specifies their initial settings at  the  start  of  
      compilation  and  execution. The PCRE_ANCHORED option can be  
      set at the time of matching as well as at compile time.  
   
      If errptr is NULL, pcre_compile() returns NULL  immediately.  
      Otherwise, if compilation of a pattern fails, pcre_compile()  
      returns NULL, and sets the variable pointed to by errptr  to  
      point  to a textual error message. The offset from the start  
      of  the  pattern  to  the  character  where  the  error  was  
      discovered   is   placed  in  the  variable  pointed  to  by  
      erroffset, which must not be NULL. If it  is,  an  immediate  
      error is given.  
   
      If the final  argument,  tableptr,  is  NULL,  PCRE  uses  a  
      default  set  of character tables which are built when it is  
      compiled, using the default C  locale.  Otherwise,  tableptr  
      must  be  the result of a call to pcre_maketables(). See the  
      section on locale support below.  
   
      This code fragment shows a typical straightforward  call  to  
      pcre_compile():  
   
        pcre *re;  
        const char *error;  
        int erroffset;  
        re = pcre_compile(  
          "^A.*Z",          /* the pattern */  
          0,                /* default options */  
          &error,           /* for error message */  
          &erroffset,       /* for error offset */  
          NULL);            /* use default character tables */  
   
      The following option bits are defined in the header file:  
   
        PCRE_ANCHORED  
   
      If this bit is set, the pattern is forced to be  "anchored",  
      that is, it is constrained to match only at the start of the  
      string which is being searched (the "subject string").  This  
      effect can also be achieved by appropriate constructs in the  
      pattern itself, which is the only way to do it in Perl.  
   
        PCRE_CASELESS  
   
      If this bit is set, letters in the pattern match both  upper  
      and  lower  case  letters.  It  is  equivalent  to Perl's /i  
      option.  
   
        PCRE_DOLLAR_ENDONLY  
   
      If this bit is set, a dollar metacharacter  in  the  pattern  
      matches  only at the end of the subject string. Without this  
      option, a dollar also matches immediately before  the  final  
      character  if it is a newline (but not before any other new-  
      lines).  The  PCRE_DOLLAR_ENDONLY  option  is   ignored   if  
      PCRE_MULTILINE is set. There is no equivalent to this option  
      in Perl.  
   
        PCRE_DOTALL  
   
      If this bit is  set,  a  dot  metacharater  in  the  pattern  
      matches all characters, including newlines. Without it, new-  
      lines are excluded. This option is equivalent to  Perl's  /s  
      option.  A negative class such as [^a] always matches a new-  
      line character, independent of the setting of this option.  
   
        PCRE_EXTENDED  
   
      If this bit is set, whitespace data characters in  the  pat-  
      tern  are  totally  ignored  except when escaped or inside a  
      character class, and characters between an unescaped #  out-  
      side  a  character  class  and  the  next newline character,  
      inclusive, are also ignored. This is equivalent to Perl's /x  
      option,  and  makes  it  possible to include comments inside  
      complicated patterns. Note, however, that this applies  only  
      to  data  characters. Whitespace characters may never appear  
      within special character sequences in a pattern, for example  
      within  the sequence (?( which introduces a conditional sub-  
      pattern.  
   
        PCRE_EXTRA  
   
      This option was invented in  order  to  turn  on  additional  
      functionality of PCRE that is incompatible with Perl, but it  
      is currently of very little use. When set, any backslash  in  
      a  pattern  that is followed by a letter that has no special  
      meaning causes an error, thus reserving  these  combinations  
      for  future  expansion.  By default, as in Perl, a backslash  
      followed by a letter with no special meaning is treated as a  
      literal.  There  are at present no other features controlled  
      by this option. It can also be set by a (?X) option  setting  
      within a pattern.  
   
        PCRE_MULTILINE  
   
      By default, PCRE treats the subject string as consisting  of  
      a  single "line" of characters (even if it actually contains  
      several newlines). The "start  of  line"  metacharacter  (^)  
      matches  only  at the start of the string, while the "end of  
      line" metacharacter ($) matches  only  at  the  end  of  the  
      string,    or   before   a   terminating   newline   (unless  
      PCRE_DOLLAR_ENDONLY is set). This is the same as Perl.  
   
      When PCRE_MULTILINE it is set, the "start of line" and  "end  
      of  line"  constructs match immediately following or immedi-  
      ately before any newline  in  the  subject  string,  respec-  
      tively,  as  well  as  at  the  very  start and end. This is  
      equivalent to Perl's /m option. If there are no "\n" charac-  
      ters  in  a subject string, or no occurrences of ^ or $ in a  
      pattern, setting PCRE_MULTILINE has no effect.  
   
        PCRE_UNGREEDY  
   
      This option inverts the "greediness" of the  quantifiers  so  
      that  they  are  not greedy by default, but become greedy if  
      followed by "?". It is not compatible with Perl. It can also  
      be set by a (?U) option setting within the pattern.  
   
        PCRE_UTF8  
   
      This option causes PCRE to regard both the pattern  and  the  
      subject  as strings of UTF-8 characters instead of just byte  
      strings. However, it is available  only  if  PCRE  has  been  
      built  to  include  UTF-8  support.  If not, the use of this  
      option provokes an error. Support for UTF-8 is new,  experi-  
      mental,  and incomplete.  Details of exactly what it entails  
      are given below.  
922    
923           pcre *pcre_compile(const char *pattern, int options,
924                const char **errptr, int *erroffset,
925                const unsigned char *tableptr);
926    
927           pcre *pcre_compile2(const char *pattern, int options,
928                int *errorcodeptr,
929                const char **errptr, int *erroffset,
930                const unsigned char *tableptr);
931    
932           Either of the functions pcre_compile() or pcre_compile2() can be called
933           to compile a pattern into an internal form. The only difference between
934           the  two interfaces is that pcre_compile2() has an additional argument,
935           errorcodeptr, via which a numerical error code can be returned.
936    
937           The pattern is a C string terminated by a binary zero, and is passed in
938           the  pattern  argument.  A  pointer to a single block of memory that is
939           obtained via pcre_malloc is returned. This contains the  compiled  code
940           and related data. The pcre type is defined for the returned block; this
941           is a typedef for a structure whose contents are not externally defined.
942           It is up to the caller to free the memory (via pcre_free) when it is no
943           longer required.
944    
945           Although the compiled code of a PCRE regex is relocatable, that is,  it
946           does not depend on memory location, the complete pcre data block is not
947           fully relocatable, because it may contain a copy of the tableptr  argu-
948           ment, which is an address (see below).
949    
950           The options argument contains various bit settings that affect the com-
951           pilation. It should be zero if no options are required.  The  available
952           options  are  described  below. Some of them, in particular, those that
953           are compatible with Perl, can also be set and  unset  from  within  the
954           pattern  (see  the  detailed  description in the pcrepattern documenta-
955           tion). For these options, the contents of the options  argument  speci-
956           fies  their initial settings at the start of compilation and execution.
957           The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at  the  time
958           of matching as well as at compile time.
959    
960           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
961           if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
962           sets the variable pointed to by errptr to point to a textual error mes-
963           sage. This is a static string that is part of the library. You must not
964           try to free it. The offset from the start of the pattern to the charac-
965           ter where the error was discovered is placed in the variable pointed to
966           by  erroffset,  which must not be NULL. If it is, an immediate error is
967           given.
968    
969           If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
970           codeptr  argument is not NULL, a non-zero error code number is returned
971           via this argument in the event of an error. This is in addition to  the
972           textual error message. Error codes and messages are listed below.
973    
974           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
975           character tables that are  built  when  PCRE  is  compiled,  using  the
976           default  C  locale.  Otherwise, tableptr must be an address that is the
977           result of a call to pcre_maketables(). This value is  stored  with  the
978           compiled  pattern,  and used again by pcre_exec(), unless another table
979           pointer is passed to it. For more discussion, see the section on locale
980           support below.
981    
982           This  code  fragment  shows a typical straightforward call to pcre_com-
983           pile():
984    
985             pcre *re;
986             const char *error;
987             int erroffset;
988             re = pcre_compile(
989               "^A.*Z",          /* the pattern */
990               0,                /* default options */
991               &error,           /* for error message */
992               &erroffset,       /* for error offset */
993               NULL);            /* use default character tables */
994    
995           The following names for option bits are defined in  the  pcre.h  header
996           file:
997    
998             PCRE_ANCHORED
999    
1000           If this bit is set, the pattern is forced to be "anchored", that is, it
1001           is constrained to match only at the first matching point in the  string
1002           that  is being searched (the "subject string"). This effect can also be
1003           achieved by appropriate constructs in the pattern itself, which is  the
1004           only way to do it in Perl.
1005    
1006             PCRE_AUTO_CALLOUT
1007    
1008           If this bit is set, pcre_compile() automatically inserts callout items,
1009           all with number 255, before each pattern item. For  discussion  of  the
1010           callout facility, see the pcrecallout documentation.
1011    
1012             PCRE_CASELESS
1013    
1014           If  this  bit is set, letters in the pattern match both upper and lower
1015           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
1016           changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1017           always understands the concept of case for characters whose values  are
1018           less  than 128, so caseless matching is always possible. For characters
1019           with higher values, the concept of case is supported if  PCRE  is  com-
1020           piled  with Unicode property support, but not otherwise. If you want to
1021           use caseless matching for characters 128 and  above,  you  must  ensure
1022           that  PCRE  is  compiled  with Unicode property support as well as with
1023           UTF-8 support.
1024    
1025             PCRE_DOLLAR_ENDONLY
1026    
1027           If this bit is set, a dollar metacharacter in the pattern matches  only
1028           at  the  end  of the subject string. Without this option, a dollar also
1029           matches immediately before a newline at the end of the string (but  not
1030           before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1031           if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
1032           Perl, and no way to set it within a pattern.
1033    
1034             PCRE_DOTALL
1035    
1036           If this bit is set, a dot metacharater in the pattern matches all char-
1037           acters, including those that indicate newline. Without it, a  dot  does
1038           not  match  when  the  current position is at a newline. This option is
1039           equivalent to Perl's /s option, and it can be changed within a  pattern
1040           by  a (?s) option setting. A negative class such as [^a] always matches
1041           newline characters, independent of the setting of this option.
1042    
1043             PCRE_DUPNAMES
1044    
1045           If this bit is set, names used to identify capturing  subpatterns  need
1046           not be unique. This can be helpful for certain types of pattern when it
1047           is known that only one instance of the named  subpattern  can  ever  be
1048           matched.  There  are  more details of named subpatterns below; see also
1049           the pcrepattern documentation.
1050    
1051             PCRE_EXTENDED
1052    
1053           If this bit is set, whitespace  data  characters  in  the  pattern  are
1054           totally ignored except when escaped or inside a character class. White-
1055           space does not include the VT character (code 11). In addition, charac-
1056           ters between an unescaped # outside a character class and the next new-
1057           line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
1058           option,  and  it  can be changed within a pattern by a (?x) option set-
1059           ting.
1060    
1061           This option makes it possible to include  comments  inside  complicated
1062           patterns.   Note,  however,  that this applies only to data characters.
1063           Whitespace  characters  may  never  appear  within  special   character
1064           sequences  in  a  pattern,  for  example  within the sequence (?( which
1065           introduces a conditional subpattern.
1066    
1067             PCRE_EXTRA
1068    
1069           This option was invented in order to turn on  additional  functionality
1070           of  PCRE  that  is  incompatible with Perl, but it is currently of very
1071           little use. When set, any backslash in a pattern that is followed by  a
1072           letter  that  has  no  special  meaning causes an error, thus reserving
1073           these combinations for future expansion. By  default,  as  in  Perl,  a
1074           backslash  followed by a letter with no special meaning is treated as a
1075           literal. (Perl can, however, be persuaded to give a warning for  this.)
1076           There  are  at  present no other features controlled by this option. It
1077           can also be set by a (?X) option setting within a pattern.
1078    
1079             PCRE_FIRSTLINE
1080    
1081           If this option is set, an  unanchored  pattern  is  required  to  match
1082           before  or  at  the  first  newline  in  the subject string, though the
1083           matched text may continue over the newline.
1084    
1085             PCRE_MULTILINE
1086    
1087           By default, PCRE treats the subject string as consisting  of  a  single
1088           line  of characters (even if it actually contains newlines). The "start
1089           of line" metacharacter (^) matches only at the  start  of  the  string,
1090           while  the  "end  of line" metacharacter ($) matches only at the end of
1091           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1092           is set). This is the same as Perl.
1093    
1094           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
1095           constructs match immediately following or immediately  before  internal
1096           newlines  in  the  subject string, respectively, as well as at the very
1097           start and end. This is equivalent to Perl's /m option, and  it  can  be
1098           changed within a pattern by a (?m) option setting. If there are no new-
1099           lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
1100           setting PCRE_MULTILINE has no effect.
1101    
1102             PCRE_NEWLINE_CR
1103             PCRE_NEWLINE_LF
1104             PCRE_NEWLINE_CRLF
1105             PCRE_NEWLINE_ANY
1106    
1107           These  options  override the default newline definition that was chosen
1108           when PCRE was built. Setting the first or the second specifies  that  a
1109           newline  is  indicated  by a single character (CR or LF, respectively).
1110           Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
1111           two-character  CRLF  sequence.  Setting PCRE_NEWLINE_ANY specifies that
1112           any Unicode newline sequence should be recognized. The Unicode  newline
1113           sequences  are  the three just mentioned, plus the single characters VT
1114           (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085),
1115           LS  (line separator, U+2028), and PS (paragraph separator, U+2029). The
1116           last two are recognized only in UTF-8 mode.
1117    
1118           The newline setting in the  options  word  uses  three  bits  that  are
1119           treated  as  a  number, giving eight possibilities. Currently only five
1120           are used (default plus the four values above). This means that  if  you
1121           set  more  than  one  newline option, the combination may or may not be
1122           sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is  equiva-
1123           lent  to PCRE_NEWLINE_CRLF, but other combinations yield unused numbers
1124           and cause an error.
1125    
1126           The only time that a line break is specially recognized when  compiling
1127           a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
1128           character class is encountered. This indicates  a  comment  that  lasts
1129           until  after the next line break sequence. In other circumstances, line
1130           break  sequences  are  treated  as  literal  data,   except   that   in
1131           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1132           and are therefore ignored.
1133    
1134           The newline option that is set at compile time becomes the default that
1135           is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1136    
1137             PCRE_NO_AUTO_CAPTURE
1138    
1139           If this option is set, it disables the use of numbered capturing paren-
1140           theses  in the pattern. Any opening parenthesis that is not followed by
1141           ? behaves as if it were followed by ?: but named parentheses can  still
1142           be  used  for  capturing  (and  they acquire numbers in the usual way).
1143           There is no equivalent of this option in Perl.
1144    
1145             PCRE_UNGREEDY
1146    
1147           This option inverts the "greediness" of the quantifiers  so  that  they
1148           are  not greedy by default, but become greedy if followed by "?". It is
1149           not compatible with Perl. It can also be set by a (?U)  option  setting
1150           within the pattern.
1151    
1152             PCRE_UTF8
1153    
1154           This  option  causes PCRE to regard both the pattern and the subject as
1155           strings of UTF-8 characters instead of single-byte  character  strings.
1156           However,  it is available only when PCRE is built to include UTF-8 sup-
1157           port. If not, the use of this option provokes an error. Details of  how
1158           this  option  changes the behaviour of PCRE are given in the section on
1159           UTF-8 support in the main pcre page.
1160    
1161             PCRE_NO_UTF8_CHECK
1162    
1163           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1164           automatically  checked. If an invalid UTF-8 sequence of bytes is found,
1165           pcre_compile() returns an error. If you already know that your  pattern
1166           is  valid, and you want to skip this check for performance reasons, you
1167           can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
1168           passing an invalid UTF-8 string as a pattern is undefined. It may cause
1169           your program to crash.  Note that this option can  also  be  passed  to
1170           pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1171           ing of subject strings.
1172    
1173    
1174    COMPILATION ERROR CODES
1175    
1176           The following table lists the error  codes  than  may  be  returned  by
1177           pcre_compile2(),  along with the error messages that may be returned by
1178           both compiling functions. As PCRE has developed, some error codes  have
1179           fallen out of use. To avoid confusion, they have not been re-used.
1180    
1181              0  no error
1182              1  \ at end of pattern
1183              2  \c at end of pattern
1184              3  unrecognized character follows \
1185              4  numbers out of order in {} quantifier
1186              5  number too big in {} quantifier
1187              6  missing terminating ] for character class
1188              7  invalid escape sequence in character class
1189              8  range out of order in character class
1190              9  nothing to repeat
1191             10  [this code is not in use]
1192             11  internal error: unexpected repeat
1193             12  unrecognized character after (?
1194             13  POSIX named classes are supported only within a class
1195             14  missing )
1196             15  reference to non-existent subpattern
1197             16  erroffset passed as NULL
1198             17  unknown option bit(s) set
1199             18  missing ) after comment
1200             19  [this code is not in use]
1201             20  regular expression too large
1202             21  failed to get memory
1203             22  unmatched parentheses
1204             23  internal error: code overflow
1205             24  unrecognized character after (?<
1206             25  lookbehind assertion is not fixed length
1207             26  malformed number or name after (?(
1208             27  conditional group contains more than two branches
1209             28  assertion expected after (?(
1210             29  (?R or (?digits must be followed by )
1211             30  unknown POSIX class name
1212             31  POSIX collating elements are not supported
1213             32  this version of PCRE is not compiled with PCRE_UTF8 support
1214             33  [this code is not in use]
1215             34  character value in \x{...} sequence is too large
1216             35  invalid condition (?(0)
1217             36  \C not allowed in lookbehind assertion
1218             37  PCRE does not support \L, \l, \N, \U, or \u
1219             38  number after (?C is > 255
1220             39  closing ) for (?C expected
1221             40  recursive call could loop indefinitely
1222             41  unrecognized character after (?P
1223             42  syntax error in subpattern name (missing terminator)
1224             43  two named subpatterns have the same name
1225             44  invalid UTF-8 string
1226             45  support for \P, \p, and \X has not been compiled
1227             46  malformed \P or \p sequence
1228             47  unknown property name after \P or \p
1229             48  subpattern name is too long (maximum 32 characters)
1230             49  too many named subpatterns (maximum 10,000)
1231             50  repeated subpattern is too long
1232             51  octal value is greater than \377 (not in UTF-8 mode)
1233             52  internal error: overran compiling workspace
1234             53   internal  error:  previously-checked  referenced  subpattern not
1235           found
1236             54  DEFINE group contains more than one branch
1237             55  repeating a DEFINE group is not allowed
1238             56  inconsistent NEWLINE options"
1239    
1240    
1241  STUDYING A PATTERN  STUDYING A PATTERN
      When a pattern is going to be  used  several  times,  it  is  
      worth  spending  more time analyzing it in order to speed up  
      the time taken for matching. The function pcre_study() takes  
      a  pointer  to a compiled pattern as its first argument, and  
      returns a pointer to a pcre_extra block (another typedef for  
      a  structure  with  hidden  contents)  containing additional  
      information  about  the  pattern;  this  can  be  passed  to  
      pcre_exec(). If no additional information is available, NULL  
      is returned.  
   
