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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             pcre-config       show PCRE installation configuration information
76             pcreapi           details of PCRE's native C API
77             pcrebuild         options for building PCRE
78             pcrecallout       details of the callout feature
79             pcrecompat        discussion of Perl compatibility
80             pcrecpp           details of the C++ wrapper
81             pcregrep          description of the pcregrep command
82             pcrematching      discussion of the two matching algorithms
83             pcrepartial       details of the partial matching facility
84             pcrepattern       syntax and semantics of supported
85                                 regular expressions
86             pcreperform       discussion of performance issues
87             pcreposix         the POSIX-compatible C API
88             pcreprecompile    details of saving and re-using precompiled patterns
89             pcresample        discussion of the sample program
90             pcrestack         discussion of stack usage
91             pcretest          description of the pcretest testing command
92    
93       pcre *pcre_compile(const char *pattern, int options,         In addition, in the "man" and HTML formats, there is a short  page  for
94            const char **errptr, int *erroffset,         each C library function, listing its arguments and results.
           const unsigned char *tableptr);  
95    
      pcre_extra *pcre_study(const pcre *code, int options,  
           const char **errptr);  
96    
97       int pcre_exec(const pcre *code, const pcre_extra *extra,  LIMITATIONS
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
98    
99       int pcre_copy_substring(const char *subject, int *ovector,         There  are some size limitations in PCRE but it is hoped that they will
100            int stringcount, int stringnumber, char *buffer,         never in practice be relevant.
           int buffersize);  
101    
102       int pcre_get_substring(const char *subject, int *ovector,         The maximum length of a compiled pattern is 65539 (sic) bytes  if  PCRE
103            int stringcount, int stringnumber,         is compiled with the default internal linkage size of 2. If you want to
104            const char **stringptr);         process regular expressions that are truly enormous,  you  can  compile
105           PCRE  with  an  internal linkage size of 3 or 4 (see the README file in
106           the source distribution and the pcrebuild documentation  for  details).
107           In  these  cases the limit is substantially larger.  However, the speed
108           of execution is slower.
109    
110           All values in repeating quantifiers must be less than 65536. The  maxi-
111           mum  compiled  length  of  subpattern  with an explicit repeat count is
112           30000 bytes. The maximum number of capturing subpatterns is 65535.
113    
114           There is no limit to the number of parenthesized subpatterns, but there
115           can be no more than 65535 capturing subpatterns.
116    
117           The maximum length of name for a named subpattern is 32 characters, and
118           the maximum number of named subpatterns is 10000.
119    
120           The maximum length of a subject string is the largest  positive  number
121           that  an integer variable can hold. However, when using the traditional
122           matching function, PCRE uses recursion to handle subpatterns and indef-
123           inite  repetition.  This means that the available stack space may limit
124           the size of a subject string that can be processed by certain patterns.
125           For a discussion of stack issues, see the pcrestack documentation.
126    
127    
128    UTF-8 AND UNICODE PROPERTY SUPPORT
129    
130           From  release  3.3,  PCRE  has  had  some support for character strings
131           encoded in the UTF-8 format. For release 4.0 this was greatly  extended
132           to  cover  most common requirements, and in release 5.0 additional sup-
133           port for Unicode general category properties was added.
134    
135           In order process UTF-8 strings, you must build PCRE  to  include  UTF-8
136           support  in  the  code,  and, in addition, you must call pcre_compile()
137           with the PCRE_UTF8 option flag. When you do this, both the pattern  and
138           any  subject  strings  that are matched against it are treated as UTF-8
139           strings instead of just strings of bytes.
140    
141           If you compile PCRE with UTF-8 support, but do not use it at run  time,
142           the  library will be a bit bigger, but the additional run time overhead
143           is limited to testing the PCRE_UTF8 flag occasionally, so should not be
144           very big.
145    
146           If PCRE is built with Unicode character property support (which implies
147           UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
148           ported.  The available properties that can be tested are limited to the
149           general category properties such as Lu for an upper case letter  or  Nd
150           for  a  decimal number, the Unicode script names such as Arabic or Han,
151           and the derived properties Any and L&. A full  list  is  given  in  the
152           pcrepattern documentation. Only the short names for properties are sup-
153           ported. For example, \p{L} matches a letter. Its Perl synonym,  \p{Let-
154           ter},  is  not  supported.   Furthermore,  in Perl, many properties may
155           optionally be prefixed by "Is", for compatibility with Perl  5.6.  PCRE
156           does not support this.
157    
158           The following comments apply when PCRE is running in UTF-8 mode:
159    
160           1.  When you set the PCRE_UTF8 flag, the strings passed as patterns and
161           subjects are checked for validity on entry to the  relevant  functions.
162           If an invalid UTF-8 string is passed, an error return is given. In some
163           situations, you may already know  that  your  strings  are  valid,  and
164           therefore want to skip these checks in order to improve performance. If
165           you set the PCRE_NO_UTF8_CHECK flag at compile time  or  at  run  time,
166           PCRE  assumes  that  the  pattern or subject it is given (respectively)
167           contains only valid UTF-8 codes. In this case, it does not diagnose  an
168           invalid  UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
169           PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  may
170           crash.
171    
172           2.  An  unbraced  hexadecimal  escape sequence (such as \xb3) matches a
173           two-byte UTF-8 character if the value is greater than 127.
174    
175           3. Octal numbers up to \777 are recognized, and  match  two-byte  UTF-8
176           characters for values greater than \177.
177    
178           4.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
179           vidual bytes, for example: \x{100}{3}.
180    
181           5. The dot metacharacter matches one UTF-8 character instead of a  sin-
182           gle byte.
183    
184           6.  The  escape sequence \C can be used to match a single byte in UTF-8
185           mode, but its use can lead to some strange effects.  This  facility  is
186           not available in the alternative matching function, pcre_dfa_exec().
187    
188           7.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
189           test characters of any code value, but the characters that PCRE  recog-
190           nizes  as  digits,  spaces,  or  word characters remain the same set as
191           before, all with values less than 256. This remains true even when PCRE
192           includes  Unicode  property support, because to do otherwise would slow
193           down PCRE in many common cases. If you really want to test for a  wider
194           sense  of,  say,  "digit",  you must use Unicode property tests such as
195           \p{Nd}.
196    
197           8. Similarly, characters that match the POSIX named  character  classes
198           are all low-valued characters.
199    
200           9.  Case-insensitive  matching  applies only to characters whose values
201           are less than 128, unless PCRE is built with Unicode property  support.
202           Even  when  Unicode  property support is available, PCRE still uses its
203           own character tables when checking the case of  low-valued  characters,
204           so  as not to degrade performance.  The Unicode property information is
205           used only for characters with higher values. Even when Unicode property
206           support is available, PCRE supports case-insensitive matching only when
207           there is a one-to-one mapping between a letter's  cases.  There  are  a
208           small  number  of  many-to-one  mappings in Unicode; these are not sup-
209           ported by PCRE.
210    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
211    
212       const unsigned char *pcre_maketables(void);  AUTHOR
213    
214       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,         Philip Hazel
215            int what, void *where);         University Computing Service
216           Cambridge CB2 3QH, England.
217    
218       int pcre_info(const pcre *code, int *optptr, *firstcharptr);         Putting an actual email address here seems to have been a spam  magnet,
219           so  I've  taken  it away. If you want to email me, use my two initials,
220           followed by the two digits 10, at the domain cam.ac.uk.
221    
      char *pcre_version(void);  
222    
223       void *(*pcre_malloc)(size_t);  REVISION
224    
225       void (*pcre_free)(void *);         Last updated: 18 April 2007
226           Copyright (c) 1997-2007 University of Cambridge.
227    ------------------------------------------------------------------------------
228    
229    
230    PCREBUILD(3)                                                      PCREBUILD(3)
231    
232    
233  DESCRIPTION  NAME
234       The PCRE library is a set of functions that implement  regu-         PCRE - Perl-compatible regular expressions
235       lar  expression  pattern  matching using the same syntax and  
236       semantics as Perl  5,  with  just  a  few  differences  (see  
237       below).  The  current  implementation  corresponds  to  Perl  PCRE BUILD-TIME OPTIONS
238       5.005, with some additional features from the Perl  develop-  
239       ment release.         This  document  describes  the  optional  features  of PCRE that can be
240           selected when the library is compiled. They are all selected, or  dese-
241       PCRE has its own native API,  which  is  described  in  this         lected, by providing options to the configure script that is run before
242       document.  There  is  also  a  set of wrapper functions that         the make command. The complete list of  options  for  configure  (which
243       correspond to the POSIX regular expression API.   These  are         includes  the  standard  ones such as the selection of the installation
244       described in the pcreposix documentation.         directory) can be obtained by running
245    
246       The native API function prototypes are defined in the header           ./configure --help
247       file  pcre.h,  and  on  Unix  systems  the library itself is  
248       called libpcre.a, so can be accessed by adding -lpcre to the         The following sections include  descriptions  of  options  whose  names
249       command  for  linking  an  application  which  calls it. The         begin with --enable or --disable. These settings specify changes to the
250       header file defines the macros PCRE_MAJOR and PCRE_MINOR  to         defaults for the configure command. Because of the way  that  configure
251       contain the major and minor release numbers for the library.         works,  --enable  and --disable always come in pairs, so the complemen-
252       Applications can use these to include support for  different         tary option always exists as well, but as it specifies the default,  it
253       releases.         is not described.
254    
255       The functions pcre_compile(), pcre_study(), and  pcre_exec()  
256       are  used  for  compiling  and matching regular expressions,  C++ SUPPORT
257       while   pcre_copy_substring(),   pcre_get_substring(),   and  
258       pcre_get_substring_list()   are  convenience  functions  for         By default, the configure script will search for a C++ compiler and C++
259       extracting  captured  substrings  from  a  matched   subject         header files. If it finds them, it automatically builds the C++ wrapper
260       string.  The function pcre_maketables() is used (optionally)         library for PCRE. You can disable this by adding
261       to build a set of character tables in the current locale for  
262       passing to pcre_compile().           --disable-cpp
263    
264       The function pcre_fullinfo() is used to find out information         to the configure command.
265       about a compiled pattern; pcre_info() is an obsolete version  
266       which returns only some of the available information, but is  
267       retained   for   backwards   compatibility.    The  function  UTF-8 SUPPORT
268       pcre_version() returns a pointer to a string containing  the  
269       version of PCRE and its date of release.         To build PCRE with support for UTF-8 character strings, add
270    
271       The global variables  pcre_malloc  and  pcre_free  initially           --enable-utf8
272       contain the entry points of the standard malloc() and free()  
273       functions respectively. PCRE  calls  the  memory  management         to  the  configure  command.  Of  itself, this does not make PCRE treat
274       functions  via  these  variables,  so  a calling program can         strings as UTF-8. As well as compiling PCRE with this option, you  also
275       replace them if it  wishes  to  intercept  the  calls.  This         have  have to set the PCRE_UTF8 option when you call the pcre_compile()
276       should be done before calling any PCRE functions.         function.
277    
278    
279    UNICODE CHARACTER PROPERTY SUPPORT
280    
281           UTF-8 support allows PCRE to process character values greater than  255
282           in  the  strings that it handles. On its own, however, it does not pro-
283           vide any facilities for accessing the properties of such characters. If
284           you  want  to  be able to use the pattern escapes \P, \p, and \X, which
285           refer to Unicode character properties, you must add
286    
287             --enable-unicode-properties
288    
289           to the configure command. This implies UTF-8 support, even if you  have
290           not explicitly requested it.
291    
292           Including  Unicode  property  support  adds around 30K of tables to the
293           PCRE library. Only the general category properties such as  Lu  and  Nd
294           are supported. Details are given in the pcrepattern documentation.
295    
296    
297    CODE VALUE OF NEWLINE
298    
299           By  default,  PCRE interprets character 10 (linefeed, LF) as indicating
300           the end of a line. This is the normal newline  character  on  Unix-like
301           systems. You can compile PCRE to use character 13 (carriage return, CR)
302           instead, by adding
303    
304             --enable-newline-is-cr
305    
306           to the  configure  command.  There  is  also  a  --enable-newline-is-lf
307           option, which explicitly specifies linefeed as the newline character.
308    
309           Alternatively, you can specify that line endings are to be indicated by
310           the two character sequence CRLF. If you want this, add
311    
312             --enable-newline-is-crlf
313    
314           to the configure command. There is a fourth option, specified by
315    
316             --enable-newline-is-anycrlf
317    
318           which causes PCRE to recognize any of the three sequences  CR,  LF,  or
319           CRLF as indicating a line ending. Finally, a fifth option, specified by
320    
321             --enable-newline-is-any
322    
323           causes PCRE to recognize any Unicode newline sequence.
324    
325           Whatever line ending convention is selected when PCRE is built  can  be
326           overridden  when  the library functions are called. At build time it is
327           conventional to use the standard for your operating system.
328    
329    
330    BUILDING SHARED AND STATIC LIBRARIES
331    
332           The PCRE building process uses libtool to build both shared and  static
333           Unix  libraries by default. You can suppress one of these by adding one
334           of
335    
336             --disable-shared
337             --disable-static
338    
339           to the configure command, as required.
340    
341    
342    POSIX MALLOC USAGE
343    
344           When PCRE is called through the POSIX interface (see the pcreposix doc-
345           umentation),  additional  working  storage  is required for holding the
346           pointers to capturing substrings, because PCRE requires three  integers
347           per  substring,  whereas  the POSIX interface provides only two. If the
348           number of expected substrings is small, the wrapper function uses space
349           on the stack, because this is faster than using malloc() for each call.
350           The default threshold above which the stack is no longer used is 10; it
351           can be changed by adding a setting such as
352    
353             --with-posix-malloc-threshold=20
354    
355           to the configure command.
356    
357    
358    HANDLING VERY LARGE PATTERNS
359    
360           Within  a  compiled  pattern,  offset values are used to point from one
361           part to another (for example, from an opening parenthesis to an  alter-
362           nation  metacharacter).  By default, two-byte values are used for these
363           offsets, leading to a maximum size for a  compiled  pattern  of  around
364           64K.  This  is sufficient to handle all but the most gigantic patterns.