      The second argument contains option  bits.  At  present,  no  
      options  are  defined  for  pcre_study(),  and this argument  
      should always be zero.  
   
      The third argument for pcre_study() is a pointer to an error  
      message. If studying succeeds (even if no data is returned),  
      the variable it points to  is  set  to  NULL.  Otherwise  it  
      points to a textual error message.  
   
      This is a typical call to pcre_study():  
   
        pcre_extra *pe;  
        pe = pcre_study(  
          re,             /* result of pcre_compile() */  
          0,              /* no options exist */  
          &error);        /* set to NULL or points to a message */  
   
      At present, studying a  pattern  is  useful  only  for  non-  
      anchored  patterns  that do not have a single fixed starting  
      character. A  bitmap  of  possible  starting  characters  is  
      created.  
1242    
1243           pcre_extra *pcre_study(const pcre *code, int options
1244                const char **errptr);
1245    
1246           If a compiled pattern is going to be used several times,  it  is  worth
1247           spending more time analyzing it in order to speed up the time taken for
1248           matching. The function pcre_study() takes a pointer to a compiled  pat-
1249           tern as its first argument. If studying the pattern produces additional
1250           information that will help speed up matching,  pcre_study()  returns  a
1251           pointer  to a pcre_extra block, in which the study_data field points to
1252           the results of the study.
1253    
1254           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1255           pcre_exec().  However,  a  pcre_extra  block also contains other fields
1256           that can be set by the caller before the block  is  passed;  these  are
1257           described below in the section on matching a pattern.
1258    
1259           If  studying  the  pattern  does not produce any additional information
1260           pcre_study() returns NULL. In that circumstance, if the calling program
1261           wants  to  pass  any of the other fields to pcre_exec(), it must set up
1262           its own pcre_extra block.
1263    
1264           The second argument of pcre_study() contains option bits.  At  present,
1265           no options are defined, and this argument should always be zero.
1266    
1267           The  third argument for pcre_study() is a pointer for an error message.
1268           If studying succeeds (even if no data is  returned),  the  variable  it
1269           points  to  is  set  to NULL. Otherwise it is set to point to a textual
1270           error message. This is a static string that is part of the library. You
1271           must  not  try  to  free it. You should test the error pointer for NULL
1272           after calling pcre_study(), to be sure that it has run successfully.
1273    
1274           This is a typical call to pcre_study():
1275    
1276             pcre_extra *pe;
1277             pe = pcre_study(
1278               re,             /* result of pcre_compile() */
1279               0,              /* no options exist */
1280               &error);        /* set to NULL or points to a message */
1281    
1282           At present, studying a pattern is useful only for non-anchored patterns
1283           that  do not have a single fixed starting character. A bitmap of possi-
1284           ble starting bytes is created.
1285    
1286    
1287  LOCALE SUPPORT  LOCALE SUPPORT
      PCRE handles caseless matching, and determines whether char-  
      acters  are  letters, digits, or whatever, by reference to a  
      set of tables. The library contains a default set of  tables  
      which  is  created in the default C locale when PCRE is com-  
      piled.  This  is   used   when   the   final   argument   of  
      pcre_compile()  is NULL, and is sufficient for many applica-  
      tions.  
   
      An alternative set of tables can, however, be supplied. Such  
      tables  are built by calling the pcre_maketables() function,  
      which has no arguments, in the relevant locale.  The  result  
      can  then be passed to pcre_compile() as often as necessary.  
      For example, to build and use tables  that  are  appropriate  
      for  the French locale (where accented characters with codes  
      greater than 128 are treated as letters), the following code  
      could be used:  
   
        setlocale(LC_CTYPE, "fr");  
        tables = pcre_maketables();  
        re = pcre_compile(..., tables);  
   
      The  tables  are  built  in  memory  that  is  obtained  via  
      pcre_malloc.  The  pointer that is passed to pcre_compile is  
      saved with the compiled pattern, and  the  same  tables  are  
      used  via this pointer by pcre_study() and pcre_exec(). Thus  
      for any single pattern, compilation, studying  and  matching  
      all happen in the same locale, but different patterns can be  
      compiled in different locales. It is the caller's  responsi-  
      bility  to  ensure  that  the  memory  containing the tables  
      remains available for as long as it is needed.  
1288    
1289           PCRE handles caseless matching, and determines whether  characters  are
1290           letters  digits,  or whatever, by reference to a set of tables, indexed
1291           by character value. When running in UTF-8 mode, this  applies  only  to
1292           characters  with  codes  less than 128. Higher-valued codes never match
1293           escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
1294           with  Unicode  character property support. The use of locales with Uni-
1295           code is discouraged.
1296    
1297           An internal set of tables is created in the default C locale when  PCRE
1298           is  built.  This  is  used when the final argument of pcre_compile() is
1299           NULL, and is sufficient for many applications. An  alternative  set  of
1300           tables  can,  however, be supplied. These may be created in a different
1301           locale from the default. As more and more applications change to  using
1302           Unicode, the need for this locale support is expected to die away.
1303    
1304           External  tables  are  built by calling the pcre_maketables() function,
1305           which has no arguments, in the relevant locale. The result can then  be
1306           passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
1307           example, to build and use tables that are appropriate  for  the  French
1308           locale  (where  accented  characters  with  values greater than 128 are
1309           treated as letters), the following code could be used:
1310    
1311             setlocale(LC_CTYPE, "fr_FR");
1312             tables = pcre_maketables();
1313             re = pcre_compile(..., tables);
1314    
1315           When pcre_maketables() runs, the tables are built  in  memory  that  is
1316           obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1317           that the memory containing the tables remains available for as long  as
1318           it is needed.
1319    
1320           The pointer that is passed to pcre_compile() is saved with the compiled
1321           pattern, and the same tables are used via this pointer by  pcre_study()
1322           and normally also by pcre_exec(). Thus, by default, for any single pat-
1323           tern, compilation, studying and matching all happen in the same locale,
1324           but different patterns can be compiled in different locales.
1325    
1326           It  is  possible to pass a table pointer or NULL (indicating the use of
1327           the internal tables) to pcre_exec(). Although  not  intended  for  this
1328           purpose,  this facility could be used to match a pattern in a different
1329           locale from the one in which it was compiled. Passing table pointers at
1330           run time is discussed below in the section on matching a pattern.
1331    
1332    
1333  INFORMATION ABOUT A PATTERN  INFORMATION ABOUT A PATTERN
      The pcre_fullinfo() function  returns  information  about  a  
      compiled pattern. It replaces the obsolete pcre_info() func-  
      tion, which is nevertheless retained for backwards compabil-  
      ity (and is documented below).  
   
      The first argument for pcre_fullinfo() is a pointer  to  the  
      compiled  pattern.  The  second  argument  is  the result of  
      pcre_study(), or NULL if the pattern was  not  studied.  The  
      third  argument  specifies  which  piece  of  information is  
      required, while the fourth argument is a pointer to a  vari-  
      able  to receive the data. The yield of the function is zero  
      for success, or one of the following negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
                              the argument where was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
        PCRE_ERROR_BADOPTION  the value of what was invalid  
   
      Here is a typical call of  pcre_fullinfo(),  to  obtain  the  
      length of the compiled pattern:  
   
        int rc;  
        unsigned long int length;  
        rc = pcre_fullinfo(  
          re,               /* result of pcre_compile() */  
          pe,               /* result of pcre_study(), or NULL */  
          PCRE_INFO_SIZE,   /* what is required */  
          &length);         /* where to put the data */  
   
      The possible values for the third argument  are  defined  in  
      pcre.h, and are as follows:  
   
        PCRE_INFO_OPTIONS  
   
      Return a copy of the options with which the pattern was com-  
      piled.  The fourth argument should point to an unsigned long  
      int variable. These option bits are those specified  in  the  
      call  to  pcre_compile(),  modified  by any top-level option  
      settings  within  the   pattern   itself,   and   with   the  
      PCRE_ANCHORED  bit  forcibly  set if the form of the pattern  
      implies that it can match only at the  start  of  a  subject  
      string.  
   
        PCRE_INFO_SIZE  
   
      Return the size of the compiled pattern, that is, the  value  
      that  was  passed as the argument to pcre_malloc() when PCRE  
      was getting memory in which to place the compiled data.  The  
      fourth argument should point to a size_t variable.  
   
        PCRE_INFO_CAPTURECOUNT  
   
      Return the number of capturing subpatterns in  the  pattern.  
      The fourth argument should point to an int variable.  
   
        PCRE_INFO_BACKREFMAX  
   
      Return the number of the highest back reference in the  pat-  
      tern.  The  fourth argument should point to an int variable.  
      Zero is returned if there are no back references.  
   
        PCRE_INFO_FIRSTCHAR  
   
      Return information about the first character of any  matched  
      string,  for  a  non-anchored  pattern.  If there is a fixed  
      first   character,   e.g.   from   a   pattern    such    as  
      (cat|cow|coyote),  it  is returned in the integer pointed to  
      by where. Otherwise, if either  
   
      (a) the pattern was compiled with the PCRE_MULTILINE option,  
      and every branch starts with "^", or  
   
      (b) every  branch  of  the  pattern  starts  with  ".*"  and  
      PCRE_DOTALL is not set (if it were set, the pattern would be  
      anchored),  
   
      -1 is returned, indicating that the pattern matches only  at  
      the  start  of a subject string or after any "\n" within the  
      string. Otherwise -2 is returned.  For anchored patterns, -2  
      is returned.  
   
        PCRE_INFO_FIRSTTABLE  
   
      If the pattern was studied, and this resulted  in  the  con-  
      struction of a 256-bit table indicating a fixed set of char-  
      acters for the first character in  any  matching  string,  a  
      pointer   to  the  table  is  returned.  Otherwise  NULL  is  
      returned. The fourth argument should point  to  an  unsigned  
      char * variable.  
   
        PCRE_INFO_LASTLITERAL  
   
      For a non-anchored pattern, return the value of  the  right-  
      most  literal  character  which  must  exist  in any matched  
      string, other than at its start. The fourth argument  should  
      point  to an int variable. If there is no such character, or  
      if the pattern is anchored, -1 is returned. For example, for  
      the pattern /a\d+z\d+/ the returned value is 'z'.  
   
      The pcre_info() function is now obsolete because its  inter-  
      face  is  too  restrictive  to return all the available data  
      about  a  compiled  pattern.   New   programs   should   use  
      pcre_fullinfo()  instead.  The  yield  of pcre_info() is the  
      number of capturing subpatterns, or  one  of  the  following  
      negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
   
      If the optptr argument is not NULL, a copy  of  the  options  
      with which the pattern was compiled is placed in the integer  
      it points to (see PCRE_INFO_OPTIONS above).  
   
      If the pattern is not anchored and the firstcharptr argument  
      is  not  NULL, it is used to pass back information about the  
      first    character    of    any    matched    string    (see  
      PCRE_INFO_FIRSTCHAR above).  
1334    
1335           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1336                int what, void *where);
1337    
1338           The  pcre_fullinfo() function returns information about a compiled pat-
1339           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1340           less retained for backwards compability (and is documented below).
1341    
1342           The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1343           pattern. The second argument is the result of pcre_study(), or NULL  if
1344           the  pattern  was not studied. The third argument specifies which piece
1345           of information is required, and the fourth argument is a pointer  to  a
1346           variable  to  receive  the  data. The yield of the function is zero for
1347           success, or one of the following negative numbers:
1348    
1349             PCRE_ERROR_NULL       the argument code was NULL
1350                                   the argument where was NULL
1351             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1352             PCRE_ERROR_BADOPTION  the value of what was invalid
1353    
1354           The "magic number" is placed at the start of each compiled  pattern  as
1355           an  simple check against passing an arbitrary memory pointer. Here is a
1356           typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1357           pattern:
1358    
1359             int rc;
1360             size_t length;
1361             rc = pcre_fullinfo(
1362               re,               /* result of pcre_compile() */
1363               pe,               /* result of pcre_study(), or NULL */
1364               PCRE_INFO_SIZE,   /* what is required */
1365               &length);         /* where to put the data */
1366    
1367           The  possible  values for the third argument are defined in pcre.h, and
1368           are as follows:
1369    
1370             PCRE_INFO_BACKREFMAX
1371    
1372           Return the number of the highest back reference  in  the  pattern.  The
1373           fourth  argument  should  point to an int variable. Zero is returned if
1374           there are no back references.
1375    
1376             PCRE_INFO_CAPTURECOUNT
1377    
1378           Return the number of capturing subpatterns in the pattern.  The  fourth
1379           argument should point to an int variable.
1380    
1381             PCRE_INFO_DEFAULT_TABLES
1382    
1383           Return  a pointer to the internal default character tables within PCRE.
1384           The fourth argument should point to an unsigned char *  variable.  This
1385           information call is provided for internal use by the pcre_study() func-
1386           tion. External callers can cause PCRE to use  its  internal  tables  by
1387           passing a NULL table pointer.
1388    
1389             PCRE_INFO_FIRSTBYTE
1390    
1391           Return  information  about  the first byte of any matched string, for a
1392           non-anchored pattern. The fourth argument should point to an int  vari-
1393           able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1394           is still recognized for backwards compatibility.)
1395    
1396           If there is a fixed first byte, for example, from  a  pattern  such  as
1397           (cat|cow|coyote), its value is returned. Otherwise, if either
1398    
1399           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1400           branch starts with "^", or
1401    
1402           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1403           set (if it were set, the pattern would be anchored),
1404    
1405           -1  is  returned, indicating that the pattern matches only at the start
1406           of a subject string or after any newline within the  string.  Otherwise
1407           -2 is returned. For anchored patterns, -2 is returned.
1408    
1409             PCRE_INFO_FIRSTTABLE
1410    
1411           If  the pattern was studied, and this resulted in the construction of a
1412           256-bit table indicating a fixed set of bytes for the first byte in any
1413           matching  string, a pointer to the table is returned. Otherwise NULL is
1414           returned. The fourth argument should point to an unsigned char *  vari-
1415           able.
1416    
1417             PCRE_INFO_LASTLITERAL
1418    
1419           Return  the  value of the rightmost literal byte that must exist in any
1420           matched string, other than at its  start,  if  such  a  byte  has  been
1421           recorded. The fourth argument should point to an int variable. If there
1422           is no such byte, -1 is returned. For anchored patterns, a last  literal
1423           byte  is  recorded only if it follows something of variable length. For
1424           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1425           /^a\dz\d/ the returned value is -1.
1426    
1427             PCRE_INFO_NAMECOUNT
1428             PCRE_INFO_NAMEENTRYSIZE
1429             PCRE_INFO_NAMETABLE
1430    
1431           PCRE  supports the use of named as well as numbered capturing parenthe-
1432           ses. The names are just an additional way of identifying the  parenthe-
1433           ses, which still acquire numbers. Several convenience functions such as
1434           pcre_get_named_substring() are provided for  extracting  captured  sub-
1435           strings  by  name. It is also possible to extract the data directly, by
1436           first converting the name to a number in order to  access  the  correct
1437           pointers in the output vector (described with pcre_exec() below). To do
1438           the conversion, you need  to  use  the  name-to-number  map,  which  is
1439           described by these three values.
1440    
1441           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1442           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1443           of  each  entry;  both  of  these  return  an int value. The entry size
1444           depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
1445           a  pointer  to  the  first  entry of the table (a pointer to char). The
1446           first two bytes of each entry are the number of the capturing parenthe-
1447           sis,  most  significant byte first. The rest of the entry is the corre-
1448           sponding name, zero terminated. The names are  in  alphabetical  order.
1449           When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1450           theses numbers. For example, consider  the  following  pattern  (assume
1451           PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
1452           ignored):
1453    
1454             (?<date> (?<year>(\d\d)?\d\d) -
1455             (?<month>\d\d) - (?<day>\d\d) )
1456    
1457           There are four named subpatterns, so the table has  four  entries,  and
1458           each  entry  in the table is eight bytes long. The table is as follows,
1459           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1460           as ??:
1461    
1462             00 01 d  a  t  e  00 ??
1463             00 05 d  a  y  00 ?? ??
1464             00 04 m  o  n  t  h  00
1465             00 02 y  e  a  r  00 ??
1466    
1467           When  writing  code  to  extract  data from named subpatterns using the
1468           name-to-number map, remember that the length of the entries  is  likely
1469           to be different for each compiled pattern.
1470    
1471             PCRE_INFO_OPTIONS
1472    
1473           Return  a  copy of the options with which the pattern was compiled. The
1474           fourth argument should point to an unsigned long  int  variable.  These
1475           option bits are those specified in the call to pcre_compile(), modified
1476           by any top-level option settings within the pattern itself.
1477    
1478           A pattern is automatically anchored by PCRE if  all  of  its  top-level
1479           alternatives begin with one of the following:
1480    
1481             ^     unless PCRE_MULTILINE is set
1482             \A    always
1483             \G    always
1484             .*    if PCRE_DOTALL is set and there are no back
1485                     references to the subpattern in which .* appears
1486    
1487           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1488           by pcre_fullinfo().
1489    
1490             PCRE_INFO_SIZE
1491    
1492           Return the size of the compiled pattern, that is, the  value  that  was
1493           passed as the argument to pcre_malloc() when PCRE was getting memory in
1494           which to place the compiled data. The fourth argument should point to a
1495           size_t variable.
1496    
1497             PCRE_INFO_STUDYSIZE
1498    
1499           Return the size of the data block pointed to by the study_data field in
1500           a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1501           pcre_malloc() when PCRE was getting memory into which to place the data
1502           created by pcre_study(). The fourth argument should point to  a  size_t
1503           variable.
1504    
1505    
1506    OBSOLETE INFO FUNCTION
1507    
1508           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1509    
1510           The  pcre_info()  function is now obsolete because its interface is too
1511           restrictive to return all the available data about a compiled  pattern.
1512           New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1513           pcre_info() is the number of capturing subpatterns, or one of the  fol-
1514           lowing negative numbers:
1515    
1516             PCRE_ERROR_NULL       the argument code was NULL
1517             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1518    
1519           If  the  optptr  argument is not NULL, a copy of the options with which
1520           the pattern was compiled is placed in the integer  it  points  to  (see
1521           PCRE_INFO_OPTIONS above).
1522    
1523           If  the  pattern  is  not anchored and the firstcharptr argument is not
1524           NULL, it is used to pass back information about the first character  of
1525           any matched string (see PCRE_INFO_FIRSTBYTE above).
1526    
1527    
1528    REFERENCE COUNTS
1529    
1530           int pcre_refcount(pcre *code, int adjust);
1531    
1532           The  pcre_refcount()  function is used to maintain a reference count in
1533           the data block that contains a compiled pattern. It is provided for the
1534           benefit  of  applications  that  operate  in an object-oriented manner,
1535           where different parts of the application may be using the same compiled
1536           pattern, but you want to free the block when they are all done.
1537    
1538           When a pattern is compiled, the reference count field is initialized to
1539           zero.  It is changed only by calling this function, whose action is  to
1540           add  the  adjust  value  (which may be positive or negative) to it. The
1541           yield of the function is the new value. However, the value of the count
1542           is  constrained to lie between 0 and 65535, inclusive. If the new value
1543           is outside these limits, it is forced to the appropriate limit value.
1544    
1545           Except when it is zero, the reference count is not correctly  preserved
1546           if  a  pattern  is  compiled on one host and then transferred to a host
1547           whose byte-order is different. (This seems a highly unlikely scenario.)
1548    
1549    
1550    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1551    
1552           int pcre_exec(const pcre *code, const pcre_extra *extra,
1553                const char *subject, int length, int startoffset,
1554                int options, int *ovector, int ovecsize);
1555    
1556           The  function pcre_exec() is called to match a subject string against a
1557           compiled pattern, which is passed in the code argument. If the  pattern
1558           has been studied, the result of the study should be passed in the extra
1559           argument. This function is the main matching facility of  the  library,
1560           and it operates in a Perl-like manner. For specialist use there is also
1561           an alternative matching function, which is described below in the  sec-
1562           tion about the pcre_dfa_exec() function.
1563    
1564           In  most applications, the pattern will have been compiled (and option-
1565           ally studied) in the same process that calls pcre_exec().  However,  it
1566           is possible to save compiled patterns and study data, and then use them
1567           later in different processes, possibly even on different hosts.  For  a
1568           discussion about this, see the pcreprecompile documentation.
1569    
1570           Here is an example of a simple call to pcre_exec():
1571    
1572             int rc;
1573             int ovector[30];
1574             rc = pcre_exec(
1575               re,             /* result of pcre_compile() */
1576               NULL,           /* we didn't study the pattern */
1577               "some string",  /* the subject string */
1578               11,             /* the length of the subject string */
1579               0,              /* start at offset 0 in the subject */
1580               0,              /* default options */
1581               ovector,        /* vector of integers for substring information */
1582               30);            /* number of elements (NOT size in bytes) */
1583    
1584       Extra data for pcre_exec()
1585    
1586           If  the  extra argument is not NULL, it must point to a pcre_extra data
1587           block. The pcre_study() function returns such a block (when it  doesn't
1588           return  NULL), but you can also create one for yourself, and pass addi-
1589           tional information in it. The pcre_extra block contains  the  following
1590           fields (not necessarily in this order):
1591    
1592             unsigned long int flags;
1593             void *study_data;
1594             unsigned long int match_limit;
1595             unsigned long int match_limit_recursion;
1596             void *callout_data;
1597             const unsigned char *tables;
1598    
1599           The  flags  field  is a bitmap that specifies which of the other fields
1600           are set. The flag bits are:
1601    
1602             PCRE_EXTRA_STUDY_DATA
1603             PCRE_EXTRA_MATCH_LIMIT
1604             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1605             PCRE_EXTRA_CALLOUT_DATA
1606             PCRE_EXTRA_TABLES
1607    
1608           Other flag bits should be set to zero. The study_data field is  set  in
1609           the  pcre_extra  block  that is returned by pcre_study(), together with
1610           the appropriate flag bit. You should not set this yourself, but you may
1611           add  to  the  block by setting the other fields and their corresponding
1612           flag bits.
1613    
1614           The match_limit field provides a means of preventing PCRE from using up
1615           a  vast amount of resources when running patterns that are not going to
1616           match, but which have a very large number  of  possibilities  in  their
1617           search  trees.  The  classic  example  is  the  use of nested unlimited
1618           repeats.
1619    
1620           Internally, PCRE uses a function called match() which it calls  repeat-
1621           edly  (sometimes  recursively). The limit set by match_limit is imposed
1622           on the number of times this function is called during  a  match,  which
1623           has  the  effect  of  limiting the amount of backtracking that can take
1624           place. For patterns that are not anchored, the count restarts from zero
1625           for each position in the subject string.
1626    
1627           The  default  value  for  the  limit can be set when PCRE is built; the
1628           default default is 10 million, which handles all but the  most  extreme
1629           cases.  You  can  override  the  default by suppling pcre_exec() with a
1630           pcre_extra    block    in    which    match_limit    is    set,     and
1631           PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1632           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1633    
1634           The match_limit_recursion field is similar to match_limit, but  instead
1635           of limiting the total number of times that match() is called, it limits
1636           the depth of recursion. The recursion depth is a  smaller  number  than
1637           the  total number of calls, because not all calls to match() are recur-
1638           sive.  This limit is of use only if it is set smaller than match_limit.
1639    
1640           Limiting  the  recursion  depth  limits the amount of stack that can be
1641           used, or, when PCRE has been compiled to use memory on the heap instead
1642           of the stack, the amount of heap memory that can be used.
1643    
1644           The  default  value  for  match_limit_recursion can be set when PCRE is
1645           built; the default default  is  the  same  value  as  the  default  for
1646           match_limit.  You can override the default by suppling pcre_exec() with
1647           a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
1648           PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
1649           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1650    
1651           The pcre_callout field is used in conjunction with the  "callout"  fea-
1652           ture, which is described in the pcrecallout documentation.
1653    
1654           The  tables  field  is  used  to  pass  a  character  tables pointer to
1655           pcre_exec(); this overrides the value that is stored with the  compiled
1656           pattern.  A  non-NULL value is stored with the compiled pattern only if
1657           custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1658           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1659           PCRE's internal tables to be used. This facility is  helpful  when  re-
1660           using  patterns  that  have been saved after compiling with an external
1661           set of tables, because the external tables  might  be  at  a  different
1662           address  when  pcre_exec() is called. See the pcreprecompile documenta-
1663           tion for a discussion of saving compiled patterns for later use.
1664    
1665       Option bits for pcre_exec()
1666    
1667           The unused bits of the options argument for pcre_exec() must  be  zero.
1668           The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1669           PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1670           PCRE_PARTIAL.
1671    
1672             PCRE_ANCHORED
1673    
1674           The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
1675           matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
1676           turned  out to be anchored by virtue of its contents, it cannot be made
1677           unachored at matching time.
1678    
1679             PCRE_NEWLINE_CR
1680             PCRE_NEWLINE_LF
1681             PCRE_NEWLINE_CRLF
1682             PCRE_NEWLINE_ANY
1683    
1684           These options override  the  newline  definition  that  was  chosen  or
1685           defaulted  when the pattern was compiled. For details, see the descrip-
1686           tion of pcre_compile()  above.  During  matching,  the  newline  choice
1687           affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
1688           ters. It may also alter the way the match position is advanced after  a
1689           match  failure  for  an  unanchored  pattern. When PCRE_NEWLINE_CRLF or
1690           PCRE_NEWLINE_ANY is set, and a match attempt  fails  when  the  current
1691           position  is  at a CRLF sequence, the match position is advanced by two
1692           characters instead of one, in other words, to after the CRLF.
1693    
1694             PCRE_NOTBOL
1695    
1696           This option specifies that first character of the subject string is not
1697           the  beginning  of  a  line, so the circumflex metacharacter should not
1698           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1699           causes  circumflex  never to match. This option affects only the behav-
1700           iour of the circumflex metacharacter. It does not affect \A.
1701    
1702             PCRE_NOTEOL
1703    
1704           This option specifies that the end of the subject string is not the end
1705           of  a line, so the dollar metacharacter should not match it nor (except
1706           in multiline mode) a newline immediately before it. Setting this  with-
1707           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1708           option affects only the behaviour of the dollar metacharacter. It  does
1709           not affect \Z or \z.
1710    
1711             PCRE_NOTEMPTY
1712    
1713           An empty string is not considered to be a valid match if this option is
1714           set. If there are alternatives in the pattern, they are tried.  If  all
1715           the  alternatives  match  the empty string, the entire match fails. For
1716           example, if the pattern
1717    
1718             a?b?