365           Nevertheless, some people do want to process enormous patterns,  so  it
366           is  possible  to compile PCRE to use three-byte or four-byte offsets by
367           adding a setting such as
368    
369             --with-link-size=3
370    
371           to the configure command. The value given must be 2,  3,  or  4.  Using
372           longer  offsets slows down the operation of PCRE because it has to load
373           additional bytes when handling them.
374    
375    
376    AVOIDING EXCESSIVE STACK USAGE
377    
378           When matching with the pcre_exec() function, PCRE implements backtrack-
379           ing  by  making recursive calls to an internal function called match().
380           In environments where the size of the stack is limited,  this  can  se-
381           verely  limit  PCRE's operation. (The Unix environment does not usually
382           suffer from this problem, but it may sometimes be necessary to increase
383           the  maximum  stack size.  There is a discussion in the pcrestack docu-
384           mentation.) An alternative approach to recursion that uses memory  from
385           the  heap  to remember data, instead of using recursive function calls,
386           has been implemented to work round the problem of limited  stack  size.
387           If you want to build a version of PCRE that works this way, add
388    
389             --disable-stack-for-recursion
390    
391           to  the  configure  command. With this configuration, PCRE will use the
392           pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
393           ment  functions.  Separate  functions are provided because the usage is
394           very predictable: the block sizes requested are always  the  same,  and
395           the  blocks  are always freed in reverse order. A calling program might
396           be able to implement optimized functions that perform better  than  the
397           standard  malloc()  and  free()  functions.  PCRE  runs noticeably more
398           slowly when built in this way. This option affects only the pcre_exec()
399           function; it is not relevant for the the pcre_dfa_exec() function.
400    
401    
402    LIMITING PCRE RESOURCE USAGE
403    
404           Internally,  PCRE has a function called match(), which it calls repeat-
405           edly  (sometimes  recursively)  when  matching  a  pattern   with   the
406           pcre_exec()  function.  By controlling the maximum number of times this
407           function may be called during a single matching operation, a limit  can
408           be  placed  on  the resources used by a single call to pcre_exec(). The
409           limit can be changed at run time, as described in the pcreapi  documen-
410           tation.  The default is 10 million, but this can be changed by adding a
411           setting such as
412    
413             --with-match-limit=500000
414    
415           to  the  configure  command.  This  setting  has  no  effect   on   the
416           pcre_dfa_exec() matching function.
417    
418           In  some  environments  it is desirable to limit the depth of recursive
419           calls of match() more strictly than the total number of calls, in order
420           to  restrict  the maximum amount of stack (or heap, if --disable-stack-
421           for-recursion is specified) that is used. A second limit controls this;
422           it  defaults  to  the  value  that is set for --with-match-limit, which
423           imposes no additional constraints. However, you can set a  lower  limit
424           by adding, for example,
425    
426             --with-match-limit-recursion=10000
427    
428           to  the  configure  command.  This  value can also be overridden at run
429           time.
430    
431    
432    CREATING CHARACTER TABLES AT BUILD TIME
433    
434           PCRE uses fixed tables for processing characters whose code values  are
435           less  than 256. By default, PCRE is built with a set of tables that are
436           distributed in the file pcre_chartables.c.dist. These  tables  are  for
437           ASCII codes only. If you add
438    
439             --enable-rebuild-chartables
440    
441           to  the  configure  command, the distributed tables are no longer used.
442           Instead, a program called dftables is compiled and  run.  This  outputs
443           the source for new set of tables, created in the default locale of your
444           C runtime system. (This method of replacing the tables does not work if
445           you  are cross compiling, because dftables is run on the local host. If
446           you need to create alternative tables when cross  compiling,  you  will
447           have to do so "by hand".)
448    
449    
450    USING EBCDIC CODE
451    
452           PCRE  assumes  by  default that it will run in an environment where the
453           character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
454           PCRE  can,  however,  be  compiled  to  run in an EBCDIC environment by
455           adding
456    
457             --enable-ebcdic
458    
459           to the configure command. This setting implies --enable-rebuild-charta-
460           bles.
461    
462    
463    SEE ALSO
464    
465           pcreapi(3), pcre_config(3).
466    
467    
468    AUTHOR
469    
470           Philip Hazel
471           University Computing Service
472           Cambridge CB2 3QH, England.
473    
474    
475    REVISION
476    
477           Last updated: 16 April 2007
478           Copyright (c) 1997-2007 University of Cambridge.
479    ------------------------------------------------------------------------------
480    
481    
482    PCREMATCHING(3)                                                PCREMATCHING(3)
483    
484    
485    NAME
486           PCRE - Perl-compatible regular expressions
487    
488    
489    PCRE MATCHING ALGORITHMS
490    
491           This document describes the two different algorithms that are available
492           in PCRE for matching a compiled regular expression against a given sub-
493           ject  string.  The  "standard"  algorithm  is  the  one provided by the
494           pcre_exec() function.  This works in the same was  as  Perl's  matching
495           function, and provides a Perl-compatible matching operation.
496    
497           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
498           this operates in a different way, and is not  Perl-compatible.  It  has
499           advantages  and disadvantages compared with the standard algorithm, and
500           these are described below.
501    
502           When there is only one possible way in which a given subject string can
503           match  a pattern, the two algorithms give the same answer. A difference
504           arises, however, when there are multiple possibilities. For example, if
505           the pattern
506    
507             ^<.*>
508    
509           is matched against the string
510    
511             <something> <something else> <something further>
512    
513           there are three possible answers. The standard algorithm finds only one
514           of them, whereas the alternative algorithm finds all three.
515    
516    
517    REGULAR EXPRESSIONS AS TREES
518    
519           The set of strings that are matched by a regular expression can be rep-
520           resented  as  a  tree structure. An unlimited repetition in the pattern
521           makes the tree of infinite size, but it is still a tree.  Matching  the
522           pattern  to a given subject string (from a given starting point) can be
523           thought of as a search of the tree.  There are two  ways  to  search  a
524           tree:  depth-first  and  breadth-first, and these correspond to the two
525           matching algorithms provided by PCRE.
526    
527    
528    THE STANDARD MATCHING ALGORITHM
529    
530           In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
531           sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
532           depth-first search of the pattern tree. That is, it  proceeds  along  a
533           single path through the tree, checking that the subject matches what is
534           required. When there is a mismatch, the algorithm  tries  any  alterna-
535           tives  at  the  current point, and if they all fail, it backs up to the
536           previous branch point in the  tree,  and  tries  the  next  alternative
537           branch  at  that  level.  This often involves backing up (moving to the
538           left) in the subject string as well.  The  order  in  which  repetition
539           branches  are  tried  is controlled by the greedy or ungreedy nature of
540           the quantifier.
541    
542           If a leaf node is reached, a matching string has  been  found,  and  at
543           that  point the algorithm stops. Thus, if there is more than one possi-
544           ble match, this algorithm returns the first one that it finds.  Whether
545           this  is the shortest, the longest, or some intermediate length depends
546           on the way the greedy and ungreedy repetition quantifiers are specified
547           in the pattern.
548    
549           Because  it  ends  up  with a single path through the tree, it is rela-
550           tively straightforward for this algorithm to keep  track  of  the  sub-
551           strings  that  are  matched  by portions of the pattern in parentheses.
552           This provides support for capturing parentheses and back references.
553    
554    
555    THE ALTERNATIVE MATCHING ALGORITHM
556    
557           This algorithm conducts a breadth-first search of  the  tree.  Starting
558           from  the  first  matching  point  in the subject, it scans the subject
559           string from left to right, once, character by character, and as it does
560           this,  it remembers all the paths through the tree that represent valid
561           matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
562           though  it is not implemented as a traditional finite state machine (it
563           keeps multiple states active simultaneously).
564    
565           The scan continues until either the end of the subject is  reached,  or
566           there  are  no more unterminated paths. At this point, terminated paths
567           represent the different matching possibilities (if there are none,  the
568           match  has  failed).   Thus,  if there is more than one possible match,
569           this algorithm finds all of them, and in particular, it finds the long-
570           est.  In PCRE, there is an option to stop the algorithm after the first
571           match (which is necessarily the shortest) has been found.
572    
573           Note that all the matches that are found start at the same point in the
574           subject. If the pattern
575    
576             cat(er(pillar)?)
577    
578           is  matched  against the string "the caterpillar catchment", the result
579           will be the three strings "cat", "cater", and "caterpillar" that  start
580           at the fourth character of the subject. The algorithm does not automat-
581           ically move on to find matches that start at later positions.
582    
583           There are a number of features of PCRE regular expressions that are not
584           supported by the alternative matching algorithm. They are as follows:
585    
586           1.  Because  the  algorithm  finds  all possible matches, the greedy or
587           ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
588           ungreedy quantifiers are treated in exactly the same way. However, pos-
589           sessive quantifiers can make a difference when what follows could  also
590           match what is quantified, for example in a pattern like this:
591    
592             ^a++\w!
593    
594           This  pattern matches "aaab!" but not "aaa!", which would be matched by
595           a non-possessive quantifier. Similarly, if an atomic group is  present,
596           it  is matched as if it were a standalone pattern at the current point,
597           and the longest match is then "locked in" for the rest of  the  overall
598           pattern.
599    
600           2. When dealing with multiple paths through the tree simultaneously, it
601           is not straightforward to keep track of  captured  substrings  for  the
602           different  matching  possibilities,  and  PCRE's implementation of this
603           algorithm does not attempt to do this. This means that no captured sub-
604           strings are available.
605    
606           3.  Because no substrings are captured, back references within the pat-
607           tern are not supported, and cause errors if encountered.
608    
609           4. For the same reason, conditional expressions that use  a  backrefer-
610           ence  as  the  condition or test for a specific group recursion are not
611           supported.
612    
613           5. Because many paths through the tree may be  active,  the  \K  escape
614           sequence, which resets the start of the match when encountered (but may
615           be on some paths and not on others), is not  supported.  It  causes  an
616           error if encountered.
617    
618           6.  Callouts  are  supported, but the value of the capture_top field is
619           always 1, and the value of the capture_last field is always -1.
620    
621           7.  The \C escape sequence, which (in the standard algorithm) matches a
622           single  byte, even in UTF-8 mode, is not supported because the alterna-
623           tive algorithm moves through the subject  string  one  character  at  a
624           time, for all active paths through the tree.
625    
626    
627  MULTI-THREADING  ADVANTAGES OF THE ALTERNATIVE ALGORITHM
      The PCRE functions can be used in  multi-threading  applica-  
      tions, with the proviso that the memory management functions  
      pointed to by pcre_malloc and pcre_free are  shared  by  all  
      threads.  
   
      The compiled form of a regular  expression  is  not  altered  
      during  matching, so the same compiled pattern can safely be  
      used by several threads at once.  
628    
629           Using  the alternative matching algorithm provides the following advan-
630           tages:
631    
632           1. All possible matches (at a single point in the subject) are automat-
633           ically  found,  and  in particular, the longest match is found. To find
634           more than one match using the standard algorithm, you have to do kludgy
635           things with callouts.
636    
637           2.  There is much better support for partial matching. The restrictions
638           on the content of the pattern that apply when using the standard  algo-
639           rithm  for  partial matching do not apply to the alternative algorithm.
640           For non-anchored patterns, the starting position of a partial match  is
641           available.
642    
643           3.  Because  the  alternative  algorithm  scans the subject string just
644           once, and never needs to backtrack, it is possible to  pass  very  long
645           subject  strings  to  the matching function in several pieces, checking
646           for partial matching each time.
647    
648    
649    DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
650    
651           The alternative algorithm suffers from a number of disadvantages:
652    
653           1. It is substantially slower than  the  standard  algorithm.  This  is
654           partly  because  it has to search for all possible matches, but is also
655           because it is less susceptible to optimization.
656    
657           2. Capturing parentheses and back references are not supported.
658    
659           3. Although atomic groups are supported, their use does not provide the
660           performance advantage that it does for the standard algorithm.
661    
662    
663    AUTHOR
664    
665           Philip Hazel
666           University Computing Service
667           Cambridge CB2 3QH, England.
668    
669    
670    REVISION
671    
672           Last updated: 29 May 2007
673           Copyright (c) 1997-2007 University of Cambridge.
674    ------------------------------------------------------------------------------
675    
676    
677    PCREAPI(3)                                                          PCREAPI(3)
678    
679    
680    NAME
681           PCRE - Perl-compatible regular expressions
682    
683    
684    PCRE NATIVE API
685    
686           #include <pcre.h>
687    
688           pcre *pcre_compile(const char *pattern, int options,
689                const char **errptr, int *erroffset,
690                const unsigned char *tableptr);
691    
692           pcre *pcre_compile2(const char *pattern, int options,
693                int *errorcodeptr,
694                const char **errptr, int *erroffset,
695                const unsigned char *tableptr);
696    
697           pcre_extra *pcre_study(const pcre *code, int options,
698                const char **errptr);
699    
700           int pcre_exec(const pcre *code, const pcre_extra *extra,
701                const char *subject, int length, int startoffset,
702                int options, int *ovector, int ovecsize);
703    
704           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
705                const char *subject, int length, int startoffset,
706                int options, int *ovector, int ovecsize,
707                int *workspace, int wscount);
708    
709           int pcre_copy_named_substring(const pcre *code,
710                const char *subject, int *ovector,
711                int stringcount, const char *stringname,
712                char *buffer, int buffersize);
713    
714           int pcre_copy_substring(const char *subject, int *ovector,
715                int stringcount, int stringnumber, char *buffer,
716                int buffersize);
717    
718           int pcre_get_named_substring(const pcre *code,
719                const char *subject, int *ovector,
720                int stringcount, const char *stringname,
721                const char **stringptr);
722    
723           int pcre_get_stringnumber(const pcre *code,
724                const char *name);
725    
726           int pcre_get_stringtable_entries(const pcre *code,
727                const char *name, char **first, char **last);
728    
729           int pcre_get_substring(const char *subject, int *ovector,
730                int stringcount, int stringnumber,
731                const char **stringptr);
732    
733           int pcre_get_substring_list(const char *subject,
734                int *ovector, int stringcount, const char ***listptr);
735    
736           void pcre_free_substring(const char *stringptr);
737    
738           void pcre_free_substring_list(const char **stringptr);
739    
740           const unsigned char *pcre_maketables(void);
741    
742           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
743                int what, void *where);
744    
745           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
746    
747           int pcre_refcount(pcre *code, int adjust);
748    
749           int pcre_config(int what, void *where);
750    
751           char *pcre_version(void);
752    
753           void *(*pcre_malloc)(size_t);
754    
755           void (*pcre_free)(void *);
756    
757           void *(*pcre_stack_malloc)(size_t);
758    
759           void (*pcre_stack_free)(void *);
760    
761           int (*pcre_callout)(pcre_callout_block *);
762    
763    
764    PCRE API OVERVIEW
765    
766           PCRE has its own native API, which is described in this document. There
767           are also some wrapper functions that correspond to  the  POSIX  regular
768           expression  API.  These  are  described in the pcreposix documentation.
769           Both of these APIs define a set of C function calls. A C++  wrapper  is
770           distributed with PCRE. It is documented in the pcrecpp page.
771    
772           The  native  API  C  function prototypes are defined in the header file
773           pcre.h, and on Unix systems the library itself is called  libpcre.   It
774           can normally be accessed by adding -lpcre to the command for linking an
775           application  that  uses  PCRE.  The  header  file  defines  the  macros
776           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
777           bers for the library.  Applications can use these  to  include  support
778           for different releases of PCRE.
779    
780           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
781           pcre_exec() are used for compiling and matching regular expressions  in
782           a  Perl-compatible  manner. A sample program that demonstrates the sim-
783           plest way of using them is provided in the file  called  pcredemo.c  in
784           the  source distribution. The pcresample documentation describes how to
785           run it.
786    
787           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
788           ble,  is  also provided. This uses a different algorithm for the match-
789           ing. The alternative algorithm finds all possible matches (at  a  given
790           point  in  the subject), and scans the subject just once. However, this
791           algorithm does not return captured substrings. A description of the two
792           matching  algorithms and their advantages and disadvantages is given in
793           the pcrematching documentation.
794    
795           In addition to the main compiling and  matching  functions,  there  are
796           convenience functions for extracting captured substrings from a subject
797           string that is matched by pcre_exec(). They are:
798    
799             pcre_copy_substring()
800             pcre_copy_named_substring()
801             pcre_get_substring()
802             pcre_get_named_substring()
803             pcre_get_substring_list()
804             pcre_get_stringnumber()
805             pcre_get_stringtable_entries()
806    
807           pcre_free_substring() and pcre_free_substring_list() are also provided,
808           to free the memory used for extracted strings.
809    
810           The  function  pcre_maketables()  is  used  to build a set of character
811           tables  in  the  current  locale   for   passing   to   pcre_compile(),
812           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
813           provided for specialist use.  Most  commonly,  no  special  tables  are
814           passed,  in  which case internal tables that are generated when PCRE is
815           built are used.
816    
817           The function pcre_fullinfo() is used to find out  information  about  a
818           compiled  pattern; pcre_info() is an obsolete version that returns only
819           some of the available information, but is retained for  backwards  com-
820           patibility.   The function pcre_version() returns a pointer to a string
821           containing the version of PCRE and its date of release.
822    
823           The function pcre_refcount() maintains a  reference  count  in  a  data
824           block  containing  a compiled pattern. This is provided for the benefit
825           of object-oriented applications.
826    
827           The global variables pcre_malloc and pcre_free  initially  contain  the
828           entry  points  of  the  standard malloc() and free() functions, respec-
829           tively. PCRE calls the memory management functions via these variables,
830           so  a  calling  program  can replace them if it wishes to intercept the
831           calls. This should be done before calling any PCRE functions.
832    
833           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
834           indirections  to  memory  management functions. These special functions
835           are used only when PCRE is compiled to use  the  heap  for  remembering
836           data, instead of recursive function calls, when running the pcre_exec()
837           function. See the pcrebuild documentation for  details  of  how  to  do
838           this.  It  is  a non-standard way of building PCRE, for use in environ-
839           ments that have limited stacks. Because of the greater  use  of  memory
840           management,  it  runs  more  slowly. Separate functions are provided so
841           that special-purpose external code can be  used  for  this  case.  When
842           used,  these  functions  are always called in a stack-like manner (last
843           obtained, first freed), and always for memory blocks of the same  size.
844           There  is  a discussion about PCRE's stack usage in the pcrestack docu-
845           mentation.
846    
847           The global variable pcre_callout initially contains NULL. It can be set
848           by  the  caller  to  a "callout" function, which PCRE will then call at
849           specified points during a matching operation. Details are given in  the
850           pcrecallout documentation.
851    
852    
853    NEWLINES
854    
855           PCRE  supports five different conventions for indicating line breaks in
856           strings: a single CR (carriage return) character, a  single  LF  (line-
857           feed) character, the two-character sequence CRLF, any of the three pre-
858           ceding, or any Unicode newline sequence. The Unicode newline  sequences
859           are  the  three just mentioned, plus the single characters VT (vertical
860           tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
861           separator, U+2028), and PS (paragraph separator, U+2029).
862    
863           Each  of  the first three conventions is used by at least one operating
864           system as its standard newline sequence. When PCRE is built, a  default
865           can  be  specified.  The default default is LF, which is the Unix stan-
866           dard. When PCRE is run, the default can be overridden,  either  when  a
867           pattern is compiled, or when it is matched.
868    
869           In the PCRE documentation the word "newline" is used to mean "the char-
870           acter or pair of characters that indicate a line break". The choice  of
871           newline  convention  affects  the  handling of the dot, circumflex, and
872           dollar metacharacters, the handling of #-comments in /x mode, and, when
873           CRLF  is a recognized line ending sequence, the match position advance-
874           ment for a non-anchored pattern. The choice of newline convention  does
875           not affect the interpretation of the \n or \r escape sequences.
876    
877    
878    MULTITHREADING
879    
880           The  PCRE  functions  can be used in multi-threading applications, with
881           the  proviso  that  the  memory  management  functions  pointed  to  by
882           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
883           callout function pointed to by pcre_callout, are shared by all threads.
884    
885           The  compiled form of a regular expression is not altered during match-
886           ing, so the same compiled pattern can safely be used by several threads
887           at once.
888    
889    
890    SAVING PRECOMPILED PATTERNS FOR LATER USE
891    
892           The compiled form of a regular expression can be saved and re-used at a
893           later time, possibly by a different program, and even on a  host  other
894           than  the  one  on  which  it  was  compiled.  Details are given in the
895           pcreprecompile documentation. However, compiling a  regular  expression
896           with  one version of PCRE for use with a different version is not guar-
897           anteed to work and may cause crashes.
898    
899    
900    CHECKING BUILD-TIME OPTIONS
901    
902           int pcre_config(int what, void *where);
903    
904           The function pcre_config() makes it possible for a PCRE client to  dis-
905           cover which optional features have been compiled into the PCRE library.
906           The pcrebuild documentation has more details about these optional  fea-
907           tures.
908    
909           The  first  argument  for pcre_config() is an integer, specifying which
910           information is required; the second argument is a pointer to a variable
911           into  which  the  information  is  placed. The following information is
912           available:
913    
914             PCRE_CONFIG_UTF8
915    
916           The output is an integer that is set to one if UTF-8 support is  avail-
917           able; otherwise it is set to zero.
918    
919             PCRE_CONFIG_UNICODE_PROPERTIES
920    
921           The  output  is  an  integer  that is set to one if support for Unicode
922           character properties is available; otherwise it is set to zero.
923    
924             PCRE_CONFIG_NEWLINE
925    
926           The output is an integer whose value specifies  the  default  character
927           sequence  that is recognized as meaning "newline". The four values that
928           are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
929           and  -1  for  ANY. The default should normally be the standard sequence
930           for your operating system.
931    
932             PCRE_CONFIG_LINK_SIZE
933    
934           The output is an integer that contains the number  of  bytes  used  for
935           internal linkage in compiled regular expressions. The value is 2, 3, or
936           4. Larger values allow larger regular expressions to  be  compiled,  at
937           the  expense  of  slower matching. The default value of 2 is sufficient
938           for all but the most massive patterns, since  it  allows  the  compiled
939           pattern to be up to 64K in size.
940    
941             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
942    
943           The  output  is  an integer that contains the threshold above which the
944           POSIX interface uses malloc() for output vectors. Further  details  are
945           given in the pcreposix documentation.
946    
947             PCRE_CONFIG_MATCH_LIMIT
948    
949           The output is an integer that gives the default limit for the number of
950           internal matching function calls in a  pcre_exec()  execution.  Further
951           details are given with pcre_exec() below.
952    
953             PCRE_CONFIG_MATCH_LIMIT_RECURSION
954    
955           The  output is an integer that gives the default limit for the depth of
956           recursion when calling the internal matching function in a  pcre_exec()
957           execution. Further details are given with pcre_exec() below.
958    
959             PCRE_CONFIG_STACKRECURSE
960    
961           The  output is an integer that is set to one if internal recursion when
962           running pcre_exec() is implemented by recursive function calls that use
963           the  stack  to remember their state. This is the usual way that PCRE is
964           compiled. The output is zero if PCRE was compiled to use blocks of data
965           on  the  heap  instead  of  recursive  function  calls.  In  this case,
966           pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
967           blocks on the heap, thus avoiding the use of the stack.
968    
969    
970  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 this for conveni-  
      ence, but in fact pcre is just a typedef for void, since the  
      contents  of  the block are not externally defined. It is up  
      to the caller to free  the  memory  when  it  is  no  longer  
      required.  
   
      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.  
   
      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.  
971    
972           pcre *pcre_compile(const char *pattern, int options,
973                const char **errptr, int *erroffset,
974                const unsigned char *tableptr);
975    
976           pcre *pcre_compile2(const char *pattern, int options,
977                int *errorcodeptr,
978                const char **errptr, int *erroffset,
979                const unsigned char *tableptr);
980    
981           Either of the functions pcre_compile() or pcre_compile2() can be called
982           to compile a pattern into an internal form. The only difference between
983           the  two interfaces is that pcre_compile2() has an additional argument,
984           errorcodeptr, via which a numerical error code can be returned.
985    
986           The pattern is a C string terminated by a binary zero, and is passed in
987           the  pattern  argument.  A  pointer to a single block of memory that is
988           obtained via pcre_malloc is returned. This contains the  compiled  code
989           and related data. The pcre type is defined for the returned block; this
990           is a typedef for a structure whose contents are not externally defined.
991           It is up to the caller to free the memory (via pcre_free) when it is no
992           longer required.
993    
994           Although the compiled code of a PCRE regex is relocatable, that is,  it
995           does not depend on memory location, the complete pcre data block is not
996           fully relocatable, because it may contain a copy of the tableptr  argu-
997           ment, which is an address (see below).
998    
999           The options argument contains various bit settings that affect the com-
1000           pilation. It should be zero if no options are required.  The  available
1001           options  are  described  below. Some of them, in particular, those that
1002           are compatible with Perl, can also be set and  unset  from  within  the
1003           pattern  (see  the  detailed  description in the pcrepattern documenta-
1004           tion). For these options, the contents of the options  argument  speci-
1005           fies  their initial settings at the start of compilation and execution.
1006           The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at  the  time
1007           of matching as well as at compile time.
1008    
1009           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
1010           if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
1011           sets the variable pointed to by errptr to point to a textual error mes-
1012           sage. This is a static string that is part of the library. You must not
1013           try to free it. The offset from the start of the pattern to the charac-
1014           ter where the error was discovered is placed in the variable pointed to
1015           by  erroffset,  which must not be NULL. If it is, an immediate error is
1016           given.
1017    
1018           If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
1019           codeptr  argument is not NULL, a non-zero error code number is returned
1020           via this argument in the event of an error. This is in addition to  the
1021           textual error message. Error codes and messages are listed below.
1022    
1023           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
1024           character tables that are  built  when  PCRE  is  compiled,  using  the
1025           default  C  locale.  Otherwise, tableptr must be an address that is the
1026           result of a call to pcre_maketables(). This value is  stored  with  the
1027           compiled  pattern,  and used again by pcre_exec(), unless another table
1028           pointer is passed to it. For more discussion, see the section on locale
1029           support below.
1030    
1031           This  code  fragment  shows a typical straightforward call to pcre_com-
1032           pile():
1033    
1034             pcre *re;
1035             const char *error;
1036             int erroffset;
1037             re = pcre_compile(
1038               "^A.*Z",          /* the pattern */
1039               0,                /* default options */
1040               &error,           /* for error message */
1041               &erroffset,       /* for error offset */
1042               NULL);            /* use default character tables */
1043    
1044           The following names for option bits are defined in  the  pcre.h  header
1045           file:
1046    
1047             PCRE_ANCHORED
1048    
1049           If this bit is set, the pattern is forced to be "anchored", that is, it
1050           is constrained to match only at the first matching point in the  string
1051           that  is being searched (the "subject string"). This effect can also be
1052           achieved by appropriate constructs in the pattern itself, which is  the
1053           only way to do it in Perl.
1054    
1055             PCRE_AUTO_CALLOUT
1056    
1057           If this bit is set, pcre_compile() automatically inserts callout items,
1058           all with number 255, before each pattern item. For  discussion  of  the
1059           callout facility, see the pcrecallout documentation.