1719    
1720           is applied to a string not beginning with "a" or "b",  it  matches  the
1721           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1722           match is not valid, so PCRE searches further into the string for occur-
1723           rences of "a" or "b".
1724    
1725           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1726           cial case of a pattern match of the empty  string  within  its  split()
1727           function,  and  when  using  the /g modifier. It is possible to emulate
1728           Perl's behaviour after matching a null string by first trying the match
1729           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1730           if that fails by advancing the starting offset (see below)  and  trying
1731           an ordinary match again. There is some code that demonstrates how to do
1732           this in the pcredemo.c sample program.
1733    
1734             PCRE_NO_UTF8_CHECK
1735    
1736           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1737           UTF-8  string is automatically checked when pcre_exec() is subsequently
1738           called.  The value of startoffset is also checked  to  ensure  that  it
1739           points  to the start of a UTF-8 character. If an invalid UTF-8 sequence
1740           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1741           startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is
1742           returned.
1743    
1744           If you already know that your subject is valid, and you  want  to  skip
1745           these    checks    for   performance   reasons,   you   can   set   the
1746           PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
1747           do  this  for the second and subsequent calls to pcre_exec() if you are
1748           making repeated calls to find all  the  matches  in  a  single  subject
1749           string.  However,  you  should  be  sure  that the value of startoffset
1750           points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
1751           set,  the  effect of passing an invalid UTF-8 string as a subject, or a
1752           value of startoffset that does not point to the start of a UTF-8  char-
1753           acter, is undefined. Your program may crash.
1754    
1755             PCRE_PARTIAL
1756    
1757           This  option  turns  on  the  partial  matching feature. If the subject
1758           string fails to match the pattern, but at some point during the  match-
1759           ing  process  the  end of the subject was reached (that is, the subject
1760           partially matches the pattern and the failure to  match  occurred  only
1761           because  there were not enough subject characters), pcre_exec() returns
1762           PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
1763           used,  there  are restrictions on what may appear in the pattern. These
1764           are discussed in the pcrepartial documentation.
1765    
1766       The string to be matched by pcre_exec()
1767    
1768           The subject string is passed to pcre_exec() as a pointer in subject,  a
1769           length  in  length, and a starting byte offset in startoffset. In UTF-8
1770           mode, the byte offset must point to the start  of  a  UTF-8  character.
1771           Unlike  the  pattern string, the subject may contain binary zero bytes.
1772           When the starting offset is zero, the search for a match starts at  the
1773           beginning of the subject, and this is by far the most common case.
1774    
1775           A  non-zero  starting offset is useful when searching for another match
1776           in the same subject by calling pcre_exec() again after a previous  suc-
1777           cess.   Setting  startoffset differs from just passing over a shortened
1778           string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
1779           with any kind of lookbehind. For example, consider the pattern
1780    
1781             \Biss\B
1782    
1783           which  finds  occurrences  of "iss" in the middle of words. (\B matches
1784           only if the current position in the subject is not  a  word  boundary.)
1785           When  applied  to the string "Mississipi" the first call to pcre_exec()
1786           finds the first occurrence. If pcre_exec() is called  again  with  just
1787           the  remainder  of  the  subject,  namely  "issipi", it does not match,
1788           because \B is always false at the start of the subject, which is deemed
1789           to  be  a  word  boundary. However, if pcre_exec() is passed the entire
1790           string again, but with startoffset set to 4, it finds the second occur-
1791           rence  of "iss" because it is able to look behind the starting point to
1792           discover that it is preceded by a letter.
1793    
1794           If a non-zero starting offset is passed when the pattern  is  anchored,
1795           one attempt to match at the given offset is made. This can only succeed
1796           if the pattern does not require the match to be at  the  start  of  the
1797           subject.
1798    
1799       How pcre_exec() returns captured substrings
1800    
1801           In  general, a pattern matches a certain portion of the subject, and in
1802           addition, further substrings from the subject  may  be  picked  out  by
1803           parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
1804           this is called "capturing" in what follows, and the  phrase  "capturing
1805           subpattern"  is  used for a fragment of a pattern that picks out a sub-
1806           string. PCRE supports several other kinds of  parenthesized  subpattern
1807           that do not cause substrings to be captured.
1808    
1809           Captured  substrings are returned to the caller via a vector of integer
1810           offsets whose address is passed in ovector. The number of  elements  in
1811           the  vector is passed in ovecsize, which must be a non-negative number.
1812           Note: this argument is NOT the size of ovector in bytes.
1813    
1814           The first two-thirds of the vector is used to pass back  captured  sub-
1815           strings,  each  substring using a pair of integers. The remaining third
1816           of the vector is used as workspace by pcre_exec() while  matching  cap-
1817           turing  subpatterns, and is not available for passing back information.
1818           The length passed in ovecsize should always be a multiple of three.  If
1819           it is not, it is rounded down.
1820    
1821           When  a  match  is successful, information about captured substrings is
1822           returned in pairs of integers, starting at the  beginning  of  ovector,
1823           and  continuing  up  to two-thirds of its length at the most. The first
1824           element of a pair is set to the offset of the first character in a sub-
1825           string,  and  the  second  is  set to the offset of the first character
1826           after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
1827           tor[1],  identify  the  portion  of  the  subject string matched by the
1828           entire pattern. The next pair is used for the first  capturing  subpat-
1829           tern, and so on. The value returned by pcre_exec() is one more than the
1830           highest numbered pair that has been set. For example, if two substrings
1831           have  been captured, the returned value is 3. If there are no capturing
1832           subpatterns, the return value from a successful match is 1,  indicating
1833           that just the first pair of offsets has been set.
1834    
1835           If a capturing subpattern is matched repeatedly, it is the last portion
1836           of the string that it matched that is returned.
1837    
1838           If the vector is too small to hold all the captured substring  offsets,
1839           it is used as far as possible (up to two-thirds of its length), and the
1840           function returns a value of zero. In particular, if the substring  off-
1841           sets are not of interest, pcre_exec() may be called with ovector passed
1842           as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1843           references  and  the  ovector is not big enough to remember the related
1844           substrings, PCRE has to get additional memory for use during  matching.
1845           Thus it is usually advisable to supply an ovector.
1846    
1847           The  pcre_info()  function  can  be used to find out how many capturing
1848           subpatterns there are in a compiled  pattern.  The  smallest  size  for
1849           ovector  that  will allow for n captured substrings, in addition to the
1850           offsets of the substring matched by the whole pattern, is (n+1)*3.
1851    
1852           It is possible for capturing subpattern number n+1 to match  some  part
1853           of the subject when subpattern n has not been used at all. For example,
1854           if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
1855           return from the function is 4, and subpatterns 1 and 3 are matched, but
1856           2 is not. When this happens, both values in  the  offset  pairs  corre-
1857           sponding to unused subpatterns are set to -1.
1858    
1859           Offset  values  that correspond to unused subpatterns at the end of the
1860           expression are also set to -1. For example,  if  the  string  "abc"  is
1861           matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1862           matched. The return from the function is 2, because  the  highest  used
1863           capturing subpattern number is 1. However, you can refer to the offsets
1864           for the second and third capturing subpatterns if  you  wish  (assuming
1865           the vector is large enough, of course).
1866    
1867           Some  convenience  functions  are  provided for extracting the captured
1868           substrings as separate strings. These are described below.
1869    
1870       Error return values from pcre_exec()
1871    
1872           If pcre_exec() fails, it returns a negative number. The  following  are
1873           defined in the header file:
1874    
1875             PCRE_ERROR_NOMATCH        (-1)
1876    
1877           The subject string did not match the pattern.
1878    
1879             PCRE_ERROR_NULL           (-2)
1880    
1881           Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1882           ovecsize was not zero.
1883    
1884             PCRE_ERROR_BADOPTION      (-3)
1885    
1886           An unrecognized bit was set in the options argument.
1887    
1888             PCRE_ERROR_BADMAGIC       (-4)
1889    
1890           PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1891           to catch the case when it is passed a junk pointer and to detect when a
1892           pattern that was compiled in an environment of one endianness is run in
1893           an  environment  with the other endianness. This is the error that PCRE
1894           gives when the magic number is not present.
1895    
1896             PCRE_ERROR_UNKNOWN_OPCODE (-5)
1897    
1898           While running the pattern match, an unknown item was encountered in the
1899           compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1900           overwriting of the compiled pattern.
1901    
1902             PCRE_ERROR_NOMEMORY       (-6)
1903    
1904           If a pattern contains back references, but the ovector that  is  passed
1905           to pcre_exec() is not big enough to remember the referenced substrings,
1906           PCRE gets a block of memory at the start of matching to  use  for  this
1907           purpose.  If the call via pcre_malloc() fails, this error is given. The
1908           memory is automatically freed at the end of matching.
1909    
1910             PCRE_ERROR_NOSUBSTRING    (-7)
1911    
1912           This error is used by the pcre_copy_substring(),  pcre_get_substring(),
1913           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1914           returned by pcre_exec().
1915    
1916             PCRE_ERROR_MATCHLIMIT     (-8)
1917    
1918           The backtracking limit, as specified by  the  match_limit  field  in  a
1919           pcre_extra  structure  (or  defaulted) was reached. See the description
1920           above.
1921    
1922             PCRE_ERROR_CALLOUT        (-9)
1923    
1924           This error is never generated by pcre_exec() itself. It is provided for
1925           use  by  callout functions that want to yield a distinctive error code.
1926           See the pcrecallout documentation for details.
1927    
1928             PCRE_ERROR_BADUTF8        (-10)
1929    
1930           A string that contains an invalid UTF-8 byte sequence was passed  as  a
1931           subject.
1932    
1933             PCRE_ERROR_BADUTF8_OFFSET (-11)
1934    
1935           The UTF-8 byte sequence that was passed as a subject was valid, but the
1936           value of startoffset did not point to the beginning of a UTF-8  charac-
1937           ter.
1938    
1939             PCRE_ERROR_PARTIAL        (-12)
1940    
1941           The  subject  string did not match, but it did match partially. See the
1942           pcrepartial documentation for details of partial matching.
1943    
1944             PCRE_ERROR_BADPARTIAL     (-13)
1945    
1946           The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
1947           items  that are not supported for partial matching. See the pcrepartial
1948           documentation for details of partial matching.
1949    
1950             PCRE_ERROR_INTERNAL       (-14)
1951    
1952           An unexpected internal error has occurred. This error could  be  caused
1953           by a bug in PCRE or by overwriting of the compiled pattern.
1954    
1955             PCRE_ERROR_BADCOUNT       (-15)
1956    
1957           This  error is given if the value of the ovecsize argument is negative.
1958    
1959             PCRE_ERROR_RECURSIONLIMIT (-21)
1960    
1961           The internal recursion limit, as specified by the match_limit_recursion
1962           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
1963           description above.
1964    
1965             PCRE_ERROR_NULLWSLIMIT    (-22)
1966    
1967           When a group that can match an empty  substring  is  repeated  with  an
1968           unbounded  upper  limit, the subject position at the start of the group
1969           must be remembered, so that a test for an empty string can be made when
1970           the  end  of the group is reached. Some workspace is required for this;
1971           if it runs out, this error is given.
1972    
1973             PCRE_ERROR_BADNEWLINE     (-23)
1974    
1975           An invalid combination of PCRE_NEWLINE_xxx options was given.
1976    
1977           Error numbers -16 to -20 are not used by pcre_exec().
1978    
1979    
1980    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
1981    
1982           int pcre_copy_substring(const char *subject, int *ovector,
1983                int stringcount, int stringnumber, char *buffer,
1984                int buffersize);
1985    
1986           int pcre_get_substring(const char *subject, int *ovector,
1987                int stringcount, int stringnumber,
1988                const char **stringptr);
1989    
1990           int pcre_get_substring_list(const char *subject,
1991                int *ovector, int stringcount, const char ***listptr);
1992    
1993           Captured substrings can be  accessed  directly  by  using  the  offsets
1994           returned  by  pcre_exec()  in  ovector.  For convenience, the functions
1995           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
1996           string_list()  are  provided for extracting captured substrings as new,
1997           separate, zero-terminated strings. These functions identify  substrings
1998           by  number.  The  next section describes functions for extracting named
1999           substrings.
2000    
2001           A substring that contains a binary zero is correctly extracted and  has
2002           a  further zero added on the end, but the result is not, of course, a C
2003           string.  However, you can process such a string  by  referring  to  the
2004           length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
2005           string().  Unfortunately, the interface to pcre_get_substring_list() is
2006           not  adequate for handling strings containing binary zeros, because the
2007           end of the final string is not independently indicated.
2008    
2009           The first three arguments are the same for all  three  of  these  func-
2010           tions:  subject  is  the subject string that has just been successfully
2011           matched, ovector is a pointer to the vector of integer offsets that was
2012           passed to pcre_exec(), and stringcount is the number of substrings that
2013           were captured by the match, including the substring  that  matched  the
2014           entire regular expression. This is the value returned by pcre_exec() if
2015           it is greater than zero. If pcre_exec() returned zero, indicating  that
2016           it  ran out of space in ovector, the value passed as stringcount should
2017           be the number of elements in the vector divided by three.
2018    
2019           The functions pcre_copy_substring() and pcre_get_substring() extract  a
2020           single  substring,  whose  number  is given as stringnumber. A value of
2021           zero extracts the substring that matched the  entire  pattern,  whereas
2022           higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
2023           string(), the string is placed in buffer,  whose  length  is  given  by
2024           buffersize,  while  for  pcre_get_substring()  a new block of memory is
2025           obtained via pcre_malloc, and its address is  returned  via  stringptr.
2026           The  yield  of  the function is the length of the string, not including
2027           the terminating zero, or one of these error codes:
2028    
2029             PCRE_ERROR_NOMEMORY       (-6)
2030    
2031           The buffer was too small for pcre_copy_substring(), or the  attempt  to
2032           get memory failed for pcre_get_substring().
2033    
2034             PCRE_ERROR_NOSUBSTRING    (-7)
2035    
2036           There is no substring whose number is stringnumber.
2037    
2038           The  pcre_get_substring_list()  function  extracts  all  available sub-
2039           strings and builds a list of pointers to them. All this is  done  in  a
2040           single block of memory that is obtained via pcre_malloc. The address of
2041           the memory block is returned via listptr, which is also  the  start  of
2042           the  list  of  string pointers. The end of the list is marked by a NULL
2043           pointer. The yield of the function is zero if all  went  well,  or  the
2044           error code
2045    
2046             PCRE_ERROR_NOMEMORY       (-6)
2047    
2048           if the attempt to get the memory block failed.
2049    
2050           When  any of these functions encounter a substring that is unset, which
2051           can happen when capturing subpattern number n+1 matches  some  part  of
2052           the  subject, but subpattern n has not been used at all, they return an
2053           empty string. This can be distinguished from a genuine zero-length sub-
2054           string  by inspecting the appropriate offset in ovector, which is nega-
2055           tive for unset substrings.
2056    
2057           The two convenience functions pcre_free_substring() and  pcre_free_sub-
2058           string_list()  can  be  used  to free the memory returned by a previous
2059           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2060           tively.  They  do  nothing  more  than  call the function pointed to by
2061           pcre_free, which of course could be called directly from a  C  program.
2062           However,  PCRE is used in some situations where it is linked via a spe-
2063           cial  interface  to  another  programming  language  that  cannot   use
2064           pcre_free  directly;  it is for these cases that the functions are pro-
2065           vided.
2066    
2067    
2068    EXTRACTING CAPTURED SUBSTRINGS BY NAME
2069    
2070           int pcre_get_stringnumber(const pcre *code,
2071                const char *name);
2072    
2073           int pcre_copy_named_substring(const pcre *code,
2074                const char *subject, int *ovector,
2075                int stringcount, const char *stringname,
2076                char *buffer, int buffersize);
2077    
2078           int pcre_get_named_substring(const pcre *code,
2079                const char *subject, int *ovector,
2080                int stringcount, const char *stringname,
2081                const char **stringptr);
2082    
2083           To extract a substring by name, you first have to find associated  num-
2084           ber.  For example, for this pattern
2085    
2086             (a+)b(?<xxx>\d+)...
2087    
2088           the number of the subpattern called "xxx" is 2. If the name is known to
2089           be unique (PCRE_DUPNAMES was not set), you can find the number from the
2090           name by calling pcre_get_stringnumber(). The first argument is the com-
2091           piled pattern, and the second is the name. The yield of the function is
2092           the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2093           subpattern of that name.