1060    
1061             PCRE_CASELESS
1062    
1063           If  this  bit is set, letters in the pattern match both upper and lower
1064           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
1065           changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1066           always understands the concept of case for characters whose values  are
1067           less  than 128, so caseless matching is always possible. For characters
1068           with higher values, the concept of case is supported if  PCRE  is  com-
1069           piled  with Unicode property support, but not otherwise. If you want to
1070           use caseless matching for characters 128 and  above,  you  must  ensure
1071           that  PCRE  is  compiled  with Unicode property support as well as with
1072           UTF-8 support.
1073    
1074             PCRE_DOLLAR_ENDONLY
1075    
1076           If this bit is set, a dollar metacharacter in the pattern matches  only
1077           at  the  end  of the subject string. Without this option, a dollar also
1078           matches immediately before a newline at the end of the string (but  not
1079           before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1080           if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
1081           Perl, and no way to set it within a pattern.
1082    
1083             PCRE_DOTALL
1084    
1085           If this bit is set, a dot metacharater in the pattern matches all char-
1086           acters, including those that indicate newline. Without it, a  dot  does
1087           not  match  when  the  current position is at a newline. This option is
1088           equivalent to Perl's /s option, and it can be changed within a  pattern
1089           by  a (?s) option setting. A negative class such as [^a] always matches
1090           newline characters, independent of the setting of this option.
1091    
1092             PCRE_DUPNAMES
1093    
1094           If this bit is set, names used to identify capturing  subpatterns  need
1095           not be unique. This can be helpful for certain types of pattern when it
1096           is known that only one instance of the named  subpattern  can  ever  be
1097           matched.  There  are  more details of named subpatterns below; see also
1098           the pcrepattern documentation.
1099    
1100             PCRE_EXTENDED
1101    
1102           If this bit is set, whitespace  data  characters  in  the  pattern  are
1103           totally ignored except when escaped or inside a character class. White-
1104           space does not include the VT character (code 11). In addition, charac-
1105           ters between an unescaped # outside a character class and the next new-
1106           line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
1107           option,  and  it  can be changed within a pattern by a (?x) option set-
1108           ting.
1109    
1110           This option makes it possible to include  comments  inside  complicated
1111           patterns.   Note,  however,  that this applies only to data characters.
1112           Whitespace  characters  may  never  appear  within  special   character
1113           sequences  in  a  pattern,  for  example  within the sequence (?( which
1114           introduces a conditional subpattern.
1115    
1116             PCRE_EXTRA
1117    
1118           This option was invented in order to turn on  additional  functionality
1119           of  PCRE  that  is  incompatible with Perl, but it is currently of very
1120           little use. When set, any backslash in a pattern that is followed by  a
1121           letter  that  has  no  special  meaning causes an error, thus reserving
1122           these combinations for future expansion. By  default,  as  in  Perl,  a
1123           backslash  followed by a letter with no special meaning is treated as a
1124           literal. (Perl can, however, be persuaded to give a warning for  this.)
1125           There  are  at  present no other features controlled by this option. It
1126           can also be set by a (?X) option setting within a pattern.
1127    
1128             PCRE_FIRSTLINE
1129    
1130           If this option is set, an  unanchored  pattern  is  required  to  match
1131           before  or  at  the  first  newline  in  the subject string, though the
1132           matched text may continue over the newline.
1133    
1134             PCRE_MULTILINE
1135    
1136           By default, PCRE treats the subject string as consisting  of  a  single
1137           line  of characters (even if it actually contains newlines). The "start
1138           of line" metacharacter (^) matches only at the  start  of  the  string,
1139           while  the  "end  of line" metacharacter ($) matches only at the end of
1140           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1141           is set). This is the same as Perl.
1142    
1143           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
1144           constructs match immediately following or immediately  before  internal
1145           newlines  in  the  subject string, respectively, as well as at the very
1146           start and end. This is equivalent to Perl's /m option, and  it  can  be
1147           changed within a pattern by a (?m) option setting. If there are no new-
1148           lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
1149           setting PCRE_MULTILINE has no effect.
1150    
1151             PCRE_NEWLINE_CR
1152             PCRE_NEWLINE_LF
1153             PCRE_NEWLINE_CRLF
1154             PCRE_NEWLINE_ANYCRLF
1155             PCRE_NEWLINE_ANY
1156    
1157           These  options  override the default newline definition that was chosen
1158           when PCRE was built. Setting the first or the second specifies  that  a
1159           newline  is  indicated  by a single character (CR or LF, respectively).
1160           Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
1161           two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
1162           that any of the three preceding sequences should be recognized. Setting
1163           PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
1164           recognized. The Unicode newline sequences are the three just mentioned,
1165           plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
1166           U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
1167           (paragraph  separator,  U+2029).  The  last  two are recognized only in
1168           UTF-8 mode.
1169    
1170           The newline setting in the  options  word  uses  three  bits  that  are
1171           treated as a number, giving eight possibilities. Currently only six are
1172           used (default plus the five values above). This means that if  you  set
1173           more  than one newline option, the combination may or may not be sensi-
1174           ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1175           PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
1176           cause an error.
1177    
1178           The only time that a line break is specially recognized when  compiling
1179           a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
1180           character class is encountered. This indicates  a  comment  that  lasts
1181           until  after the next line break sequence. In other circumstances, line
1182           break  sequences  are  treated  as  literal  data,   except   that   in
1183           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1184           and are therefore ignored.
1185    
1186           The newline option that is set at compile time becomes the default that
1187           is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1188    
1189             PCRE_NO_AUTO_CAPTURE
1190    
1191           If this option is set, it disables the use of numbered capturing paren-
1192           theses  in the pattern. Any opening parenthesis that is not followed by
1193           ? behaves as if it were followed by ?: but named parentheses can  still
1194           be  used  for  capturing  (and  they acquire numbers in the usual way).
1195           There is no equivalent of this option in Perl.
1196    
1197             PCRE_UNGREEDY
1198    
1199           This option inverts the "greediness" of the quantifiers  so  that  they
1200           are  not greedy by default, but become greedy if followed by "?". It is
1201           not compatible with Perl. It can also be set by a (?U)  option  setting
1202           within the pattern.
1203    
1204             PCRE_UTF8
1205    
1206           This  option  causes PCRE to regard both the pattern and the subject as
1207           strings of UTF-8 characters instead of single-byte  character  strings.
1208           However,  it is available only when PCRE is built to include UTF-8 sup-
1209           port. If not, the use of this option provokes an error. Details of  how
1210           this  option  changes the behaviour of PCRE are given in the section on
1211           UTF-8 support in the main pcre page.
1212    
1213             PCRE_NO_UTF8_CHECK
1214    
1215           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1216           automatically  checked. If an invalid UTF-8 sequence of bytes is found,
1217           pcre_compile() returns an error. If you already know that your  pattern
1218           is  valid, and you want to skip this check for performance reasons, you
1219           can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
1220           passing an invalid UTF-8 string as a pattern is undefined. It may cause
1221           your program to crash.  Note that this option can  also  be  passed  to
1222           pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1223           ing of subject strings.
1224    
1225    
1226    COMPILATION ERROR CODES
1227    
1228           The following table lists the error  codes  than  may  be  returned  by
1229           pcre_compile2(),  along with the error messages that may be returned by
1230           both compiling functions. As PCRE has developed, some error codes  have
1231           fallen out of use. To avoid confusion, they have not been re-used.
1232    
1233              0  no error
1234              1  \ at end of pattern
1235              2  \c at end of pattern
1236              3  unrecognized character follows \
1237              4  numbers out of order in {} quantifier
1238              5  number too big in {} quantifier
1239              6  missing terminating ] for character class
1240              7  invalid escape sequence in character class
1241              8  range out of order in character class
1242              9  nothing to repeat
1243             10  [this code is not in use]
1244             11  internal error: unexpected repeat
1245             12  unrecognized character after (?
1246             13  POSIX named classes are supported only within a class
1247             14  missing )
1248             15  reference to non-existent subpattern
1249             16  erroffset passed as NULL
1250             17  unknown option bit(s) set
1251             18  missing ) after comment
1252             19  [this code is not in use]
1253             20  regular expression too large
1254             21  failed to get memory
1255             22  unmatched parentheses
1256             23  internal error: code overflow
1257             24  unrecognized character after (?<
1258             25  lookbehind assertion is not fixed length
1259             26  malformed number or name after (?(
1260             27  conditional group contains more than two branches
1261             28  assertion expected after (?(
1262             29  (?R or (?digits must be followed by )
1263             30  unknown POSIX class name
1264             31  POSIX collating elements are not supported
1265             32  this version of PCRE is not compiled with PCRE_UTF8 support
1266             33  [this code is not in use]
1267             34  character value in \x{...} sequence is too large
1268             35  invalid condition (?(0)
1269             36  \C not allowed in lookbehind assertion
1270             37  PCRE does not support \L, \l, \N, \U, or \u
1271             38  number after (?C is > 255
1272             39  closing ) for (?C expected
1273             40  recursive call could loop indefinitely
1274             41  unrecognized character after (?P
1275             42  syntax error in subpattern name (missing terminator)
1276             43  two named subpatterns have the same name
1277             44  invalid UTF-8 string
1278             45  support for \P, \p, and \X has not been compiled
1279             46  malformed \P or \p sequence
1280             47  unknown property name after \P or \p
1281             48  subpattern name is too long (maximum 32 characters)
1282             49  too many named subpatterns (maximum 10,000)
1283             50  repeated subpattern is too long
1284             51  octal value is greater than \377 (not in UTF-8 mode)
1285             52  internal error: overran compiling workspace
1286             53   internal  error:  previously-checked  referenced  subpattern not
1287           found
1288             54  DEFINE group contains more than one branch
1289             55  repeating a DEFINE group is not allowed
1290             56  inconsistent NEWLINE options"
1291    
1292    
1293  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  void  
      typedef)  containing  additional  information about the pat-  
      tern; 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.  
   
      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.  
1294    
1295           pcre_extra *pcre_study(const pcre *code, int options
1296                const char **errptr);
1297    
1298           If a compiled pattern is going to be used several times,  it  is  worth
1299           spending more time analyzing it in order to speed up the time taken for
1300           matching. The function pcre_study() takes a pointer to a compiled  pat-
1301           tern as its first argument. If studying the pattern produces additional
1302           information that will help speed up matching,  pcre_study()  returns  a
1303           pointer  to a pcre_extra block, in which the study_data field points to
1304           the results of the study.
1305    
1306           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1307           pcre_exec().  However,  a  pcre_extra  block also contains other fields
1308           that can be set by the caller before the block  is  passed;  these  are
1309           described below in the section on matching a pattern.
1310    
1311           If  studying  the  pattern  does not produce any additional information
1312           pcre_study() returns NULL. In that circumstance, if the calling program
1313           wants  to  pass  any of the other fields to pcre_exec(), it must set up
1314           its own pcre_extra block.
1315    
1316           The second argument of pcre_study() contains option bits.  At  present,
1317           no options are defined, and this argument should always be zero.
1318    
1319           The  third argument for pcre_study() is a pointer for an error message.
1320           If studying succeeds (even if no data is  returned),  the  variable  it
1321           points  to  is  set  to NULL. Otherwise it is set to point to a textual
1322           error message. This is a static string that is part of the library. You
1323           must  not  try  to  free it. You should test the error pointer for NULL
1324           after calling pcre_study(), to be sure that it has run successfully.
1325    
1326           This is a typical call to pcre_study():
1327    
1328             pcre_extra *pe;
1329             pe = pcre_study(
1330               re,             /* result of pcre_compile() */
1331               0,              /* no options exist */
1332               &error);        /* set to NULL or points to a message */
1333    
1334           At present, studying a pattern is useful only for non-anchored patterns
1335           that  do not have a single fixed starting character. A bitmap of possi-
1336           ble starting bytes is created.
1337    
1338    
1339  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.  
1340    
1341           PCRE handles caseless matching, and determines whether  characters  are
1342           letters,  digits, or whatever, by reference to a set of tables, indexed
1343           by character value. When running in UTF-8 mode, this  applies  only  to
1344           characters  with  codes  less than 128. Higher-valued codes never match
1345           escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
1346           with  Unicode  character property support. The use of locales with Uni-
1347           code is discouraged. If you are handling characters with codes  greater
1348           than  128, you should either use UTF-8 and Unicode, or use locales, but
1349           not try to mix the two.
1350    
1351           PCRE contains an internal set of tables that are used  when  the  final
1352           argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
1353           applications.  Normally, the internal tables recognize only ASCII char-
1354           acters. However, when PCRE is built, it is possible to cause the inter-
1355           nal tables to be rebuilt in the default "C" locale of the local system,
1356           which may cause them to be different.
1357    
1358           The  internal tables can always be overridden by tables supplied by the
1359           application that calls PCRE. These may be created in a different locale
1360           from  the  default.  As more and more applications change to using Uni-
1361           code, the need for this locale support is expected to die away.
1362    
1363           External tables are built by calling  the  pcre_maketables()  function,
1364           which  has no arguments, in the relevant locale. The result can then be
1365           passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
1366           example,  to  build  and use tables that are appropriate for the French
1367           locale (where accented characters with  values  greater  than  128  are
1368           treated as letters), the following code could be used:
1369    
1370             setlocale(LC_CTYPE, "fr_FR");
1371             tables = pcre_maketables();
1372             re = pcre_compile(..., tables);
1373    
1374           The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
1375           if you are using Windows, the name for the French locale is "french".
1376    
1377           When pcre_maketables() runs, the tables are built  in  memory  that  is
1378           obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1379           that the memory containing the tables remains available for as long  as
1380           it is needed.
1381    
1382           The pointer that is passed to pcre_compile() is saved with the compiled
1383           pattern, and the same tables are used via this pointer by  pcre_study()
1384           and normally also by pcre_exec(). Thus, by default, for any single pat-
1385           tern, compilation, studying and matching all happen in the same locale,
1386           but different patterns can be compiled in different locales.
1387    
1388           It  is  possible to pass a table pointer or NULL (indicating the use of
1389           the internal tables) to pcre_exec(). Although  not  intended  for  this
1390           purpose,  this facility could be used to match a pattern in a different
1391           locale from the one in which it was compiled. Passing table pointers at
1392           run time is discussed below in the section on matching a pattern.
1393    
1394    
1395  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  
   
      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 au 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), then 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),  
   
      then -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).  
1396    
1397           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1398                int what, void *where);
1399    
1400           The  pcre_fullinfo() function returns information about a compiled pat-
1401           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1402           less retained for backwards compability (and is documented below).