2094    
2095           Given the number, you can extract the substring directly, or use one of
2096           the functions described in the previous section. For convenience, there
2097           are also two functions that do the whole job.
2098    
2099           Most   of   the   arguments    of    pcre_copy_named_substring()    and
2100           pcre_get_named_substring()  are  the  same  as  those for the similarly
2101           named functions that extract by number. As these are described  in  the
2102           previous  section,  they  are not re-described here. There are just two
2103           differences:
2104    
2105           First, instead of a substring number, a substring name is  given.  Sec-
2106           ond, there is an extra argument, given at the start, which is a pointer
2107           to the compiled pattern. This is needed in order to gain access to  the
2108           name-to-number translation table.
2109    
2110           These  functions call pcre_get_stringnumber(), and if it succeeds, they
2111           then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
2112           ate.
2113    
2114    
2115    DUPLICATE SUBPATTERN NAMES
2116    
2117           int pcre_get_stringtable_entries(const pcre *code,
2118                const char *name, char **first, char **last);
2119    
2120           When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
2121           subpatterns are not required to  be  unique.  Normally,  patterns  with
2122           duplicate  names  are such that in any one match, only one of the named
2123           subpatterns participates. An example is shown in the pcrepattern  docu-
2124           mentation. When duplicates are present, pcre_copy_named_substring() and
2125           pcre_get_named_substring() return the first substring corresponding  to
2126           the  given  name  that  is  set.  If  none  are set, an empty string is
2127           returned.  The pcre_get_stringnumber() function returns one of the num-
2128           bers  that are associated with the name, but it is not defined which it
2129           is.
2130    
2131           If you want to get full details of all captured substrings for a  given
2132           name,  you  must  use  the pcre_get_stringtable_entries() function. The
2133           first argument is the compiled pattern, and the second is the name. The
2134           third  and  fourth  are  pointers to variables which are updated by the
2135           function. After it has run, they point to the first and last entries in
2136           the  name-to-number  table  for  the  given  name.  The function itself
2137           returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
2138           there  are none. The format of the table is described above in the sec-
2139           tion entitled Information about a  pattern.   Given  all  the  relevant
2140           entries  for the name, you can extract each of their numbers, and hence
2141           the captured data, if any.
2142    
2143    
2144    FINDING ALL POSSIBLE MATCHES
2145    
2146           The traditional matching function uses a  similar  algorithm  to  Perl,
2147           which stops when it finds the first match, starting at a given point in
2148           the subject. If you want to find all possible matches, or  the  longest
2149           possible  match,  consider using the alternative matching function (see
2150           below) instead. If you cannot use the alternative function,  but  still
2151           need  to  find all possible matches, you can kludge it up by making use
2152           of the callout facility, which is described in the pcrecallout documen-
2153           tation.
2154    
2155           What you have to do is to insert a callout right at the end of the pat-
2156           tern.  When your callout function is called, extract and save the  cur-
2157           rent  matched  substring.  Then  return  1, which forces pcre_exec() to
2158           backtrack and try other alternatives. Ultimately, when it runs  out  of
2159           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2160    
2161    
2162    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2163    
2164           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2165                const char *subject, int length, int startoffset,
2166                int options, int *ovector, int ovecsize,
2167                int *workspace, int wscount);
2168    
2169           The  function  pcre_dfa_exec()  is  called  to  match  a subject string
2170           against a compiled pattern, using a matching algorithm that  scans  the
2171           subject  string  just  once, and does not backtrack. This has different
2172           characteristics to the normal algorithm, and  is  not  compatible  with
2173           Perl.  Some  of the features of PCRE patterns are not supported. Never-
2174           theless, there are times when this kind of matching can be useful.  For
2175           a discussion of the two matching algorithms, see the pcrematching docu-
2176           mentation.
2177    
2178           The arguments for the pcre_dfa_exec() function  are  the  same  as  for
2179           pcre_exec(), plus two extras. The ovector argument is used in a differ-
2180           ent way, and this is described below. The other  common  arguments  are
2181           used  in  the  same way as for pcre_exec(), so their description is not
2182           repeated here.
2183    
2184           The two additional arguments provide workspace for  the  function.  The
2185           workspace  vector  should  contain at least 20 elements. It is used for
2186           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2187           workspace  will  be  needed for patterns and subjects where there are a
2188           lot of potential matches.
2189    
2190           Here is an example of a simple call to pcre_dfa_exec():
2191    
2192             int rc;
2193             int ovector[10];
2194             int wspace[20];
2195             rc = pcre_dfa_exec(
2196               re,             /* result of pcre_compile() */
2197               NULL,           /* we didn't study the pattern */
2198               "some string",  /* the subject string */
2199               11,             /* the length of the subject string */
2200               0,              /* start at offset 0 in the subject */
2201               0,              /* default options */
2202               ovector,        /* vector of integers for substring information */
2203               10,             /* number of elements (NOT size in bytes) */
2204               wspace,         /* working space vector */
2205               20);            /* number of elements (NOT size in bytes) */
2206    
2207       Option bits for pcre_dfa_exec()
2208    
2209           The unused bits of the options argument  for  pcre_dfa_exec()  must  be
2210           zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
2211           LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,
2212           PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2213           three of these are the same as for pcre_exec(), so their description is
2214           not repeated here.
2215    
2216             PCRE_PARTIAL
2217    
2218           This  has  the  same general effect as it does for pcre_exec(), but the
2219           details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
2220           pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
2221           PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
2222           been no complete matches, but there is still at least one matching pos-
2223           sibility. The portion of the string that provided the partial match  is
2224           set as the first matching string.
2225    
2226             PCRE_DFA_SHORTEST
2227    
2228           Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
2229           stop as soon as it has found one match. Because of the way the alterna-
2230           tive  algorithm  works, this is necessarily the shortest possible match
2231           at the first possible matching point in the subject string.
2232    
2233             PCRE_DFA_RESTART
2234    
2235           When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
2236           returns  a  partial  match, it is possible to call it again, with addi-
2237           tional subject characters, and have it continue with  the  same  match.
2238           The  PCRE_DFA_RESTART  option requests this action; when it is set, the
2239           workspace and wscount options must reference the same vector as  before
2240           because  data  about  the  match so far is left in them after a partial
2241           match. There is more discussion of this  facility  in  the  pcrepartial
2242           documentation.
2243    
2244       Successful returns from pcre_dfa_exec()
2245    
2246           When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
2247           string in the subject. Note, however, that all the matches from one run
2248           of  the  function  start  at the same point in the subject. The shorter
2249           matches are all initial substrings of the longer matches. For  example,
2250           if the pattern
2251    
2252             <.*>
2253    
2254           is matched against the string
2255    
2256             This is <something> <something else> <something further> no more
2257    
2258           the three matched strings are
2259    
2260             <something>
2261             <something> <something else>
2262             <something> <something else> <something further>
2263    
2264           On  success,  the  yield of the function is a number greater than zero,
2265           which is the number of matched substrings.  The  substrings  themselves
2266           are  returned  in  ovector. Each string uses two elements; the first is
2267           the offset to the start, and the second is the offset to  the  end.  In
2268           fact,  all  the  strings  have the same start offset. (Space could have
2269           been saved by giving this only once, but it was decided to retain  some
2270           compatibility  with  the  way pcre_exec() returns data, even though the
2271           meaning of the strings is different.)
2272    
2273           The strings are returned in reverse order of length; that is, the long-
2274           est  matching  string is given first. If there were too many matches to
2275           fit into ovector, the yield of the function is zero, and the vector  is
2276           filled with the longest matches.
2277    
2278       Error returns from pcre_dfa_exec()
2279    
2280           The  pcre_dfa_exec()  function returns a negative number when it fails.
2281           Many of the errors are the same  as  for  pcre_exec(),  and  these  are
2282           described  above.   There are in addition the following errors that are
2283           specific to pcre_dfa_exec():
2284    
2285             PCRE_ERROR_DFA_UITEM      (-16)
2286    
2287           This return is given if pcre_dfa_exec() encounters an item in the  pat-
2288           tern  that  it  does not support, for instance, the use of \C or a back
2289           reference.
2290    
2291             PCRE_ERROR_DFA_UCOND      (-17)
2292    
2293           This return is given if pcre_dfa_exec()  encounters  a  condition  item
2294           that  uses  a back reference for the condition, or a test for recursion
2295           in a specific group. These are not supported.
2296    
2297             PCRE_ERROR_DFA_UMLIMIT    (-18)
2298    
2299           This return is given if pcre_dfa_exec() is called with an  extra  block
2300           that contains a setting of the match_limit field. This is not supported
2301           (it is meaningless).
2302    
2303             PCRE_ERROR_DFA_WSSIZE     (-19)
2304    
2305           This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
2306           workspace vector.
2307    
2308             PCRE_ERROR_DFA_RECURSE    (-20)
2309    
2310           When  a  recursive subpattern is processed, the matching function calls
2311           itself recursively, using private vectors for  ovector  and  workspace.
2312           This  error  is  given  if  the output vector is not large enough. This
2313           should be extremely rare, as a vector of size 1000 is used.
2314    
2315    
2316    SEE ALSO
2317    
2318           pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
2319           tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2320    
2321    Last updated: 30 November 2006
2322    Copyright (c) 1997-2006 University of Cambridge.
2323    ------------------------------------------------------------------------------
2324    
 MATCHING A PATTERN  
      The function pcre_exec() is called to match a subject string  
2325    
2326    PCRECALLOUT(3)                                                  PCRECALLOUT(3)
2327    
2328    
2329    NAME
2330           PCRE - Perl-compatible regular expressions
2331    
2332    
2333  SunOS 5.8                 Last change:                          9  PCRE CALLOUTS
2334    
2335           int (*pcre_callout)(pcre_callout_block *);
2336    
2337           PCRE provides a feature called "callout", which is a means of temporar-
2338           ily passing control to the caller of PCRE  in  the  middle  of  pattern
2339           matching.  The  caller of PCRE provides an external function by putting
2340           its entry point in the global variable pcre_callout. By  default,  this
2341           variable contains NULL, which disables all calling out.
2342    
2343           Within  a  regular  expression,  (?C) indicates the points at which the
2344           external function is to be called.  Different  callout  points  can  be
2345           identified  by  putting  a number less than 256 after the letter C. The
2346           default value is zero.  For  example,  this  pattern  has  two  callout
2347           points:
2348    
2349             (?C1)eabc(?C2)def
2350    
2351           If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2352           called, PCRE automatically  inserts  callouts,  all  with  number  255,
2353           before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2354           used with the pattern
2355    
2356             A(\d{2}|--)
2357    
2358           it is processed as if it were
2359    
2360           (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2361    
2362           Notice that there is a callout before and after  each  parenthesis  and
2363           alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2364           progress of pattern matching. The pcretest command has an  option  that
2365           sets  automatic callouts; when it is used, the output indicates how the
2366           pattern is matched. This is useful information when you are  trying  to
2367           optimize the performance of a particular pattern.
2368    
2369    
2370    MISSING CALLOUTS
2371    
2372           You  should  be  aware  that,  because of optimizations in the way PCRE
2373           matches patterns, callouts sometimes do not happen. For example, if the
2374           pattern is
2375    
2376             ab(?C4)cd
2377    
2378           PCRE knows that any matching string must contain the letter "d". If the
2379           subject string is "abyz", the lack of "d" means that  matching  doesn't
2380           ever  start,  and  the  callout is never reached. However, with "abyd",
2381           though the result is still no match, the callout is obeyed.
2382    
2383    
2384    THE CALLOUT INTERFACE
2385    
2386           During matching, when PCRE reaches a callout point, the external  func-
2387           tion  defined by pcre_callout is called (if it is set). This applies to
2388           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2389           only  argument  to  the callout function is a pointer to a pcre_callout
2390           block. This structure contains the following fields:
2391    
2392             int          version;
2393             int          callout_number;
2394             int         *offset_vector;
2395             const char  *subject;
2396             int          subject_length;
2397             int          start_match;
2398             int          current_position;
2399             int          capture_top;
2400             int          capture_last;
2401             void        *callout_data;
2402             int          pattern_position;
2403             int          next_item_length;
2404    
2405           The version field is an integer containing the version  number  of  the
2406           block  format. The initial version was 0; the current version is 1. The
2407           version number will change again in future  if  additional  fields  are
2408           added, but the intention is never to remove any of the existing fields.
2409    
2410           The callout_number field contains the number of the  callout,  as  com-
2411           piled  into  the pattern (that is, the number after ?C for manual call-
2412           outs, and 255 for automatically generated callouts).
2413    
2414           The offset_vector field is a pointer to the vector of offsets that  was
2415           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2416           pcre_exec() is used, the contents can be inspected in order to  extract
2417           substrings  that  have  been  matched  so  far,  in the same way as for
2418           extracting substrings after a match has completed. For  pcre_dfa_exec()
2419           this field is not useful.
2420    
2421           The subject and subject_length fields contain copies of the values that
2422           were passed to pcre_exec().
2423    
2424           The start_match field contains the offset within the subject  at  which
2425           the  current match attempt started. If the pattern is not anchored, the
2426           callout function may be called several times from the same point in the
2427           pattern for different starting points in the subject.
2428    
2429           The  current_position  field  contains the offset within the subject of
2430           the current match pointer.
2431    
2432           When the pcre_exec() function is used, the capture_top  field  contains
2433           one  more than the number of the highest numbered captured substring so
2434           far. If no substrings have been captured, the value of  capture_top  is
2435           one.  This  is always the case when pcre_dfa_exec() is used, because it
2436           does not support captured substrings.
2437    
2438           The capture_last field contains the number of the  most  recently  cap-
2439           tured  substring. If no substrings have been captured, its value is -1.
2440           This is always the case when pcre_dfa_exec() is used.
2441    
2442           The callout_data field contains a value that is passed  to  pcre_exec()
2443           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2444           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2445           structure.  If  no such data was passed, the value of callout_data in a
2446           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2447           structure in the pcreapi documentation.
2448    
2449           The  pattern_position field is present from version 1 of the pcre_call-
2450           out structure. It contains the offset to the next item to be matched in
2451           the pattern string.
2452    
2453           The  next_item_length field is present from version 1 of the pcre_call-
2454           out structure. It contains the length of the next item to be matched in
2455           the  pattern  string. When the callout immediately precedes an alterna-
2456           tion bar, a closing parenthesis, or the end of the pattern, the  length
2457           is  zero.  When the callout precedes an opening parenthesis, the length
2458           is that of the entire subpattern.
2459    
2460           The pattern_position and next_item_length fields are intended  to  help
2461           in  distinguishing between different automatic callouts, which all have
2462           the same callout number. However, they are set for all callouts.
2463    
2464    
2465    RETURN VALUES
2466    
2467           The external callout function returns an integer to PCRE. If the  value
2468           is  zero,  matching  proceeds  as  normal. If the value is greater than
2469           zero, matching fails at the current point, but  the  testing  of  other
2470           matching possibilities goes ahead, just as if a lookahead assertion had
2471           failed. If the value is less than zero, the  match  is  abandoned,  and
2472           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2473    
2474           Negative   values   should   normally   be   chosen  from  the  set  of
2475           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2476           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2477           reserved for use by callout functions; it will never be  used  by  PCRE
2478           itself.
2479    
2480    Last updated: 28 February 2005
2481    Copyright (c) 1997-2005 University of Cambridge.
2482    ------------------------------------------------------------------------------
2483    
      against  a pre-compiled pattern, which is passed in the code  
      argument. If the pattern has been studied, the result of the  
      study should be passed in the extra argument. Otherwise this  
      must be NULL.  
   
      Here is an example of a simple call to pcre_exec():  
   
        int rc;  
        int ovector[30];  
        rc = pcre_exec(  
          re,             /* result of pcre_compile() */  
          NULL,           /* we didn't study the pattern */  
          "some string",  /* the subject string */  
          11,             /* the length of the subject string */  
          0,              /* start at offset 0 in the subject */  
          0,              /* default options */  
          ovector,        /* vector for substring information */  
          30);            /* number of elements in the vector */  
   
      The PCRE_ANCHORED option can be passed in the options  argu-  
      ment,  whose unused bits must be zero. However, if a pattern  
      was  compiled  with  PCRE_ANCHORED,  or  turned  out  to  be  
      anchored  by  virtue  of  its  contents,  it  cannot be made  
      unachored at matching time.  
   
      There are also three further options that can be set only at  
      matching time:  
   
        PCRE_NOTBOL  
   
      The first character of the string is not the beginning of  a  
      line,  so  the  circumflex  metacharacter  should  not match  
      before it. Setting this without PCRE_MULTILINE  (at  compile  
      time) causes circumflex never to match.  
   
        PCRE_NOTEOL  
   
      The end of the string is not the end of a line, so the  dol-  
      lar  metacharacter should not match it nor (except in multi-  
      line mode) a newline immediately  before  it.  Setting  this  
      without PCRE_MULTILINE (at compile time) causes dollar never  
      to match.  
   
        PCRE_NOTEMPTY  
   
      An empty string is not considered to be  a  valid  match  if  
      this  option  is  set. If there are alternatives in the pat-  
      tern, they are tried. If  all  the  alternatives  match  the  
      empty  string,  the  entire match fails. For example, if the  
      pattern  
   
        a?b?  
   
      is applied to a string not beginning with  "a"  or  "b",  it  
      matches  the  empty string at the start of the subject. With  
      PCRE_NOTEMPTY set, this match is not valid, so PCRE searches  
      further into the string for occurrences of "a" or "b".  
   
      Perl has no direct equivalent of PCRE_NOTEMPTY, but it  does  
      make  a  special case of a pattern match of the empty string  
      within its split() function, and when using the /g modifier.  
      It  is possible to emulate Perl's behaviour after matching a  
      null string by first trying the  match  again  at  the  same  
      offset  with  PCRE_NOTEMPTY  set,  and then if that fails by  
      advancing the starting offset  (see  below)  and  trying  an  
      ordinary match again.  
   
      The subject string is passed as  a  pointer  in  subject,  a  
      length  in  length,  and  a  starting offset in startoffset.  
      Unlike the pattern string, the subject  may  contain  binary  
      zero  characters.  When  the  starting  offset  is zero, the  
      search for a match starts at the beginning of  the  subject,  
      and this is by far the most common case.  
   
      A non-zero starting offset  is  useful  when  searching  for  
      another  match  in  the  same subject by calling pcre_exec()  
      again after a previous success.  Setting startoffset differs  
      from  just  passing  over  a  shortened  string  and setting  
      PCRE_NOTBOL in the case of a pattern that  begins  with  any  
      kind of lookbehind. For example, consider the pattern  
   
        \Biss\B  
   
      which finds occurrences of "iss" in the middle of words. (\B  
      matches only if the current position in the subject is not a  
      word boundary.) When applied to the string "Mississipi"  the  
      first  call  to  pcre_exec()  finds the first occurrence. If  
      pcre_exec() is called again with just the remainder  of  the  
      subject,  namely  "issipi", it does not match, because \B is  
      always false at the start of the subject, which is deemed to  
      be  a  word  boundary. However, if pcre_exec() is passed the  
      entire string again, but with startoffset set to 4, it finds  
      the  second  occurrence  of "iss" because it is able to look  
      behind the starting point to discover that it is preceded by  
      a letter.  
   
      If a non-zero starting offset is passed when the pattern  is  
      anchored, one attempt to match at the given offset is tried.  
      This can only succeed if the pattern does  not  require  the  
      match to be at the start of the subject.  
   
      In general, a pattern matches a certain portion of the  sub-  
      ject,  and  in addition, further substrings from the subject  
      may be picked out by parts of  the  pattern.  Following  the  
      usage  in  Jeffrey Friedl's book, this is called "capturing"  
      in what follows, and the phrase  "capturing  subpattern"  is  
      used for a fragment of a pattern that picks out a substring.  
      PCRE supports several other kinds of  parenthesized  subpat-  
      tern that do not cause substrings to be captured.  
   
      Captured substrings are returned to the caller via a  vector  
      of  integer  offsets whose address is passed in ovector. The  
      number of elements in the vector is passed in ovecsize.  The  
      first two-thirds of the vector is used to pass back captured  
      substrings, each substring using a  pair  of  integers.  The  
      remaining  third  of  the  vector  is  used  as workspace by  
      pcre_exec() while matching capturing subpatterns, and is not  
      available for passing back information. The length passed in  
      ovecsize should always be a multiple of three. If it is not,  
      it is rounded down.  
   
      When a match has been successful, information about captured  
      substrings is returned in pairs of integers, starting at the  
      beginning of ovector, and continuing up to two-thirds of its  
      length  at  the  most. The first element of a pair is set to  
      the offset of the first character in a  substring,  and  the  
      second is set to the offset of the first character after the  
      end of a substring. The first  pair,  ovector[0]  and  ovec-  
      tor[1],  identify  the portion of the subject string matched  
      by the entire pattern. The next pair is used for  the  first  
      capturing  subpattern,  and  so  on.  The  value returned by  
      pcre_exec() is the number of pairs that have  been  set.  If  
      there  are no capturing subpatterns, the return value from a  
      successful match is 1, indicating that just the  first  pair  
      of offsets has been set.  
   
      Some convenience functions are provided for  extracting  the  
      captured substrings as separate strings. These are described  
      in the following section.  
   
      It is possible for an capturing  subpattern  number  n+1  to  
      match  some  part  of  the subject when subpattern n has not  
      been used at all.  For  example,  if  the  string  "abc"  is  
      matched  against the pattern (a|(z))(bc) subpatterns 1 and 3  
      are matched, but 2 is not. When this  happens,  both  offset  
      values corresponding to the unused subpattern are set to -1.  
   
      If a capturing subpattern is matched repeatedly, it  is  the  
      last  portion  of  the  string  that  it  matched  that gets  
      returned.  
   
      If the vector is too small to hold  all  the  captured  sub-  
      strings,  it is used as far as possible (up to two-thirds of  
      its length), and the function returns a value  of  zero.  In  
      particular,  if  the  substring offsets are not of interest,  
      pcre_exec() may be called with ovector passed  as  NULL  and  
      ovecsize  as  zero.  However,  if  the pattern contains back  
      references and the ovector isn't big enough to remember  the  
      related  substrings,  PCRE  has to get additional memory for  
      use during matching. Thus it is usually advisable to  supply  
      an ovector.  
   
      Note that pcre_info() can be used to find out how many  cap-  
      turing  subpatterns  there  are  in  a compiled pattern. The  
      smallest size for ovector that will  allow  for  n  captured  
      substrings  in  addition  to  the  offsets  of the substring  
      matched by the whole pattern is (n+1)*3.  
   
      If pcre_exec() fails, it returns a negative number. The fol-  
      lowing are defined in the header file:  
   
        PCRE_ERROR_NOMATCH        (-1)  
   
      The subject string did not match the pattern.  
   
        PCRE_ERROR_NULL           (-2)  
   
      Either code or subject was passed as NULL,  or  ovector  was  
      NULL and ovecsize was not zero.  
   
        PCRE_ERROR_BADOPTION      (-3)  
   
      An unrecognized bit was set in the options argument.  
   
        PCRE_ERROR_BADMAGIC       (-4)  
   
      PCRE stores a 4-byte "magic number" at the start of the com-  
      piled  code,  to  catch  the  case  when it is passed a junk  
      pointer. This is the error it gives when  the  magic  number  
      isn't present.  
   
        PCRE_ERROR_UNKNOWN_NODE   (-5)  
   
      While running the pattern match, an unknown item was encoun-  
      tered in the compiled pattern. This error could be caused by  
      a bug in PCRE or by overwriting of the compiled pattern.  
   
        PCRE_ERROR_NOMEMORY       (-6)  
   
      If a pattern contains back references, but the ovector  that  
      is  passed  to pcre_exec() is not big enough to remember the  
      referenced substrings, PCRE gets a block of  memory  at  the  
      start  of  matching to use for this purpose. If the call via  
      pcre_malloc() fails, this error  is  given.  The  memory  is  
      freed at the end of matching.  
   
   
   
   
 EXTRACTING CAPTURED SUBSTRINGS  
      Captured substrings can be accessed directly  by  using  the  
      offsets returned by pcre_exec() in ovector. For convenience,  
      the functions  pcre_copy_substring(),  pcre_get_substring(),  
      and  pcre_get_substring_list()  are  provided for extracting  
      captured  substrings  as  new,   separate,   zero-terminated  
      strings.   A  substring  that  contains  a  binary  zero  is  
      correctly extracted and has a further zero added on the end,  
      but the result does not, of course, function as a C string.  
   
      The first three arguments are the same for all  three  func-  
      tions:  subject  is  the  subject string which has just been  
      successfully matched, ovector is a pointer to the vector  of  
      integer   offsets   that  was  passed  to  pcre_exec(),  and  
      stringcount is the number of substrings that  were  captured  
      by  the  match,  including  the  substring  that matched the  
      entire regular expression. This is  the  value  returned  by  
      pcre_exec  if  it  is  greater  than  zero.  If  pcre_exec()  
      returned zero, indicating that it ran out of space in  ovec-  
      tor,  the  value passed as stringcount should be the size of  
      the vector divided by three.  
   
      The functions pcre_copy_substring() and pcre_get_substring()  
      extract a single substring, whose number is given as string-  
      number. A value of zero extracts the substring that  matched  
      the entire pattern, while higher values extract the captured  
      substrings. For pcre_copy_substring(), the string is  placed  
      in  buffer,  whose  length is given by buffersize, while for  
      pcre_get_substring() a new block of memory is  obtained  via  
      pcre_malloc,  and its address is returned via stringptr. The  
      yield of the function is  the  length  of  the  string,  not  
      including the terminating zero, or one of  
   
        PCRE_ERROR_NOMEMORY       (-6)  
   
      The buffer was too small for pcre_copy_substring(),  or  the  
      attempt to get memory failed for pcre_get_substring().  
   
        PCRE_ERROR_NOSUBSTRING    (-7)  
   
      There is no substring whose number is stringnumber.  
   
      The pcre_get_substring_list() function extracts  all  avail-  
      able  substrings  and builds a list of pointers to them. All  
      this is done in a single block of memory which  is  obtained  
      via pcre_malloc. The address of the memory block is returned  
      via listptr, which is also the start of the list  of  string  
      pointers.  The  end of the list is marked by a NULL pointer.  
      The yield of the function is zero if all went well, or  
   
        PCRE_ERROR_NOMEMORY       (-6)  
   
      if the attempt to get the memory block failed.  
   
      When any of these functions encounter a  substring  that  is  
      unset, which can happen when capturing subpattern number n+1  
      matches some part of the subject, but subpattern n  has  not  
      been  used  at all, they return an empty string. This can be  
      distinguished  from  a  genuine  zero-length  substring   by  
      inspecting the appropriate offset in ovector, which is nega-  
      tive for unset substrings.  
   
      The  two  convenience  functions  pcre_free_substring()  and  
      pcre_free_substring_list()  can  be  used to free the memory  
      returned by  a  previous  call  of  pcre_get_substring()  or  
      pcre_get_substring_list(),  respectively.  They  do  nothing  
      more than call the function pointed to by  pcre_free,  which  
      of  course  could  be called directly from a C program. How-  
      ever, PCRE is used in some situations where it is linked via  
      a  special  interface  to another programming language which  
      cannot use pcre_free directly; it is for  these  cases  that  
      the functions are provided.  
2484    
2485    PCRECOMPAT(3)                                                    PCRECOMPAT(3)
2486    
2487    
2488  LIMITATIONS  NAME
2489       There are some size limitations in PCRE but it is hoped that         PCRE - Perl-compatible regular expressions
      they will never in practice be relevant.  The maximum length  
      of a compiled pattern is 65539 (sic) bytes.  All  values  in  
      repeating  quantifiers  must be less than 65536.  There max-  
      imum number of capturing subpatterns is 65535.  There is  no  
      limit  to  the  number of non-capturing subpatterns, but the  
      maximum depth of nesting of all kinds of parenthesized  sub-  
      pattern,  including  capturing  subpatterns, assertions, and  
      other types of subpattern, is 200.  
   
      The maximum length of a subject string is the largest  posi-  
      tive number that an integer variable can hold. However, PCRE  
      uses recursion to handle subpatterns and indefinite  repeti-  
      tion.  This  means  that the available stack space may limit  
      the size of a subject string that can be processed  by  cer-  
      tain patterns.  
   
   
   
 DIFFERENCES FROM PERL  
      The differences described here  are  with  respect  to  Perl  
      5.005.  
   
      1. By default, a whitespace character is any character  that  
      the  C  library  function isspace() recognizes, though it is  
      possible to compile PCRE  with  alternative  character  type  
      tables. Normally isspace() matches space, formfeed, newline,  
      carriage return, horizontal tab, and vertical tab. Perl 5 no  
      longer  includes vertical tab in its set of whitespace char-  
      acters. The \v escape that was in the Perl documentation for  
      a long time was never in fact recognized. However, the char-  
      acter itself was treated as whitespace at least up to 5.002.  
      In 5.004 and 5.005 it does not match \s.  
   
      2. PCRE does  not  allow  repeat  quantifiers  on  lookahead  
      assertions. Perl permits them, but they do not mean what you  
      might think. For example, (?!a){3} does not assert that  the  
      next  three characters are not "a". It just asserts that the  
      next character is not "a" three times.  
   
      3. Capturing subpatterns that occur inside  negative  looka-  
      head  assertions  are  counted,  but  their  entries  in the  
      offsets vector are never set. Perl sets its numerical  vari-  
      ables  from  any  such  patterns that are matched before the  
      assertion fails to match something (thereby succeeding), but  
      only  if  the negative lookahead assertion contains just one  
      branch.  
   
      4. Though binary zero characters are supported in  the  sub-  
      ject  string,  they  are  not  allowed  in  a pattern string  
      because it is passed as a normal  C  string,  terminated  by  
      zero. The escape sequence "\0" can be used in the pattern to  
      represent a binary zero.  
   
      5. The following Perl escape sequences  are  not  supported:  
      \l,  \u,  \L,  \U,  \E, \Q. In fact these are implemented by  
      Perl's general string-handling and are not part of its  pat-  
      tern matching engine.  
   
      6. The Perl \G assertion is  not  supported  as  it  is  not  
      relevant to single pattern matches.  
   
      7. Fairly obviously, PCRE does not support the (?{code}) and  
      (?p{code})  constructions. However, there is some experimen-  
      tal support for recursive patterns using the  non-Perl  item  
      (?R).  
   
      8. There are at the time of writing some  oddities  in  Perl  
      5.005_02  concerned  with  the  settings of captured strings  
      when part of a pattern is repeated.  For  example,  matching  
      "aba"  against the pattern /^(a(b)?)+$/ sets $2 to the value  
      "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves  $2  
      unset.    However,    if   the   pattern   is   changed   to  
      /^(aa(b(b))?)+$/ then $2 (and $3) are set.  
   
      In Perl 5.004 $2 is set in both cases, and that is also true  
      of PCRE. If in the future Perl changes to a consistent state  
      that is different, PCRE may change to follow.  
   
      9. Another as yet unresolved discrepancy  is  that  in  Perl  
      5.005_02  the  pattern /^(a)?(?(1)a|b)+$/ matches the string  
      "a", whereas in PCRE it does not.  However, in both Perl and  
      PCRE /^(a)?a/ matched against "a" leaves $1 unset.  
   
      10. PCRE  provides  some  extensions  to  the  Perl  regular  
      expression facilities:  
   
      (a) Although lookbehind assertions must match  fixed  length  
      strings,  each  alternative branch of a lookbehind assertion  
      can match a different length of string. Perl 5.005  requires  
      them all to have the same length.  
   