1403    
1404           The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1405           pattern. The second argument is the result of pcre_study(), or NULL  if
1406           the  pattern  was not studied. The third argument specifies which piece
1407           of information is required, and the fourth argument is a pointer  to  a
1408           variable  to  receive  the  data. The yield of the function is zero for
1409           success, or one of the following negative numbers:
1410    
1411             PCRE_ERROR_NULL       the argument code was NULL
1412                                   the argument where was NULL
1413             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1414             PCRE_ERROR_BADOPTION  the value of what was invalid
1415    
1416           The "magic number" is placed at the start of each compiled  pattern  as
1417           an  simple check against passing an arbitrary memory pointer. Here is a
1418           typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1419           pattern:
1420    
1421             int rc;
1422             size_t length;
1423             rc = pcre_fullinfo(
1424               re,               /* result of pcre_compile() */
1425               pe,               /* result of pcre_study(), or NULL */
1426               PCRE_INFO_SIZE,   /* what is required */
1427               &length);         /* where to put the data */
1428    
1429           The  possible  values for the third argument are defined in pcre.h, and
1430           are as follows:
1431    
1432             PCRE_INFO_BACKREFMAX
1433    
1434           Return the number of the highest back reference  in  the  pattern.  The
1435           fourth  argument  should  point to an int variable. Zero is returned if
1436           there are no back references.
1437    
1438             PCRE_INFO_CAPTURECOUNT
1439    
1440           Return the number of capturing subpatterns in the pattern.  The  fourth
1441           argument should point to an int variable.
1442    
1443             PCRE_INFO_DEFAULT_TABLES
1444    
1445           Return  a pointer to the internal default character tables within PCRE.
1446           The fourth argument should point to an unsigned char *  variable.  This
1447           information call is provided for internal use by the pcre_study() func-
1448           tion. External callers can cause PCRE to use  its  internal  tables  by
1449           passing a NULL table pointer.
1450    
1451             PCRE_INFO_FIRSTBYTE
1452    
1453           Return  information  about  the first byte of any matched string, for a
1454           non-anchored pattern. The fourth argument should point to an int  vari-
1455           able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1456           is still recognized for backwards compatibility.)
1457    
1458           If there is a fixed first byte, for example, from  a  pattern  such  as
1459           (cat|cow|coyote), its value is returned. Otherwise, if either
1460    
1461           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1462           branch starts with "^", or
1463    
1464           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1465           set (if it were set, the pattern would be anchored),
1466    
1467           -1  is  returned, indicating that the pattern matches only at the start
1468           of a subject string or after any newline within the  string.  Otherwise
1469           -2 is returned. For anchored patterns, -2 is returned.
1470    
1471             PCRE_INFO_FIRSTTABLE
1472    
1473           If  the pattern was studied, and this resulted in the construction of a
1474           256-bit table indicating a fixed set of bytes for the first byte in any
1475           matching  string, a pointer to the table is returned. Otherwise NULL is
1476           returned. The fourth argument should point to an unsigned char *  vari-
1477           able.
1478    
1479             PCRE_INFO_JCHANGED
1480    
1481           Return  1  if the (?J) option setting is used in the pattern, otherwise
1482           0. The fourth argument should point to an int variable. The (?J) inter-
1483           nal option setting changes the local PCRE_DUPNAMES value.
1484    
1485             PCRE_INFO_LASTLITERAL
1486    
1487           Return  the  value of the rightmost literal byte that must exist in any
1488           matched string, other than at its  start,  if  such  a  byte  has  been
1489           recorded. The fourth argument should point to an int variable. If there
1490           is no such byte, -1 is returned. For anchored patterns, a last  literal
1491           byte  is  recorded only if it follows something of variable length. For
1492           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1493           /^a\dz\d/ the returned value is -1.
1494    
1495             PCRE_INFO_NAMECOUNT
1496             PCRE_INFO_NAMEENTRYSIZE
1497             PCRE_INFO_NAMETABLE
1498    
1499           PCRE  supports the use of named as well as numbered capturing parenthe-
1500           ses. The names are just an additional way of identifying the  parenthe-
1501           ses, which still acquire numbers. Several convenience functions such as
1502           pcre_get_named_substring() are provided for  extracting  captured  sub-
1503           strings  by  name. It is also possible to extract the data directly, by
1504           first converting the name to a number in order to  access  the  correct
1505           pointers in the output vector (described with pcre_exec() below). To do
1506           the conversion, you need  to  use  the  name-to-number  map,  which  is
1507           described by these three values.
1508    
1509           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1510           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1511           of  each  entry;  both  of  these  return  an int value. The entry size
1512           depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
1513           a  pointer  to  the  first  entry of the table (a pointer to char). The
1514           first two bytes of each entry are the number of the capturing parenthe-
1515           sis,  most  significant byte first. The rest of the entry is the corre-
1516           sponding name, zero terminated. The names are  in  alphabetical  order.
1517           When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1518           theses numbers. For example, consider  the  following  pattern  (assume
1519           PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
1520           ignored):
1521    
1522             (?<date> (?<year>(\d\d)?\d\d) -
1523             (?<month>\d\d) - (?<day>\d\d) )
1524    
1525           There are four named subpatterns, so the table has  four  entries,  and
1526           each  entry  in the table is eight bytes long. The table is as follows,
1527           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1528           as ??:
1529    
1530             00 01 d  a  t  e  00 ??
1531             00 05 d  a  y  00 ?? ??
1532             00 04 m  o  n  t  h  00
1533             00 02 y  e  a  r  00 ??
1534    
1535           When  writing  code  to  extract  data from named subpatterns using the
1536           name-to-number map, remember that the length of the entries  is  likely
1537           to be different for each compiled pattern.
1538    
1539             PCRE_INFO_OKPARTIAL
1540    
1541           Return  1 if the pattern can be used for partial matching, otherwise 0.
1542           The fourth argument should point to an int  variable.  The  pcrepartial
1543           documentation  lists  the restrictions that apply to patterns when par-
1544           tial matching is used.
1545    
1546             PCRE_INFO_OPTIONS
1547    
1548           Return a copy of the options with which the pattern was  compiled.  The
1549           fourth  argument  should  point to an unsigned long int variable. These
1550           option bits are those specified in the call to pcre_compile(), modified
1551           by any top-level option settings within the pattern itself.
1552    
1553           A  pattern  is  automatically  anchored by PCRE if all of its top-level
1554           alternatives begin with one of the following:
1555    
1556             ^     unless PCRE_MULTILINE is set
1557             \A    always
1558             \G    always
1559             .*    if PCRE_DOTALL is set and there are no back
1560                     references to the subpattern in which .* appears
1561    
1562           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1563           by pcre_fullinfo().
1564    
1565             PCRE_INFO_SIZE
1566    
1567           Return  the  size  of the compiled pattern, that is, the value that was
1568           passed as the argument to pcre_malloc() when PCRE was getting memory in
1569           which to place the compiled data. The fourth argument should point to a
1570           size_t variable.
1571    
1572             PCRE_INFO_STUDYSIZE
1573    
1574           Return the size of the data block pointed to by the study_data field in
1575           a  pcre_extra  block.  That  is,  it  is  the  value that was passed to
1576           pcre_malloc() when PCRE was getting memory into which to place the data
1577           created  by  pcre_study(). The fourth argument should point to a size_t
1578           variable.
1579    
1580    
1581    OBSOLETE INFO FUNCTION
1582    
1583           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1584    
1585           The pcre_info() function is now obsolete because its interface  is  too
1586           restrictive  to return all the available data about a compiled pattern.
1587           New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of
1588           pcre_info()  is the number of capturing subpatterns, or one of the fol-
1589           lowing negative numbers:
1590    
1591             PCRE_ERROR_NULL       the argument code was NULL
1592             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1593    
1594           If the optptr argument is not NULL, a copy of the  options  with  which
1595           the  pattern  was  compiled  is placed in the integer it points to (see
1596           PCRE_INFO_OPTIONS above).
1597    
1598           If the pattern is not anchored and the  firstcharptr  argument  is  not
1599           NULL,  it is used to pass back information about the first character of
1600           any matched string (see PCRE_INFO_FIRSTBYTE above).
1601    
1602    
1603    REFERENCE COUNTS
1604    
1605           int pcre_refcount(pcre *code, int adjust);
1606    
1607           The pcre_refcount() function is used to maintain a reference  count  in
1608           the data block that contains a compiled pattern. It is provided for the
1609           benefit of applications that  operate  in  an  object-oriented  manner,
1610           where different parts of the application may be using the same compiled
1611           pattern, but you want to free the block when they are all done.
1612    
1613           When a pattern is compiled, the reference count field is initialized to
1614           zero.   It is changed only by calling this function, whose action is to
1615           add the adjust value (which may be positive or  negative)  to  it.  The
1616           yield of the function is the new value. However, the value of the count
1617           is constrained to lie between 0 and 65535, inclusive. If the new  value
1618           is outside these limits, it is forced to the appropriate limit value.
1619    
1620           Except  when it is zero, the reference count is not correctly preserved
1621           if a pattern is compiled on one host and then  transferred  to  a  host
1622           whose byte-order is different. (This seems a highly unlikely scenario.)
1623    
1624    
1625    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1626    
1627           int pcre_exec(const pcre *code, const pcre_extra *extra,
1628                const char *subject, int length, int startoffset,
1629                int options, int *ovector, int ovecsize);
1630    
1631           The function pcre_exec() is called to match a subject string against  a
1632           compiled  pattern, which is passed in the code argument. If the pattern
1633           has been studied, the result of the study should be passed in the extra
1634           argument.  This  function is the main matching facility of the library,
1635           and it operates in a Perl-like manner. For specialist use there is also
1636           an  alternative matching function, which is described below in the sec-
1637           tion about the pcre_dfa_exec() function.
1638    
1639           In most applications, the pattern will have been compiled (and  option-
1640           ally  studied)  in the same process that calls pcre_exec(). However, it
1641           is possible to save compiled patterns and study data, and then use them
1642           later  in  different processes, possibly even on different hosts. For a
1643           discussion about this, see the pcreprecompile documentation.
1644    
1645           Here is an example of a simple call to pcre_exec():
1646    
1647             int rc;
1648             int ovector[30];
1649             rc = pcre_exec(
1650               re,             /* result of pcre_compile() */
1651               NULL,           /* we didn't study the pattern */
1652               "some string",  /* the subject string */
1653               11,             /* the length of the subject string */
1654               0,              /* start at offset 0 in the subject */
1655               0,              /* default options */
1656               ovector,        /* vector of integers for substring information */
1657               30);            /* number of elements (NOT size in bytes) */
1658    
1659       Extra data for pcre_exec()
1660    
1661           If the extra argument is not NULL, it must point to a  pcre_extra  data
1662           block.  The pcre_study() function returns such a block (when it doesn't
1663           return NULL), but you can also create one for yourself, and pass  addi-
1664           tional  information  in it. The pcre_extra block contains the following
1665           fields (not necessarily in this order):
1666    
1667             unsigned long int flags;
1668             void *study_data;
1669             unsigned long int match_limit;
1670             unsigned long int match_limit_recursion;
1671             void *callout_data;
1672             const unsigned char *tables;
1673    
1674           The flags field is a bitmap that specifies which of  the  other  fields
1675           are set. The flag bits are:
1676    
1677             PCRE_EXTRA_STUDY_DATA
1678             PCRE_EXTRA_MATCH_LIMIT
1679             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1680             PCRE_EXTRA_CALLOUT_DATA
1681             PCRE_EXTRA_TABLES
1682    
1683           Other  flag  bits should be set to zero. The study_data field is set in
1684           the pcre_extra block that is returned by  pcre_study(),  together  with
1685           the appropriate flag bit. You should not set this yourself, but you may
1686           add to the block by setting the other fields  and  their  corresponding
1687           flag bits.
1688    
1689           The match_limit field provides a means of preventing PCRE from using up
1690           a vast amount of resources when running patterns that are not going  to
1691           match,  but  which  have  a very large number of possibilities in their
1692           search trees. The classic  example  is  the  use  of  nested  unlimited
1693           repeats.
1694    
1695           Internally,  PCRE uses a function called match() which it calls repeat-
1696           edly (sometimes recursively). The limit set by match_limit  is  imposed
1697           on  the  number  of times this function is called during a match, which
1698           has the effect of limiting the amount of  backtracking  that  can  take
1699           place. For patterns that are not anchored, the count restarts from zero
1700           for each position in the subject string.
1701    
1702           The default value for the limit can be set  when  PCRE  is  built;  the
1703           default  default  is 10 million, which handles all but the most extreme
1704           cases. You can override the default  by  suppling  pcre_exec()  with  a
1705           pcre_extra     block    in    which    match_limit    is    set,    and
1706           PCRE_EXTRA_MATCH_LIMIT is set in the  flags  field.  If  the  limit  is
1707           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1708    
1709           The  match_limit_recursion field is similar to match_limit, but instead
1710           of limiting the total number of times that match() is called, it limits
1711           the  depth  of  recursion. The recursion depth is a smaller number than
1712           the total number of calls, because not all calls to match() are  recur-
1713           sive.  This limit is of use only if it is set smaller than match_limit.
1714    
1715           Limiting the recursion depth limits the amount of  stack  that  can  be
1716           used, or, when PCRE has been compiled to use memory on the heap instead
1717           of the stack, the amount of heap memory that can be used.
1718    
1719           The default value for match_limit_recursion can be  set  when  PCRE  is
1720           built;  the  default  default  is  the  same  value  as the default for
1721           match_limit. You can override the default by suppling pcre_exec()  with
1722           a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
1723           PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
1724           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1725    
1726           The  pcre_callout  field is used in conjunction with the "callout" fea-
1727           ture, which is described in the pcrecallout documentation.
1728    
1729           The tables field  is  used  to  pass  a  character  tables  pointer  to
1730           pcre_exec();  this overrides the value that is stored with the compiled
1731           pattern. A non-NULL value is stored with the compiled pattern  only  if
1732           custom  tables  were  supplied to pcre_compile() via its tableptr argu-
1733           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1734           PCRE's  internal  tables  to be used. This facility is helpful when re-
1735           using patterns that have been saved after compiling  with  an  external
1736           set  of  tables,  because  the  external tables might be at a different
1737           address when pcre_exec() is called. See the  pcreprecompile  documenta-
1738           tion for a discussion of saving compiled patterns for later use.
1739    
1740       Option bits for pcre_exec()
1741    
1742           The  unused  bits of the options argument for pcre_exec() must be zero.
1743           The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
1744           PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1745           PCRE_PARTIAL.
1746    
1747             PCRE_ANCHORED
1748    
1749           The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
1750           matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
1751           turned out to be anchored by virtue of its contents, it cannot be  made
1752           unachored at matching time.
1753    
1754             PCRE_NEWLINE_CR
1755             PCRE_NEWLINE_LF
1756             PCRE_NEWLINE_CRLF
1757             PCRE_NEWLINE_ANYCRLF
1758             PCRE_NEWLINE_ANY
1759    
1760           These  options  override  the  newline  definition  that  was chosen or
1761           defaulted when the pattern was compiled. For details, see the  descrip-
1762           tion  of  pcre_compile()  above.  During  matching,  the newline choice
1763           affects the behaviour of the dot, circumflex,  and  dollar  metacharac-
1764           ters.  It may also alter the way the match position is advanced after a
1765           match  failure  for  an  unanchored  pattern.  When  PCRE_NEWLINE_CRLF,
1766           PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY is set, and a match attempt
1767           fails when the current position is at a CRLF sequence, the match  posi-
1768           tion  is  advanced by two characters instead of one, in other words, to
1769           after the CRLF.
1770    
1771             PCRE_NOTBOL
1772    
1773           This option specifies that first character of the subject string is not
1774           the  beginning  of  a  line, so the circumflex metacharacter should not
1775           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1776           causes  circumflex  never to match. This option affects only the behav-
1777           iour of the circumflex metacharacter. It does not affect \A.
1778    
1779             PCRE_NOTEOL
1780    
1781           This option specifies that the end of the subject string is not the end
1782           of  a line, so the dollar metacharacter should not match it nor (except
1783           in multiline mode) a newline immediately before it. Setting this  with-
1784           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1785           option affects only the behaviour of the dollar metacharacter. It  does
1786           not affect \Z or \z.
1787    
1788             PCRE_NOTEMPTY
1789    
1790           An empty string is not considered to be a valid match if this option is
1791           set. If there are alternatives in the pattern, they are tried.  If  all
1792           the  alternatives  match  the empty string, the entire match fails. For
1793           example, if the pattern
1794    
1795             a?b?
1796    
1797           is applied to a string not beginning with "a" or "b",  it  matches  the
1798           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1799           match is not valid, so PCRE searches further into the string for occur-
1800           rences of "a" or "b".
1801    
1802           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1803           cial case of a pattern match of the empty  string  within  its  split()
1804           function,  and  when  using  the /g modifier. It is possible to emulate
1805           Perl's behaviour after matching a null string by first trying the match
1806           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1807           if that fails by advancing the starting offset (see below)  and  trying
1808           an ordinary match again. There is some code that demonstrates how to do
1809           this in the pcredemo.c sample program.
1810    
1811             PCRE_NO_UTF8_CHECK
1812    
1813           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1814           UTF-8  string is automatically checked when pcre_exec() is subsequently
1815           called.  The value of startoffset is also checked  to  ensure  that  it
1816           points  to the start of a UTF-8 character. If an invalid UTF-8 sequence
1817           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1818           startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is
1819           returned.
1820    
1821           If you already know that your subject is valid, and you  want  to  skip
1822           these    checks    for   performance   reasons,   you   can   set   the
1823           PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
1824           do  this  for the second and subsequent calls to pcre_exec() if you are
1825           making repeated calls to find all  the  matches  in  a  single  subject
1826           string.  However,  you  should  be  sure  that the value of startoffset
1827           points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
1828           set,  the  effect of passing an invalid UTF-8 string as a subject, or a
1829           value of startoffset that does not point to the start of a UTF-8  char-
1830           acter, is undefined. Your program may crash.
1831    
1832             PCRE_PARTIAL
1833    
1834           This  option  turns  on  the  partial  matching feature. If the subject
1835           string fails to match the pattern, but at some point during the  match-
1836           ing  process  the  end of the subject was reached (that is, the subject
1837           partially matches the pattern and the failure to  match  occurred  only
1838           because  there were not enough subject characters), pcre_exec() returns
1839           PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
1840           used,  there  are restrictions on what may appear in the pattern. These
1841           are discussed in the pcrepartial documentation.
1842    
1843       The string to be matched by pcre_exec()
1844    
1845           The subject string is passed to pcre_exec() as a pointer in subject,  a
1846           length  in  length, and a starting byte offset in startoffset. In UTF-8
1847           mode, the byte offset must point to the start  of  a  UTF-8  character.
1848           Unlike  the  pattern string, the subject may contain binary zero bytes.
1849           When the starting offset is zero, the search for a match starts at  the
1850           beginning of the subject, and this is by far the most common case.
1851    
1852           A  non-zero  starting offset is useful when searching for another match
1853           in the same subject by calling pcre_exec() again after a previous  suc-
1854           cess.   Setting  startoffset differs from just passing over a shortened
1855           string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
1856           with any kind of lookbehind. For example, consider the pattern
1857    
1858             \Biss\B
1859    
1860           which  finds  occurrences  of "iss" in the middle of words. (\B matches
1861           only if the current position in the subject is not  a  word  boundary.)
1862           When  applied  to the string "Mississipi" the first call to pcre_exec()
1863           finds the first occurrence. If pcre_exec() is called  again  with  just
1864           the  remainder  of  the  subject,  namely  "issipi", it does not match,
1865           because \B is always false at the start of the subject, which is deemed
1866           to  be  a  word  boundary. However, if pcre_exec() is passed the entire
1867           string again, but with startoffset set to 4, it finds the second occur-
1868           rence  of "iss" because it is able to look behind the starting point to
1869           discover that it is preceded by a letter.
1870    
1871           If a non-zero starting offset is passed when the pattern  is  anchored,
1872           one attempt to match at the given offset is made. This can only succeed
1873           if the pattern does not require the match to be at  the  start  of  the
1874           subject.
1875    
1876       How pcre_exec() returns captured substrings
1877    
1878           In  general, a pattern matches a certain portion of the subject, and in
1879           addition, further substrings from the subject  may  be  picked  out  by
1880           parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
1881           this is called "capturing" in what follows, and the  phrase  "capturing
1882           subpattern"  is  used for a fragment of a pattern that picks out a sub-
1883           string. PCRE supports several other kinds of  parenthesized  subpattern
1884           that do not cause substrings to be captured.
1885    
1886           Captured  substrings are returned to the caller via a vector of integer
1887           offsets whose address is passed in ovector. The number of  elements  in
1888           the  vector is passed in ovecsize, which must be a non-negative number.
1889           Note: this argument is NOT the size of ovector in bytes.
1890    
1891           The first two-thirds of the vector is used to pass back  captured  sub-
1892           strings,  each  substring using a pair of integers. The remaining third
1893           of the vector is used as workspace by pcre_exec() while  matching  cap-
1894           turing  subpatterns, and is not available for passing back information.
1895           The length passed in ovecsize should always be a multiple of three.  If
1896           it is not, it is rounded down.
1897    
1898           When  a  match  is successful, information about captured substrings is
1899           returned in pairs of integers, starting at the  beginning  of  ovector,
1900           and  continuing  up  to two-thirds of its length at the most. The first
1901           element of a pair is set to the offset of the first character in a sub-
1902           string,  and  the  second  is  set to the offset of the first character
1903           after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
1904           tor[1],  identify  the  portion  of  the  subject string matched by the
1905           entire pattern. The next pair is used for the first  capturing  subpat-
1906           tern, and so on. The value returned by pcre_exec() is one more than the
1907           highest numbered pair that has been set. For example, if two substrings
1908           have  been captured, the returned value is 3. If there are no capturing
1909           subpatterns, the return value from a successful match is 1,  indicating
1910           that just the first pair of offsets has been set.
1911    
1912           If a capturing subpattern is matched repeatedly, it is the last portion
1913           of the string that it matched that is returned.
1914    
1915           If the vector is too small to hold all the captured substring  offsets,
1916           it is used as far as possible (up to two-thirds of its length), and the
1917           function returns a value of zero. In particular, if the substring  off-
1918           sets are not of interest, pcre_exec() may be called with ovector passed
1919           as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1920           references  and  the  ovector is not big enough to remember the related
1921           substrings, PCRE has to get additional memory for use during  matching.
1922           Thus it is usually advisable to supply an ovector.
1923    
1924           The  pcre_info()  function  can  be used to find out how many capturing
1925           subpatterns there are in a compiled  pattern.  The  smallest  size  for
1926           ovector  that  will allow for n captured substrings, in addition to the
1927           offsets of the substring matched by the whole pattern, is (n+1)*3.
1928    
1929           It is possible for capturing subpattern number n+1 to match  some  part
1930           of the subject when subpattern n has not been used at all. For example,
1931           if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
1932           return from the function is 4, and subpatterns 1 and 3 are matched, but
1933           2 is not. When this happens, both values in  the  offset  pairs  corre-
1934           sponding to unused subpatterns are set to -1.
1935    
1936           Offset  values  that correspond to unused subpatterns at the end of the
1937           expression are also set to -1. For example,  if  the  string  "abc"  is
1938           matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1939           matched. The return from the function is 2, because  the  highest  used
1940           capturing subpattern number is 1. However, you can refer to the offsets
1941           for the second and third capturing subpatterns if  you  wish  (assuming
1942           the vector is large enough, of course).
1943    
1944           Some  convenience  functions  are  provided for extracting the captured
1945           substrings as separate strings. These are described below.
1946    
1947       Error return values from pcre_exec()
1948    
1949           If pcre_exec() fails, it returns a negative number. The  following  are
1950           defined in the header file:
1951    
1952             PCRE_ERROR_NOMATCH        (-1)
1953    
1954           The subject string did not match the pattern.
1955    
1956             PCRE_ERROR_NULL           (-2)
1957    
1958           Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1959           ovecsize was not zero.
1960    
1961             PCRE_ERROR_BADOPTION      (-3)
1962    
1963           An unrecognized bit was set in the options argument.
1964    
1965             PCRE_ERROR_BADMAGIC       (-4)
1966    
1967           PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1968           to catch the case when it is passed a junk pointer and to detect when a
1969           pattern that was compiled in an environment of one endianness is run in
1970           an  environment  with the other endianness. This is the error that PCRE
1971           gives when the magic number is not present.
1972    
1973             PCRE_ERROR_UNKNOWN_OPCODE (-5)
1974    
1975           While running the pattern match, an unknown item was encountered in the
1976           compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1977           overwriting of the compiled pattern.
1978    
1979             PCRE_ERROR_NOMEMORY       (-6)
1980    
1981           If a pattern contains back references, but the ovector that  is  passed
1982           to pcre_exec() is not big enough to remember the referenced substrings,
1983           PCRE gets a block of memory at the start of matching to  use  for  this
1984           purpose.  If the call via pcre_malloc() fails, this error is given. The
1985           memory is automatically freed at the end of matching.
1986    
1987             PCRE_ERROR_NOSUBSTRING    (-7)
1988    
1989           This error is used by the pcre_copy_substring(),  pcre_get_substring(),
1990           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1991           returned by pcre_exec().
1992    
1993             PCRE_ERROR_MATCHLIMIT     (-8)
1994    
1995           The backtracking limit, as specified by  the  match_limit  field  in  a
1996           pcre_extra  structure  (or  defaulted) was reached. See the description
1997           above.
1998    
1999             PCRE_ERROR_CALLOUT        (-9)
2000    
2001           This error is never generated by pcre_exec() itself. It is provided for
2002           use  by  callout functions that want to yield a distinctive error code.
2003           See the pcrecallout documentation for details.
2004    
2005             PCRE_ERROR_BADUTF8        (-10)
2006    
2007           A string that contains an invalid UTF-8 byte sequence was passed  as  a
2008           subject.
2009    
2010             PCRE_ERROR_BADUTF8_OFFSET (-11)
2011    
2012           The UTF-8 byte sequence that was passed as a subject was valid, but the
2013           value of startoffset did not point to the beginning of a UTF-8  charac-
2014           ter.
2015    
2016             PCRE_ERROR_PARTIAL        (-12)
2017    
2018           The  subject  string did not match, but it did match partially. See the
2019           pcrepartial documentation for details of partial matching.
2020    
2021             PCRE_ERROR_BADPARTIAL     (-13)
2022    
2023           The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
2024           items  that are not supported for partial matching. See the pcrepartial
2025           documentation for details of partial matching.
2026    
2027             PCRE_ERROR_INTERNAL       (-14)
2028    
2029           An unexpected internal error has occurred. This error could  be  caused
2030           by a bug in PCRE or by overwriting of the compiled pattern.
2031    
2032             PCRE_ERROR_BADCOUNT       (-15)
2033    
2034           This  error is given if the value of the ovecsize argument is negative.
2035    
2036             PCRE_ERROR_RECURSIONLIMIT (-21)
2037    
2038           The internal recursion limit, as specified by the match_limit_recursion
2039           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
2040           description above.
2041    
2042             PCRE_ERROR_NULLWSLIMIT    (-22)
2043    
2044           When a group that can match an empty  substring  is  repeated  with  an
2045           unbounded  upper  limit, the subject position at the start of the group
2046           must be remembered, so that a test for an empty string can be made when
2047           the  end  of the group is reached. Some workspace is required for this;
2048           if it runs out, this error is given.