      (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is  not  
      set,  the  $ meta- character matches only at the very end of  
      the string.  
   
      (c) If PCRE_EXTRA is set, a backslash followed by  a  letter  
      with no special meaning is faulted.  
   
      (d) If PCRE_UNGREEDY is set, the greediness of  the  repeti-  
      tion  quantifiers  is inverted, that is, by default they are  
      not greedy, but if followed by a question mark they are.  
   
      (e) PCRE_ANCHORED can be used to force a pattern to be tried  
      only at the start of the subject.  
   
      (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY  options  
      for pcre_exec() have no Perl equivalents.  
   
      (g) The (?R) construct allows for recursive pattern matching  
      (Perl  5.6 can do this using the (?p{code}) construct, which  
      PCRE cannot of course support.)  
   
   
   
 REGULAR EXPRESSION DETAILS  
      The syntax and semantics of  the  regular  expressions  sup-  
      ported  by PCRE are described below. Regular expressions are  
      also described in the Perl documentation and in a number  of  
      other  books,  some  of which have copious examples. Jeffrey  
      Friedl's  "Mastering  Regular  Expressions",  published   by  
      O'Reilly (ISBN 1-56592-257), covers them in great detail.  
   
      The description here is intended as reference documentation.  
      The basic operation of PCRE is on strings of bytes. However,  
      there is the beginnings of some support for UTF-8  character  
      strings.  To  use  this  support  you must configure PCRE to  
      include it, and then call pcre_compile() with the  PCRE_UTF8  
      option.  How  this affects the pattern matching is described  
      in the final section of this document.  
   
      A regular expression is a pattern that is matched against  a  
      subject string from left to right. Most characters stand for  
      themselves in a pattern, and match the corresponding charac-  
      ters in the subject. As a trivial example, the pattern  
   
        The quick brown fox  
   
      matches a portion of a subject string that is  identical  to  
      itself.  The  power  of  regular  expressions comes from the  
      ability to include alternatives and repetitions in the  pat-  
      tern.  These  are encoded in the pattern by the use of meta-  
      characters, which do not stand for  themselves  but  instead  
      are interpreted in some special way.  
   
      There are two different sets of meta-characters: those  that  
      are  recognized anywhere in the pattern except within square  
      brackets, and those that are recognized in square  brackets.  
      Outside square brackets, the meta-characters are as follows:  
   
        \      general escape character with several uses  
        ^      assert start of  subject  (or  line,  in  multiline  
      mode)  
        $      assert end of subject (or line, in multiline mode)  
        .      match any character except newline (by default)  
        [      start character class definition  
        |      start of alternative branch  
        (      start subpattern  
        )      end subpattern  
        ?      extends the meaning of (  
               also 0 or 1 quantifier  
               also quantifier minimizer  
        *      0 or more quantifier  
        +      1 or more quantifier  
        {      start min/max quantifier  
   
      Part of a pattern that is in square  brackets  is  called  a  
      "character  class".  In  a  character  class  the only meta-  
      characters are:  
   
        \      general escape character  
        ^      negate the class, but only if the first character  
        -      indicates character range  
        ]      terminates the character class  
2490    
      The following sections describe  the  use  of  each  of  the  
      meta-characters.  
2491    
2492    DIFFERENCES BETWEEN PCRE AND PERL
2493    
2494           This  document describes the differences in the ways that PCRE and Perl
2495           handle regular expressions. The differences described here  are  mainly
2496           with  respect  to  Perl 5.8, though PCRE version 7.0 contains some fea-
2497           tures that are expected to be in the forthcoming Perl 5.10.
2498    
2499  BACKSLASH         1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2500       The backslash character has several uses. Firstly, if it  is         of  what  it does have are given in the section on UTF-8 support in the
2501       followed  by  a  non-alphameric character, it takes away any         main pcre page.
      special  meaning  that  character  may  have.  This  use  of  
   
      backslash  as  an  escape  character applies both inside and  
      outside character classes.  
   
      For example, if you want to match a "*" character, you write  
      "\*" in the pattern. This applies whether or not the follow-  
      ing character would otherwise  be  interpreted  as  a  meta-  
      character,  so it is always safe to precede a non-alphameric  
      with "\" to specify that it stands for itself.  In  particu-  
      lar, if you want to match a backslash, you write "\\".  
   
      If a pattern is compiled with the PCRE_EXTENDED option, whi-  
      tespace in the pattern (other than in a character class) and  
      characters between a "#" outside a character class  and  the  
      next  newline  character  are ignored. An escaping backslash  
      can be used to include a whitespace or "#" character as part  
      of the pattern.  
   
      A second use of backslash provides a way  of  encoding  non-  
      printing  characters  in patterns in a visible manner. There  
      is no restriction on the appearance of non-printing  charac-  
      ters,  apart from the binary zero that terminates a pattern,  
      but when a pattern is being prepared by text editing, it  is  
      usually  easier to use one of the following escape sequences  
      than the binary character it represents:  
   
        \a     alarm, that is, the BEL character (hex 07)  
        \cx    "control-x", where x is any character  
        \e     escape (hex 1B)  
        \f     formfeed (hex 0C)  
        \n     newline (hex 0A)  
        \r     carriage return (hex 0D)  
        \t     tab (hex 09)  
        \xhh   character with hex code hh  
        \ddd   character with octal code ddd, or backreference  
   
      The precise effect of "\cx" is as follows: if "x" is a lower  
      case  letter,  it  is converted to upper case. Then bit 6 of  
      the character (hex 40) is inverted.  Thus "\cz" becomes  hex  
      1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.  
   
      After "\x", up to two hexadecimal digits are  read  (letters  
      can be in upper or lower case).  
   
      After "\0" up to two further octal digits are read. In  both  
      cases,  if  there are fewer than two digits, just those that  
      are present are used. Thus the sequence "\0\x\07"  specifies  
      two binary zeros followed by a BEL character.  Make sure you  
      supply two digits after the initial zero  if  the  character  
      that follows is itself an octal digit.  
   
      The handling of a backslash followed by a digit other than 0  
      is  complicated.   Outside  a character class, PCRE reads it  
      and any following digits as a decimal number. If the  number  
      is  less  than  10, or if there have been at least that many  
      previous capturing left parentheses in the  expression,  the  
      entire  sequence is taken as a back reference. A description  
      of how this works is given later, following  the  discussion  
      of parenthesized subpatterns.  
   
      Inside a character  class,  or  if  the  decimal  number  is  
      greater  than  9 and there have not been that many capturing  
      subpatterns, PCRE re-reads up to three octal digits  follow-  
      ing  the  backslash,  and  generates  a single byte from the  
      least significant 8 bits of the value. Any subsequent digits  
      stand for themselves.  For example:  
   
        \040   is another way of writing a space  
        \40    is the same, provided there are fewer than 40  
                  previous capturing subpatterns  
        \7     is always a back reference  
        \11    might be a back reference, or another way of  
                  writing a tab  
        \011   is always a tab  
        \0113  is a tab followed by the character "3"  
        \113   is the character with octal code 113 (since there  
                  can be no more than 99 back references)  
        \377   is a byte consisting entirely of 1 bits  
        \81    is either a back reference, or a binary zero  
                  followed by the two characters "8" and "1"  
   
      Note that octal values of 100 or greater must not be  intro-  
      duced  by  a  leading zero, because no more than three octal  
      digits are ever read.  
   
      All the sequences that define a single  byte  value  can  be  
      used both inside and outside character classes. In addition,  
      inside a character class, the sequence "\b"  is  interpreted  
      as  the  backspace  character  (hex 08). Outside a character  
      class it has a different meaning (see below).  
   
      The third use of backslash is for specifying generic charac-  
      ter types:  
   
        \d     any decimal digit  
        \D     any character that is not a decimal digit  
        \s     any whitespace character  
        \S     any character that is not a whitespace character  
        \w     any "word" character  
        \W     any "non-word" character  
   
      Each pair of escape sequences partitions the complete set of  
      characters  into  two  disjoint  sets.  Any  given character  
      matches one, and only one, of each pair.  
   
      A "word" character is any letter or digit or the  underscore  
      character,  that  is,  any  character which can be part of a  
      Perl "word". The definition of letters and  digits  is  con-  
      trolled  by PCRE's character tables, and may vary if locale-  
      specific matching is  taking  place  (see  "Locale  support"  
      above). For example, in the "fr" (French) locale, some char-  
      acter codes greater than 128 are used for accented  letters,  
      and these are matched by \w.  
   
      These character type sequences can appear  both  inside  and  
      outside  character classes. They each match one character of  
      the appropriate type. If the current matching  point  is  at  
      the end of the subject string, all of them fail, since there  
      is no character to match.  
   
      The fourth use of backslash is  for  certain  simple  asser-  
      tions. An assertion specifies a condition that has to be met  
      at a particular point in  a  match,  without  consuming  any  
      characters  from  the subject string. The use of subpatterns  
      for more complicated  assertions  is  described  below.  The  
      backslashed assertions are  
   
        \b     word boundary  
        \B     not a word boundary  
        \A     start of subject (independent of multiline mode)  
        \Z     end of subject or newline at  end  (independent  of  
      multiline mode)  
        \z     end of subject (independent of multiline mode)  
   
      These assertions may not appear in  character  classes  (but  
      note that "\b" has a different meaning, namely the backspace  
      character, inside a character class).  
   
      A word boundary is a position in the  subject  string  where  
      the current character and the previous character do not both  
      match \w or \W (i.e. one matches \w and  the  other  matches  
      \W),  or the start or end of the string if the first or last  
      character matches \w, respectively.  
   
      The \A, \Z, and \z assertions differ  from  the  traditional  
      circumflex  and  dollar  (described below) in that they only  
      ever match at the very start and end of the subject  string,  
      whatever  options  are  set.  They  are  not affected by the  
      PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-  
      ment  of  pcre_exec()  is  non-zero, \A can never match. The  
      difference between \Z and \z is that  \Z  matches  before  a  
      newline  that is the last character of the string as well as  
      at the end of the string, whereas \z  matches  only  at  the  
      end.  
2502    
2503           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2504           permits  them,  but they do not mean what you might think. For example,
2505           (?!a){3} does not assert that the next three characters are not "a". It
2506           just asserts that the next character is not "a" three times.
2507    
2508           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2509           tions are counted, but their entries in the offsets  vector  are  never
2510           set.  Perl sets its numerical variables from any such patterns that are
2511           matched before the assertion fails to match something (thereby succeed-
2512           ing),  but  only  if the negative lookahead assertion contains just one
2513           branch.
2514    
2515  CIRCUMFLEX AND DOLLAR         4. Though binary zero characters are supported in the  subject  string,
2516       Outside a character class, in the default matching mode, the         they are not allowed in a pattern string because it is passed as a nor-
2517       circumflex  character  is an assertion which is true only if         mal C string, terminated by zero. The escape sequence \0 can be used in
2518       the current matching point is at the start  of  the  subject         the pattern to represent a binary zero.
      string.  If  the startoffset argument of pcre_exec() is non-  
      zero, circumflex can never match. Inside a character  class,  
      circumflex has an entirely different meaning (see below).  
   
      Circumflex need not be the first character of the pattern if  
      a  number of alternatives are involved, but it should be the  
      first thing in each alternative in which it appears  if  the  
      pattern is ever to match that branch. If all possible alter-  
      natives start with a circumflex, that is, if the pattern  is  
      constrained to match only at the start of the subject, it is  
      said to be an "anchored" pattern. (There are also other con-  
      structs that can cause a pattern to be anchored.)  
   
      A dollar character is an assertion which is true only if the  
      current  matching point is at the end of the subject string,  
      or immediately before a newline character that is  the  last  
      character in the string (by default). Dollar need not be the  
      last character of the pattern if a  number  of  alternatives  
      are  involved,  but it should be the last item in any branch  
      in which it appears.  Dollar has no  special  meaning  in  a  
      character class.  
   
      The meaning of dollar can be changed so that it matches only  
      at   the   very   end   of   the   string,  by  setting  the  
      PCRE_DOLLAR_ENDONLY option at compile or matching time. This  
      does not affect the \Z assertion.  
   
      The meanings of the circumflex  and  dollar  characters  are  
      changed  if  the  PCRE_MULTILINE option is set. When this is  
      the case,  they  match  immediately  after  and  immediately  
      before an internal "\n" character, respectively, in addition  
      to matching at the start and end of the subject string.  For  
      example,  the  pattern  /^abc$/  matches  the subject string  
      "def\nabc" in multiline  mode,  but  not  otherwise.  Conse-  
      quently,  patterns  that  are  anchored  in single line mode  
      because all branches start with "^" are not anchored in mul-  
      tiline mode, and a match for circumflex is possible when the  
      startoffset  argument  of  pcre_exec()  is   non-zero.   The  
      PCRE_DOLLAR_ENDONLY  option  is ignored if PCRE_MULTILINE is  
      set.  
   
      Note that the sequences \A, \Z, and \z can be used to  match  
      the  start  and end of the subject in both modes, and if all  
      branches of a pattern start with \A it is  always  anchored,  
      whether PCRE_MULTILINE is set or not.  
2519    
2520           5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2521           \U, and \N. In fact these are implemented by Perl's general string-han-
2522           dling  and are not part of its pattern matching engine. If any of these
2523           are encountered by PCRE, an error is generated.
2524    
2525           6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2526           is  built  with Unicode character property support. The properties that
2527           can be tested with \p and \P are limited to the general category  prop-
2528           erties  such  as  Lu and Nd, script names such as Greek or Han, and the
2529           derived properties Any and L&.
2530    
2531  FULL STOP (PERIOD, DOT)         7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2532       Outside a character class, a dot in the pattern matches  any         ters  in  between  are  treated as literals. This is slightly different
2533       one character in the subject, including a non-printing char-         from Perl in that $ and @ are  also  handled  as  literals  inside  the
2534       acter, but not (by default)  newline.   If  the  PCRE_DOTALL         quotes.  In Perl, they cause variable interpolation (but of course PCRE
2535       option  is set, dots match newlines as well. The handling of         does not have variables). Note the following examples:
      dot is entirely independent of the  handling  of  circumflex  
      and  dollar,  the  only  relationship  being  that they both  
      involve newline characters. Dot has no special meaning in  a  
      character class.  
   
   
   
 SQUARE BRACKETS  
      An opening square bracket introduces a character class, ter-  
      minated  by  a  closing  square  bracket.  A  closing square  
      bracket on its own is  not  special.  If  a  closing  square  
      bracket  is  required as a member of the class, it should be  
      the first data character in the class (after an initial cir-  
      cumflex, if present) or escaped with a backslash.  
   
      A character class matches a single character in the subject;  
      the  character  must  be in the set of characters defined by  
      the class, unless the first character in the class is a cir-  
      cumflex,  in which case the subject character must not be in  
      the set defined by the class. If a  circumflex  is  actually  
      required  as  a  member  of  the class, ensure it is not the  
      first character, or escape it with a backslash.  
   
      For example, the character class [aeiou] matches  any  lower  
      case vowel, while [^aeiou] matches any character that is not  
      a lower case vowel. Note that a circumflex is  just  a  con-  
      venient  notation for specifying the characters which are in  
      the class by enumerating those that are not. It  is  not  an  
      assertion:  it  still  consumes a character from the subject  
      string, and fails if the current pointer is at  the  end  of  
      the string.  
   
      When caseless matching  is  set,  any  letters  in  a  class  
      represent  both their upper case and lower case versions, so  
      for example, a caseless [aeiou] matches "A" as well as  "a",  
      and  a caseless [^aeiou] does not match "A", whereas a case-  
      ful version would.  
   