2049    
2050             PCRE_ERROR_BADNEWLINE     (-23)
2051    
2052           An invalid combination of PCRE_NEWLINE_xxx options was given.
2053    
2054           Error numbers -16 to -20 are not used by pcre_exec().
2055    
2056    
2057    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2058    
2059           int pcre_copy_substring(const char *subject, int *ovector,
2060                int stringcount, int stringnumber, char *buffer,
2061                int buffersize);
2062    
2063           int pcre_get_substring(const char *subject, int *ovector,
2064                int stringcount, int stringnumber,
2065                const char **stringptr);
2066    
2067           int pcre_get_substring_list(const char *subject,
2068                int *ovector, int stringcount, const char ***listptr);
2069    
2070           Captured substrings can be  accessed  directly  by  using  the  offsets
2071           returned  by  pcre_exec()  in  ovector.  For convenience, the functions
2072           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2073           string_list()  are  provided for extracting captured substrings as new,
2074           separate, zero-terminated strings. These functions identify  substrings
2075           by  number.  The  next section describes functions for extracting named
2076           substrings.
2077    
2078           A substring that contains a binary zero is correctly extracted and  has
2079           a  further zero added on the end, but the result is not, of course, a C
2080           string.  However, you can process such a string  by  referring  to  the
2081           length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
2082           string().  Unfortunately, the interface to pcre_get_substring_list() is
2083           not  adequate for handling strings containing binary zeros, because the
2084           end of the final string is not independently indicated.
2085    
2086           The first three arguments are the same for all  three  of  these  func-
2087           tions:  subject  is  the subject string that has just been successfully
2088           matched, ovector is a pointer to the vector of integer offsets that was
2089           passed to pcre_exec(), and stringcount is the number of substrings that
2090           were captured by the match, including the substring  that  matched  the
2091           entire regular expression. This is the value returned by pcre_exec() if
2092           it is greater than zero. If pcre_exec() returned zero, indicating  that
2093           it  ran out of space in ovector, the value passed as stringcount should
2094           be the number of elements in the vector divided by three.
2095    
2096           The functions pcre_copy_substring() and pcre_get_substring() extract  a
2097           single  substring,  whose  number  is given as stringnumber. A value of
2098           zero extracts the substring that matched the  entire  pattern,  whereas
2099           higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
2100           string(), the string is placed in buffer,  whose  length  is  given  by
2101           buffersize,  while  for  pcre_get_substring()  a new block of memory is
2102           obtained via pcre_malloc, and its address is  returned  via  stringptr.
2103           The  yield  of  the function is the length of the string, not including
2104           the terminating zero, or one of these error codes:
2105    
2106             PCRE_ERROR_NOMEMORY       (-6)
2107    
2108           The buffer was too small for pcre_copy_substring(), or the  attempt  to
2109           get memory failed for pcre_get_substring().
2110    
2111             PCRE_ERROR_NOSUBSTRING    (-7)
2112    
2113           There is no substring whose number is stringnumber.
2114    
2115           The  pcre_get_substring_list()  function  extracts  all  available sub-
2116           strings and builds a list of pointers to them. All this is  done  in  a
2117           single block of memory that is obtained via pcre_malloc. The address of
2118           the memory block is returned via listptr, which is also  the  start  of
2119           the  list  of  string pointers. The end of the list is marked by a NULL
2120           pointer. The yield of the function is zero if all  went  well,  or  the
2121           error code
2122    
2123             PCRE_ERROR_NOMEMORY       (-6)
2124    
2125           if the attempt to get the memory block failed.
2126    
2127           When  any of these functions encounter a substring that is unset, which
2128           can happen when capturing subpattern number n+1 matches  some  part  of
2129           the  subject, but subpattern n has not been used at all, they return an
2130           empty string. This can be distinguished from a genuine zero-length sub-
2131           string  by inspecting the appropriate offset in ovector, which is nega-
2132           tive for unset substrings.
2133    
2134           The two convenience functions pcre_free_substring() and  pcre_free_sub-
2135           string_list()  can  be  used  to free the memory returned by a previous
2136           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2137           tively.  They  do  nothing  more  than  call the function pointed to by
2138           pcre_free, which of course could be called directly from a  C  program.
2139           However,  PCRE is used in some situations where it is linked via a spe-
2140           cial  interface  to  another  programming  language  that  cannot   use
2141           pcre_free  directly;  it is for these cases that the functions are pro-
2142           vided.
2143    
2144    
2145    EXTRACTING CAPTURED SUBSTRINGS BY NAME
2146    
2147           int pcre_get_stringnumber(const pcre *code,
2148                const char *name);
2149    
2150           int pcre_copy_named_substring(const pcre *code,
2151                const char *subject, int *ovector,
2152                int stringcount, const char *stringname,
2153                char *buffer, int buffersize);
2154    
2155           int pcre_get_named_substring(const pcre *code,
2156                const char *subject, int *ovector,
2157                int stringcount, const char *stringname,
2158                const char **stringptr);
2159    
2160           To extract a substring by name, you first have to find associated  num-
2161           ber.  For example, for this pattern
2162    
2163             (a+)b(?<xxx>\d+)...
2164    
2165           the number of the subpattern called "xxx" is 2. If the name is known to
2166           be unique (PCRE_DUPNAMES was not set), you can find the number from the
2167           name by calling pcre_get_stringnumber(). The first argument is the com-
2168           piled pattern, and the second is the name. The yield of the function is
2169           the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2170           subpattern of that name.
2171    
2172           Given the number, you can extract the substring directly, or use one of
2173           the functions described in the previous section. For convenience, there
2174           are also two functions that do the whole job.
2175    
2176           Most   of   the   arguments    of    pcre_copy_named_substring()    and
2177           pcre_get_named_substring()  are  the  same  as  those for the similarly
2178           named functions that extract by number. As these are described  in  the
2179           previous  section,  they  are not re-described here. There are just two
2180           differences:
2181    
2182           First, instead of a substring number, a substring name is  given.  Sec-
2183           ond, there is an extra argument, given at the start, which is a pointer
2184           to the compiled pattern. This is needed in order to gain access to  the
2185           name-to-number translation table.
2186    
2187           These  functions call pcre_get_stringnumber(), and if it succeeds, they
2188           then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
2189           ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
2190           behaviour may not be what you want (see the next section).
2191    
2192    
2193    DUPLICATE SUBPATTERN NAMES
2194    
2195           int pcre_get_stringtable_entries(const pcre *code,
2196                const char *name, char **first, char **last);
2197    
2198           When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
2199           subpatterns  are  not  required  to  be unique. Normally, patterns with
2200           duplicate names are such that in any one match, only one of  the  named
2201           subpatterns  participates. An example is shown in the pcrepattern docu-
2202           mentation. When duplicates are present, pcre_copy_named_substring() and
2203           pcre_get_named_substring()  return the first substring corresponding to
2204           the given name that is set.  If  none  are  set,  an  empty  string  is
2205           returned.  The pcre_get_stringnumber() function returns one of the num-
2206           bers that are associated with the name, but it is not defined which  it
2207           is.
2208    
2209           If  you want to get full details of all captured substrings for a given
2210           name, you must use  the  pcre_get_stringtable_entries()  function.  The
2211           first argument is the compiled pattern, and the second is the name. The
2212           third and fourth are pointers to variables which  are  updated  by  the
2213           function. After it has run, they point to the first and last entries in
2214           the name-to-number table  for  the  given  name.  The  function  itself
2215           returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2216           there are none. The format of the table is described above in the  sec-
2217           tion  entitled  Information  about  a  pattern.  Given all the relevant
2218           entries for the name, you can extract each of their numbers, and  hence
2219           the captured data, if any.
2220    
2221    
2222    FINDING ALL POSSIBLE MATCHES
2223    
2224           The  traditional  matching  function  uses a similar algorithm to Perl,
2225           which stops when it finds the first match, starting at a given point in
2226           the  subject.  If you want to find all possible matches, or the longest
2227           possible match, consider using the alternative matching  function  (see
2228           below)  instead.  If you cannot use the alternative function, but still
2229           need to find all possible matches, you can kludge it up by  making  use
2230           of the callout facility, which is described in the pcrecallout documen-
2231           tation.
2232    
2233           What you have to do is to insert a callout right at the end of the pat-
2234           tern.   When your callout function is called, extract and save the cur-
2235           rent matched substring. Then return  1,  which  forces  pcre_exec()  to
2236           backtrack  and  try other alternatives. Ultimately, when it runs out of
2237           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2238    
2239    
2240    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2241    
2242           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2243                const char *subject, int length, int startoffset,
2244                int options, int *ovector, int ovecsize,
2245                int *workspace, int wscount);
2246    
2247           The function pcre_dfa_exec()  is  called  to  match  a  subject  string
2248           against  a  compiled pattern, using a matching algorithm that scans the
2249           subject string just once, and does not backtrack.  This  has  different
2250           characteristics  to  the  normal  algorithm, and is not compatible with
2251           Perl. Some of the features of PCRE patterns are not  supported.  Never-
2252           theless,  there are times when this kind of matching can be useful. For
2253           a discussion of the two matching algorithms, see the pcrematching docu-
2254           mentation.
2255    
2256           The  arguments  for  the  pcre_dfa_exec()  function are the same as for
2257           pcre_exec(), plus two extras. The ovector argument is used in a differ-
2258           ent  way,  and  this is described below. The other common arguments are
2259           used in the same way as for pcre_exec(), so their  description  is  not
2260           repeated here.
2261    
2262           The  two  additional  arguments provide workspace for the function. The
2263           workspace vector should contain at least 20 elements. It  is  used  for
2264           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2265           workspace will be needed for patterns and subjects where  there  are  a
2266           lot of potential matches.
2267    
2268           Here is an example of a simple call to pcre_dfa_exec():
2269    
2270             int rc;
2271             int ovector[10];
2272             int wspace[20];
2273             rc = pcre_dfa_exec(
2274               re,             /* result of pcre_compile() */
2275               NULL,           /* we didn't study the pattern */
2276               "some string",  /* the subject string */
2277               11,             /* the length of the subject string */
2278               0,              /* start at offset 0 in the subject */
2279               0,              /* default options */
2280               ovector,        /* vector of integers for substring information */
2281               10,             /* number of elements (NOT size in bytes) */
2282               wspace,         /* working space vector */
2283               20);            /* number of elements (NOT size in bytes) */
2284    
2285       Option bits for pcre_dfa_exec()
2286    
2287           The  unused  bits  of  the options argument for pcre_dfa_exec() must be
2288           zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
2289           LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2290           PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2291           three of these are the same as for pcre_exec(), so their description is
2292           not repeated here.
2293    
2294             PCRE_PARTIAL
2295    
2296           This has the same general effect as it does for  pcre_exec(),  but  the
2297           details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
2298           pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
2299           PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
2300           been no complete matches, but there is still at least one matching pos-
2301           sibility.  The portion of the string that provided the partial match is
2302           set as the first matching string.
2303    
2304             PCRE_DFA_SHORTEST
2305    
2306           Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
2307           stop as soon as it has found one match. Because of the way the alterna-
2308           tive algorithm works, this is necessarily the shortest  possible  match
2309           at the first possible matching point in the subject string.
2310    
2311             PCRE_DFA_RESTART
2312    
2313           When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
2314           returns a partial match, it is possible to call it  again,  with  addi-
2315           tional  subject  characters,  and have it continue with the same match.
2316           The PCRE_DFA_RESTART option requests this action; when it is  set,  the
2317           workspace  and wscount options must reference the same vector as before
2318           because data about the match so far is left in  them  after  a  partial
2319           match.  There  is  more  discussion of this facility in the pcrepartial
2320           documentation.
2321    
2322       Successful returns from pcre_dfa_exec()
2323    
2324           When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
2325           string in the subject. Note, however, that all the matches from one run
2326           of the function start at the same point in  the  subject.  The  shorter
2327           matches  are all initial substrings of the longer matches. For example,
2328           if the pattern
2329    
2330             <.*>
2331    
2332           is matched against the string
2333    
2334             This is <something> <something else> <something further> no more
2335    
2336           the three matched strings are
2337    
2338             <something>
2339             <something> <something else>
2340             <something> <something else> <something further>
2341    
2342           On success, the yield of the function is a number  greater  than  zero,
2343           which  is  the  number of matched substrings. The substrings themselves
2344           are returned in ovector. Each string uses two elements;  the  first  is
2345           the  offset  to  the start, and the second is the offset to the end. In
2346           fact, all the strings have the same start  offset.  (Space  could  have
2347           been  saved by giving this only once, but it was decided to retain some
2348           compatibility with the way pcre_exec() returns data,  even  though  the
2349           meaning of the strings is different.)
2350    
2351           The strings are returned in reverse order of length; that is, the long-
2352           est matching string is given first. If there were too many  matches  to
2353           fit  into ovector, the yield of the function is zero, and the vector is
2354           filled with the longest matches.
2355    
2356       Error returns from pcre_dfa_exec()
2357    
2358           The pcre_dfa_exec() function returns a negative number when  it  fails.
2359           Many  of  the  errors  are  the  same as for pcre_exec(), and these are
2360           described above.  There are in addition the following errors  that  are
2361           specific to pcre_dfa_exec():
2362    
2363             PCRE_ERROR_DFA_UITEM      (-16)
2364    
2365           This  return is given if pcre_dfa_exec() encounters an item in the pat-
2366           tern that it does not support, for instance, the use of \C  or  a  back
2367           reference.
2368    
2369             PCRE_ERROR_DFA_UCOND      (-17)
2370    
2371           This  return  is  given  if pcre_dfa_exec() encounters a condition item
2372           that uses a back reference for the condition, or a test  for  recursion
2373           in a specific group. These are not supported.
2374    
2375             PCRE_ERROR_DFA_UMLIMIT    (-18)
2376    
2377           This  return  is given if pcre_dfa_exec() is called with an extra block
2378           that contains a setting of the match_limit field. This is not supported
2379           (it is meaningless).
2380    
2381             PCRE_ERROR_DFA_WSSIZE     (-19)
2382    
2383           This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
2384           workspace vector.
2385    
2386             PCRE_ERROR_DFA_RECURSE    (-20)
2387    
2388           When a recursive subpattern is processed, the matching  function  calls
2389           itself  recursively,  using  private vectors for ovector and workspace.
2390           This error is given if the output vector  is  not  large  enough.  This
2391           should be extremely rare, as a vector of size 1000 is used.
2392    
 MATCHING A PATTERN  
      The function pcre_exec() is called to match a subject string  
      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.  
   
      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, it may contain binary zero  char-  
      acters.  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, then 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 store  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.  
2393    
2394    SEE ALSO
2395    
2396           pcrebuild(3),  pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2397           tial(3), pcreposix(3), pcreprecompile(3), pcresample(3),  pcrestack(3).
2398    
2399    
2400  LIMITATIONS  AUTHOR
      There are some size limitations in PCRE but it is hoped that  
      they will never in practice be relevant.  The maximum length  
      of a compiled pattern is 65539 (sic) bytes.  All  values  in  
      repeating  quantifiers must be less than 65536.  The maximum  
      number of capturing subpatterns is 99.  The  maximum  number  
      of  all  parenthesized subpatterns, including capturing sub-  
      patterns, 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) get 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.  
   
      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  
2401    
2402       The following sections describe  the  use  of  each  of  the         Philip Hazel
2403       meta-characters.         University Computing Service
2404           Cambridge CB2 3QH, England.
2405    
2406    
2407    REVISION
2408    
2409  BACKSLASH         Last updated: 04 June 2007
2410       The backslash character has several uses. Firstly, if it  is         Copyright (c) 1997-2007 University of Cambridge.
2411       followed  by  a  non-alphameric character, it takes away any  ------------------------------------------------------------------------------
      special  meaning  that  character  may  have.  This  use  of  
      backslash  as  an  escape  character applies both inside and  
      outside character classes.  
   
      For example, if you want to match a "*" character, you write  
      "\*" in the pattern. This 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.  
2412    
2413    
2414    PCRECALLOUT(3)                                                  PCRECALLOUT(3)
2415    
 CIRCUMFLEX AND DOLLAR  
      Outside a character class, in the default matching mode, the  
      circumflex  character  is an assertion which is true only if  
      the current matching point is at the start  of  the  subject  
      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 is it  always  anchored,  
      whether PCRE_MULTILINE is set or not.  
2416    
2417    NAME
2418           PCRE - Perl-compatible regular expressions
2419    
2420    
2421  FULL STOP (PERIOD, DOT)  PCRE CALLOUTS
      Outside a character class, a dot in the pattern matches  any  
      one character in the subject, including a non-printing char-  
      acter, but not (by default)  newline.   If  the  PCRE_DOTALL  
      option  is  set,  then dots match newlines as well. The han-  
      dling of dot is entirely independent of the handling of cir-  
      cumflex  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.  
2422    
2423           int (*pcre_callout)(pcre_callout_block *);
2424    
2425           PCRE provides a feature called "callout", which is a means of temporar-
2426           ily passing control to the caller of PCRE  in  the  middle  of  pattern
2427           matching.  The  caller of PCRE provides an external function by putting
2428           its entry point in the global variable pcre_callout. By  default,  this
2429           variable contains NULL, which disables all calling out.
2430    
2431           Within  a  regular  expression,  (?C) indicates the points at which the
2432           external function is to be called.  Different  callout  points  can  be
2433           identified  by  putting  a number less than 256 after the letter C. The
2434           default value is zero.  For  example,  this  pattern  has  two  callout
2435           points:
2436    
2437             (?C1)abc(?C2)def
2438    
2439           If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2440           called, PCRE automatically  inserts  callouts,  all  with  number  255,
2441           before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2442           used with the pattern
2443    
2444             A(\d{2}|--)
2445    
2446           it is processed as if it were
2447    
2448           (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2449    
2450           Notice that there is a callout before and after  each  parenthesis  and
2451           alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2452           progress of pattern matching. The pcretest command has an  option  that
2453           sets  automatic callouts; when it is used, the output indicates how the
2454           pattern is matched. This is useful information when you are  trying  to
2455           optimize the performance of a particular pattern.
2456    
2457    
2458    MISSING CALLOUTS
2459    
2460           You  should  be  aware  that,  because of optimizations in the way PCRE
2461           matches patterns, callouts sometimes do not happen. For example, if the
2462           pattern is
2463    
2464             ab(?C4)cd
2465    
2466           PCRE knows that any matching string must contain the letter "d". If the
2467           subject string is "abyz", the lack of "d" means that  matching  doesn't
2468           ever  start,  and  the  callout is never reached. However, with "abyd",
2469           though the result is still no match, the callout is obeyed.
2470    
2471    
2472    THE CALLOUT INTERFACE
2473    
2474           During matching, when PCRE reaches a callout point, the external  func-
2475           tion  defined by pcre_callout is called (if it is set). This applies to
2476           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2477           only  argument  to  the callout function is a pointer to a pcre_callout
2478           block. This structure contains the following fields:
2479    
2480             int          version;
2481             int          callout_number;
2482             int         *offset_vector;
2483             const char  *subject;
2484             int          subject_length;
2485             int          start_match;
2486             int          current_position;
2487             int          capture_top;
2488             int          capture_last;
2489             void        *callout_data;
2490             int          pattern_position;
2491             int          next_item_length;
2492    
2493           The version field is an integer containing the version  number  of  the
2494           block  format. The initial version was 0; the current version is 1. The
2495           version number will change again in future  if  additional  fields  are
2496           added, but the intention is never to remove any of the existing fields.
2497    
2498           The callout_number field contains the number of the  callout,  as  com-
2499           piled  into  the pattern (that is, the number after ?C for manual call-
2500           outs, and 255 for automatically generated callouts).
2501    
2502           The offset_vector field is a pointer to the vector of offsets that  was
2503           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2504           pcre_exec() is used, the contents can be inspected in order to  extract
2505           substrings  that  have  been  matched  so  far,  in the same way as for
2506           extracting substrings after a match has completed. For  pcre_dfa_exec()
2507           this field is not useful.
2508    
2509           The subject and subject_length fields contain copies of the values that
2510           were passed to pcre_exec().
2511    
2512           The start_match field normally contains the offset within  the  subject
2513           at  which  the  current  match  attempt started. However, if the escape
2514           sequence \K has been encountered, this value is changed to reflect  the
2515           modified  starting  point.  If the pattern is not anchored, the callout
2516           function may be called several times from the same point in the pattern
2517           for different starting points in the subject.
2518    
2519           The  current_position  field  contains the offset within the subject of
2520           the current match pointer.
2521    
2522           When the pcre_exec() function is used, the capture_top  field  contains
2523           one  more than the number of the highest numbered captured substring so
2524           far. If no substrings have been captured, the value of  capture_top  is
2525           one.  This  is always the case when pcre_dfa_exec() is used, because it
2526           does not support captured substrings.
2527    
2528           The capture_last field contains the number of the  most  recently  cap-
2529           tured  substring. If no substrings have been captured, its value is -1.
2530           This is always the case when pcre_dfa_exec() is used.
2531    
2532           The callout_data field contains a value that is passed  to  pcre_exec()
2533           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2534           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2535           structure.  If  no such data was passed, the value of callout_data in a
2536           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2537           structure in the pcreapi documentation.
2538    
2539           The  pattern_position field is present from version 1 of the pcre_call-
2540           out structure. It contains the offset to the next item to be matched in
2541           the pattern string.
2542    
2543           The  next_item_length field is present from version 1 of the pcre_call-
2544           out structure. It contains the length of the next item to be matched in
2545           the  pattern  string. When the callout immediately precedes an alterna-
2546           tion bar, a closing parenthesis, or the end of the pattern, the  length
2547           is  zero.  When the callout precedes an opening parenthesis, the length
2548           is that of the entire subpattern.
2549    
2550           The pattern_position and next_item_length fields are intended  to  help
2551           in  distinguishing between different automatic callouts, which all have
2552           the same callout number. However, they are set for all callouts.
2553    
2554    
2555    RETURN VALUES
2556    
2557           The external callout function returns an integer to PCRE. If the  value
2558           is  zero,  matching  proceeds  as  normal. If the value is greater than
2559           zero, matching fails at the current point, but  the  testing  of  other
2560           matching possibilities goes ahead, just as if a lookahead assertion had
2561           failed. If the value is less than zero, the  match  is  abandoned,  and
2562           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2563    
2564           Negative   values   should   normally   be   chosen  from  the  set  of
2565           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2566           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2567           reserved for use by callout functions; it will never be  used  by  PCRE
2568           itself.
2569    
 POSIX CHARACTER CLASSES  
      Perl 5.6 (not yet released at the time of writing) is  going  
      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  
      recogize  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.  
2570    
2571    AUTHOR
2572    
2573           Philip Hazel
2574           University Computing Service
2575           Cambridge CB2 3QH, England.
2576    
 VERTICAL BAR  
      Vertical bar characters are  used  to  separate  alternative  
      patterns. For example, the pattern  
2577    
2578         gilbert|sullivan  REVISION
2579    
2580       matches either "gilbert" or "sullivan". Any number of alter-         Last updated: 29 May 2007
2581       natives  may  appear,  and an empty alternative is permitted         Copyright (c) 1997-2007 University of Cambridge.
2582       (matching the empty string).   The  matching  process  tries  ------------------------------------------------------------------------------
      each  alternative in turn, from left to right, and the first  
      one that succeeds is used. If the alternatives are within  a  
      subpattern  (defined  below),  "succeeds" means matching the  
      rest of the main pattern as well as the alternative  in  the  
      subpattern.  
2583    
2584    
2585    PCRECOMPAT(3)                                                    PCRECOMPAT(3)
2586    
 INTERNAL OPTION SETTING  
      The settings of PCRE_CASELESS, PCRE_MULTILINE,  PCRE_DOTALL,  
      and  PCRE_EXTENDED can be changed from within the pattern by  
      a sequence of Perl option letters enclosed between "(?"  and  
      ")". The option letters are  
   
        i  for PCRE_CASELESS  
        m  for PCRE_MULTILINE  
        s  for PCRE_DOTALL  
        x  for PCRE_EXTENDED  
   
      For example, (?im) sets caseless, multiline matching. It  is  
      also 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.  
2587    
2588    NAME
2589           PCRE - Perl-compatible regular expressions
2590    
2591    
2592  SUBPATTERNS  DIFFERENCES BETWEEN PCRE AND PERL
      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.  
   
      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".  
2593    
2594           This  document describes the differences in the ways that PCRE and Perl
2595           handle regular expressions. The differences described here  are  mainly
2596           with  respect  to  Perl 5.8, though PCRE version 7.0 contains some fea-
2597           tures that are expected to be in the forthcoming Perl 5.10.
2598    
2599           1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2600           of  what  it does have are given in the section on UTF-8 support in the
2601           main pcre page.
2602    
2603           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2604           permits  them,  but they do not mean what you might think. For example,
2605           (?!a){3} does not assert that the next three characters are not "a". It
2606           just asserts that the next character is not "a" three times.
2607    
2608           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2609           tions are counted, but their entries in the offsets  vector  are  never
2610           set.  Perl sets its numerical variables from any such patterns that are
2611           matched before the assertion fails to match something (thereby succeed-
2612           ing),  but  only  if the negative lookahead assertion contains just one
2613           branch.
2614    
2615           4. Though binary zero characters are supported in the  subject  string,
2616           they are not allowed in a pattern string because it is passed as a nor-
2617           mal C string, terminated by zero. The escape sequence \0 can be used in
2618           the pattern to represent a binary zero.
2619    
2620           5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2621           \U, and \N. In fact these are implemented by Perl's general string-han-
2622           dling  and are not part of its pattern matching engine. If any of these
2623           are encountered by PCRE, an error is generated.
2624    
2625           6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2626           is  built  with Unicode character property support. The properties that
2627           can be tested with \p and \P are limited to the general category  prop-
2628           erties  such  as  Lu and Nd, script names such as Greek or Han, and the
2629           derived properties Any and L&.
2630    
2631           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2632           ters  in  between  are  treated as literals. This is slightly different
2633           from Perl in that $ and @ are  also  handled  as  literals  inside  the
2634           quotes.  In Perl, they cause variable interpolation (but of course PCRE
2635           does not have variables). Note the following examples:
2636    
2637               Pattern            PCRE matches      Perl matches
2638    
2639               \Qabc$xyz\E        abc$xyz           abc followed by the
2640                                                      contents of $xyz
2641               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2642               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2643    
2644           The \Q...\E sequence is recognized both inside  and  outside  character
2645           classes.
2646    
2647           8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2648           constructions. However, there is support for recursive  patterns.  This
2649           is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2650           "callout" feature allows an external function to be called during  pat-
2651           tern matching. See the pcrecallout documentation for details.
2652    
2653           9.  Subpatterns  that  are  called  recursively or as "subroutines" are
2654           always treated as atomic groups in  PCRE.  This  is  like  Python,  but
2655           unlike Perl.
2656    
2657           10.  There are some differences that are concerned with the settings of
2658           captured strings when part of  a  pattern  is  repeated.  For  example,
2659           matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
2660           unset, but in PCRE it is set to "b".
2661    
2662           11. PCRE provides some extensions to the Perl regular expression facil-
2663           ities.   Perl  5.10  will  include new features that are not in earlier
2664           versions, some of which (such as named parentheses) have been  in  PCRE
2665           for some time. This list is with respect to Perl 5.10:
2666    
2667           (a)  Although  lookbehind  assertions  must match fixed length strings,
2668           each alternative branch of a lookbehind assertion can match a different
2669           length of string. Perl requires them all to have the same length.
2670    
2671           (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2672           meta-character matches only at the very end of the string.
2673    
2674           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2675           cial  meaning  is  faulted.  Otherwise,  like  Perl,  the  backslash is
2676           ignored. (Perl can be made to issue a warning.)
2677    
2678           (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2679           fiers is inverted, that is, by default they are not greedy, but if fol-
2680           lowed by a question mark they are.
2681    
2682           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2683           tried only at the first matching position in the subject string.
2684    
2685           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2686           TURE options for pcre_exec() have no Perl equivalents.
2687    
2688