      The newline character is never treated in any special way in  
      character  classes,  whatever the setting of the PCRE_DOTALL  
      or PCRE_MULTILINE options is. A  class  such  as  [^a]  will  
      always match a newline.  
   
      The minus (hyphen) character can be used to specify a  range  
      of  characters  in  a  character  class.  For example, [d-m]  
      matches any letter between d and m, inclusive.  If  a  minus  
      character  is required in a class, it must be escaped with a  
      backslash or appear in a position where it cannot be  inter-  
      preted as indicating a range, typically as the first or last  
      character in the class.  
   
      It is not possible to have the literal character "]" as  the  
      end  character  of  a  range.  A  pattern such as [W-]46] is  
      interpreted as a class of two characters ("W" and "-")  fol-  
      lowed by a literal string "46]", so it would match "W46]" or  
      "-46]". However, if the "]" is escaped with a  backslash  it  
      is  interpreted  as  the end of range, so [W-\]46] is inter-  
      preted as a single class containing a range followed by  two  
      separate characters. The octal or hexadecimal representation  
      of "]" can also be used to end a range.  
   
      Ranges operate in ASCII collating sequence. They can also be  
      used  for  characters  specified  numerically,  for  example  
      [\000-\037]. If a range that includes letters is  used  when  
      caseless  matching  is set, it matches the letters in either  
      case. For example, [W-c] is equivalent  to  [][\^_`wxyzabc],  
      matched  caselessly,  and  if  character tables for the "fr"  
      locale are in use, [\xc8-\xcb] matches accented E characters  
      in both cases.  
   
      The character types \d, \D, \s, \S,  \w,  and  \W  may  also  
      appear  in  a  character  class, and add the characters that  
      they match to the class. For example, [\dABCDEF] matches any  
      hexadecimal  digit.  A  circumflex  can conveniently be used  
      with the upper case character types to specify a  more  res-  
      tricted set of characters than the matching lower case type.  
      For example, the class [^\W_] matches any letter  or  digit,  
      but not underscore.  
   
      All non-alphameric characters other than \,  -,  ^  (at  the  
      start)  and  the  terminating ] are non-special in character  
      classes, but it does no harm if they are escaped.  
2536    
2537               Pattern            PCRE matches      Perl matches
2538    
2539               \Qabc$xyz\E        abc$xyz           abc followed by the
2540                                                      contents of $xyz
2541               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2542               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2543    
2544  POSIX CHARACTER CLASSES         The \Q...\E sequence is recognized both inside  and  outside  character
2545       Perl 5.6 (not yet released at the time of writing) is  going         classes.
      to  support  the POSIX notation for character classes, which  
      uses names enclosed by  [:  and  :]   within  the  enclosing  
      square brackets. PCRE supports this notation. For example,  
   
        [01[:alpha:]%]  
   
      matches "0", "1", any alphabetic character, or "%". The sup-  
      ported class names are  
   
        alnum    letters and digits  
        alpha    letters  
        ascii    character codes 0 - 127  
        cntrl    control characters  
        digit    decimal digits (same as \d)  
        graph    printing characters, excluding space  
        lower    lower case letters  
        print    printing characters, including space  
        punct    printing characters, excluding letters and digits  
        space    white space (same as \s)  
        upper    upper case letters  
        word     "word" characters (same as \w)  
        xdigit   hexadecimal digits  
   
      The names "ascii" and "word" are  Perl  extensions.  Another  
      Perl  extension is negation, which is indicated by a ^ char-  
      acter after the colon. For example,  
   
        [12[:^digit:]]  
   
      matches "1", "2", or any non-digit.  PCRE  (and  Perl)  also  
      recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a  
      "collating element", but these are  not  supported,  and  an  
      error is given if they are encountered.  
2546    
2547           8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2548           constructions. However, there is support for recursive  patterns.  This
2549           is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2550           "callout" feature allows an external function to be called during  pat-
2551           tern matching. See the pcrecallout documentation for details.
2552    
2553           9.  Subpatterns  that  are  called  recursively or as "subroutines" are
2554           always treated as atomic groups in  PCRE.  This  is  like  Python,  but
2555           unlike Perl.
2556    
2557  VERTICAL BAR         10.  There are some differences that are concerned with the settings of
2558       Vertical bar characters are  used  to  separate  alternative         captured strings when part of  a  pattern  is  repeated.  For  example,
2559       patterns. For example, the pattern         matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
2560           unset, but in PCRE it is set to "b".
2561    
2562         gilbert|sullivan         11. PCRE provides some extensions to the Perl regular expression facil-
2563           ities.   Perl  5.10  will  include new features that are not in earlier
2564           versions, some of which (such as named parentheses) have been  in  PCRE
2565           for some time. This list is with respect to Perl 5.10:
2566    
2567       matches either "gilbert" or "sullivan". Any number of alter-         (a)  Although  lookbehind  assertions  must match fixed length strings,
2568       natives  may  appear,  and an empty alternative is permitted         each alternative branch of a lookbehind assertion can match a different
2569       (matching the empty string).   The  matching  process  tries         length of string. Perl requires them all to have the same length.
      each  alternative in turn, from left to right, and the first  
      one that succeeds is used. If the alternatives are within  a  
      subpattern  (defined  below),  "succeeds" means matching the  
      rest of the main pattern as well as the alternative  in  the  
      subpattern.  
2570    
2571           (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2572           meta-character matches only at the very end of the string.
2573    
2574           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2575           cial  meaning  is  faulted.  Otherwise,  like  Perl,  the  backslash is
2576           ignored. (Perl can be made to issue a warning.)
2577    
2578  INTERNAL OPTION SETTING         (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2579       The settings of PCRE_CASELESS, PCRE_MULTILINE,  PCRE_DOTALL,         fiers is inverted, that is, by default they are not greedy, but if fol-
2580       and  PCRE_EXTENDED can be changed from within the pattern by         lowed by a question mark they are.
      a sequence of Perl option letters enclosed between "(?"  and  
      ")". The option letters are  
   
        i  for PCRE_CASELESS  
        m  for PCRE_MULTILINE  
        s  for PCRE_DOTALL  
        x  for PCRE_EXTENDED  
   
      For example, (?im) sets caseless, multiline matching. It  is  
      also possible to unset these options by preceding the letter  
      with a hyphen, and a combined setting and unsetting such  as  
      (?im-sx),  which sets PCRE_CASELESS and PCRE_MULTILINE while  
      unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  permitted.  
      If  a  letter  appears both before and after the hyphen, the  
      option is unset.  
   
      The scope of these option changes depends on  where  in  the  
      pattern  the  setting  occurs. For settings that are outside  
      any subpattern (defined below), the effect is the same as if  
      the  options were set or unset at the start of matching. The  
      following patterns all behave in exactly the same way:  
   
        (?i)abc  
        a(?i)bc  
        ab(?i)c  
        abc(?i)  
   
      which in turn is the same as compiling the pattern abc  with  
      PCRE_CASELESS  set.   In  other words, such "top level" set-  
      tings apply to the whole pattern  (unless  there  are  other  
      changes  inside subpatterns). If there is more than one set-  
      ting of the same option at top level, the rightmost  setting  
      is used.  
   
      If an option change occurs inside a subpattern,  the  effect  
      is  different.  This is a change of behaviour in Perl 5.005.  
      An option change inside a subpattern affects only that  part  
      of the subpattern that follows it, so  
   
        (a(?i)b)c  
   
      matches  abc  and  aBc  and  no  other   strings   (assuming  
      PCRE_CASELESS  is  not used).  By this means, options can be  
      made to have different settings in different  parts  of  the  
      pattern.  Any  changes  made  in one alternative do carry on  
      into subsequent branches within  the  same  subpattern.  For  
      example,  
   
        (a(?i)b|c)  
   
      matches "ab", "aB", "c", and "C", even though when  matching  
      "C" the first branch is abandoned before the option setting.  
      This is because the effects of  option  settings  happen  at  
      compile  time. There would be some very weird behaviour oth-  
      erwise.  
   
      The PCRE-specific options PCRE_UNGREEDY and  PCRE_EXTRA  can  
      be changed in the same way as the Perl-compatible options by  
      using the characters U and X  respectively.  The  (?X)  flag  
      setting  is  special in that it must always occur earlier in  
      the pattern than any of the additional features it turns on,  
      even when it is at top level. It is best put at the start.  
2581    
2582           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2583           tried only at the first matching position in the subject string.
2584    
2585           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2586           TURE options for pcre_exec() have no Perl equivalents.
2587    
2588  SUBPATTERNS         (g) The callout facility is PCRE-specific.
      Subpatterns are delimited by parentheses  (round  brackets),  
      which can be nested.  Marking part of a pattern as a subpat-  
      tern does two things:  
   
      1. It localizes a set of alternatives. For example, the pat-  
      tern  
   
        cat(aract|erpillar|)  
   
      matches one of the words "cat",  "cataract",  or  "caterpil-  
      lar".  Without  the  parentheses, it would match "cataract",  
      "erpillar" or the empty string.  
   
      2. It sets up the subpattern as a capturing  subpattern  (as  
      defined  above).   When the whole pattern matches, that por-  
      tion of the subject string that matched  the  subpattern  is  
      passed  back  to  the  caller  via  the  ovector argument of  
      pcre_exec(). Opening parentheses are counted  from  left  to  
      right (starting from 1) to obtain the numbers of the captur-  
      ing subpatterns.  
   
      For example, if the string "the red king" is matched against  
      the pattern  
   
        the ((red|white) (king|queen))  
   
      the captured substrings are "red king", "red",  and  "king",  
      and are numbered 1, 2, and 3, respectively.  
   
      The fact that plain parentheses fulfil two functions is  not  
      always  helpful.  There are often times when a grouping sub-  
      pattern is required without a capturing requirement.  If  an  
      opening parenthesis is followed by "?:", the subpattern does  
      not do any capturing, and is not counted when computing  the  
      number of any subsequent capturing subpatterns. For example,  
      if the string "the white queen" is matched against the  pat-  
      tern  
   
        the ((?:red|white) (king|queen))  
   
      the captured substrings are "white queen" and  "queen",  and  
      are  numbered  1  and 2. The maximum number of captured sub-  
      strings is 99, and the maximum number  of  all  subpatterns,  
      both capturing and non-capturing, is 200.  
   
      As a  convenient  shorthand,  if  any  option  settings  are  
      required  at  the  start  of a non-capturing subpattern, the  
      option letters may appear between the "?" and the ":".  Thus  
      the two patterns  
   
        (?i:saturday|sunday)  
        (?:(?i)saturday|sunday)  
   
      match exactly the same set of strings.  Because  alternative  
      branches  are  tried from left to right, and options are not  
      reset until the end of the subpattern is reached, an  option  
      setting  in  one  branch does affect subsequent branches, so  
      the above patterns match "SUNDAY" as well as "Saturday".  
2589    
2590           (h) The partial matching facility is PCRE-specific.
2591    
2592           (i) Patterns compiled by PCRE can be saved and re-used at a later time,
2593           even on different hosts that have the other endianness.
2594    
2595  REPETITION         (j)  The  alternative  matching function (pcre_dfa_exec()) matches in a
2596       Repetition is specified by quantifiers, which can follow any         different way and is not Perl-compatible.
      of the following items:  
2597    
2598         a single character, possibly escaped  Last updated: 28 November 2006
2599         the . metacharacter  Copyright (c) 1997-2006 University of Cambridge.
2600         a character class  ------------------------------------------------------------------------------
        a back reference (see next section)  
        a parenthesized subpattern (unless it is  an  assertion  -  
      see below)  
   
      The general repetition quantifier specifies  a  minimum  and  
      maximum  number  of  permitted  matches,  by  giving the two  
      numbers in curly brackets (braces), separated  by  a  comma.  
      The  numbers  must be less than 65536, and the first must be  
      less than or equal to the second. For example:  
   
        z{2,4}  
   
      matches "zz", "zzz", or "zzzz". A closing brace on  its  own  
      is not a special character. If the second number is omitted,  
      but the comma is present, there is no upper  limit;  if  the  
      second number and the comma are both omitted, the quantifier  
      specifies an exact number of required matches. Thus  
   
        [aeiou]{3,}  
   
      matches at least 3 successive vowels,  but  may  match  many  
      more, while  
   
        \d{8}  
   
      matches exactly 8 digits.  An  opening  curly  bracket  that  
      appears  in a position where a quantifier is not allowed, or  
      one that does not match the syntax of a quantifier, is taken  
      as  a literal character. For example, {,6} is not a quantif-  
      ier, but a literal string of four characters.  
      The quantifier {0} is permitted, causing the  expression  to  
      behave  as  if the previous item and the quantifier were not  
      present.  
   
      For convenience (and  historical  compatibility)  the  three  
      most common quantifiers have single-character abbreviations:  
   
        *    is equivalent to {0,}  
        +    is equivalent to {1,}  
        ?    is equivalent to {0,1}  
   
      It is possible to construct infinite loops  by  following  a  
      subpattern  that  can  match no characters with a quantifier  
      that has no upper limit, for example:  
   
        (a?)*  
   
      Earlier versions of Perl and PCRE used to give an  error  at  
      compile  time  for such patterns. However, because there are  
      cases where this  can  be  useful,  such  patterns  are  now  
      accepted,  but  if  any repetition of the subpattern does in  
      fact match no characters, the loop is forcibly broken.  
   
      By default, the quantifiers  are  "greedy",  that  is,  they  
      match  as much as possible (up to the maximum number of per-  
      mitted times), without causing the rest of  the  pattern  to  
      fail. The classic example of where this gives problems is in  
      trying to match comments in C programs. These appear between  
      the  sequences /* and */ and within the sequence, individual  
      * and / characters may appear. An attempt to  match  C  com-  
      ments by applying the pattern  
   
        /\*.*\*/  
   
      to the string  
   
        /* first command */  not comment  /* second comment */  
   
      fails, because it matches the entire  string  owing  to  the  
      greediness of the .*  item.  
   
      However, if a quantifier is followed by a question mark,  it  
      ceases  to be greedy, and instead matches the minimum number  
      of times possible, so the pattern  
   
        /\*.*?\*/  
   
      does the right thing with the C comments. The meaning of the  
      various  quantifiers is not otherwise changed, just the pre-  
      ferred number of matches.  Do not confuse this use of  ques-  
      tion  mark  with  its  use as a quantifier in its own right.  
      Because it has two uses, it can sometimes appear doubled, as  
      in  
   
        \d??\d  
   
      which matches one digit by preference, but can match two  if  
      that is the only way the rest of the pattern matches.  
   
      If the PCRE_UNGREEDY option is set (an option which  is  not  
      available  in  Perl),  the  quantifiers  are  not  greedy by  
      default, but individual ones can be made greedy by following  
      them  with  a  question mark. In other words, it inverts the  
      default behaviour.  
   
      When a parenthesized subpattern is quantified with a minimum  
      repeat  count  that is greater than 1 or with a limited max-  
      imum, more store is required for the  compiled  pattern,  in  
      proportion to the size of the minimum or maximum.  
   
      If a pattern starts with .* or  .{0,}  and  the  PCRE_DOTALL  
      option (equivalent to Perl's /s) is set, thus allowing the .  
      to match  newlines,  the  pattern  is  implicitly  anchored,  
      because whatever follows will be tried against every charac-  
      ter position in the subject string, so there is no point  in  
      retrying  the overall match at any position after the first.  
      PCRE treats such a pattern as though it were preceded by \A.  
      In  cases where it is known that the subject string contains  
      no newlines, it is worth setting PCRE_DOTALL when  the  pat-  
      tern begins with .* in order to obtain this optimization, or  
      alternatively using ^ to indicate anchoring explicitly.  
   
      When a capturing subpattern is repeated, the value  captured  
      is the substring that matched the final iteration. For exam-  
  &