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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 123 by ph10, Mon Mar 12 15:19:06 2007 UTC
# Line 18  INTRODUCTION Line 18  INTRODUCTION
18    
19         The  PCRE  library is a set of functions that implement regular expres-         The  PCRE  library is a set of functions that implement regular expres-
20         sion pattern matching using the same syntax and semantics as Perl, with         sion pattern matching using the same syntax and semantics as Perl, with
21         just  a  few  differences.  The current implementation of PCRE (release         just  a  few differences. (Certain features that appeared in Python and
22         6.x) corresponds approximately with Perl  5.8,  including  support  for         PCRE before they appeared in Perl are also available using  the  Python
23         UTF-8 encoded strings and Unicode general category properties. However,         syntax.)
24         this support has to be explicitly enabled; it is not the default.  
25           The  current  implementation of PCRE (release 7.x) corresponds approxi-
26         In addition to the Perl-compatible matching function,  PCRE  also  con-         mately with Perl 5.10, including support for UTF-8 encoded strings  and
27         tains  an  alternative matching function that matches the same compiled         Unicode general category properties. However, UTF-8 and Unicode support
28         patterns in a different way. In certain circumstances, the  alternative         has to be explicitly enabled; it is not the default. The Unicode tables
29         function  has  some  advantages.  For  a discussion of the two matching         correspond to Unicode release 5.0.0.
30         algorithms, see the pcrematching page.  
31           In  addition to the Perl-compatible matching function, PCRE contains an
32         PCRE is written in C and released as a C library. A  number  of  people         alternative matching function that matches the same  compiled  patterns
33         have  written  wrappers and interfaces of various kinds. In particular,         in  a different way. In certain circumstances, the alternative function
34         Google Inc.  have provided a comprehensive C++  wrapper.  This  is  now         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         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         of  this  interface.  Other  people's contributions can be found in the
42         Contrib directory at the primary FTP site, which is:         Contrib directory at the primary FTP site, which is:
43    
44         ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre         ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45    
46         Details  of  exactly which Perl regular expression features are and are         Details of exactly which Perl regular expression features are  and  are
47         not supported by PCRE are given in separate documents. See the pcrepat-         not supported by PCRE are given in separate documents. See the pcrepat-
48         tern and pcrecompat pages.         tern and pcrecompat pages.
49    
50         Some  features  of  PCRE can be included, excluded, or changed when the         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         library  is  built.  The pcre_config() function makes it possible for a
52         client  to  discover  which  features are available. The features them-         client to discover which features are  available.  The  features  them-
53         selves are described in the pcrebuild page. Documentation about  build-         selves  are described in the pcrebuild page. Documentation about build-
54         ing  PCRE for various operating systems can be found in the README file         ing PCRE for various operating systems can be found in the README  file
55         in the source distribution.         in the source distribution.
56    
57         The library contains a number of undocumented  internal  functions  and         The  library  contains  a number of undocumented internal functions and
58         data  tables  that  are  used by more than one of the exported external         data tables that are used by more than one  of  the  exported  external
59         functions, but which are not intended  for  use  by  external  callers.         functions,  but  which  are  not  intended for use by external callers.
60         Their  names  all begin with "_pcre_", which hopefully will not provoke         Their names all begin with "_pcre_", which hopefully will  not  provoke
61         any name clashes. In some environments, it is possible to control which         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         external symbols are exported when a shared library is  built,  and  in
63         these cases the undocumented symbols are not exported.         these cases the undocumented symbols are not exported.
64    
65    
66  USER DOCUMENTATION  USER DOCUMENTATION
67    
68         The user documentation for PCRE comprises a number  of  different  sec-         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         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.         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         In the plain text format, all the sections are concatenated,  for  ease
72         of searching. The sections are as follows:         of searching. The sections are as follows:
73    
74           pcre              this document           pcre              this document
# Line 84  USER DOCUMENTATION Line 89  USER DOCUMENTATION
89           pcrestack         discussion of stack usage           pcrestack         discussion of stack usage
90           pcretest          description of the pcretest testing command           pcretest          description of the pcretest testing command
91    
92         In  addition,  in the "man" and HTML formats, there is a short page for         In addition, in the "man" and HTML formats, there is a short  page  for
93         each C library function, listing its arguments and results.         each C library function, listing its arguments and results.
94    
95    
96  LIMITATIONS  LIMITATIONS
97    
98         There are some size limitations in PCRE but it is hoped that they  will         There  are some size limitations in PCRE but it is hoped that they will
99         never in practice be relevant.         never in practice be relevant.
100    
101         The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE         The maximum length of a compiled pattern is 65539 (sic) bytes  if  PCRE
102         is compiled with the default internal linkage size of 2. If you want to         is compiled with the default internal linkage size of 2. If you want to
103         process  regular  expressions  that are truly enormous, you can compile         process regular expressions that are truly enormous,  you  can  compile
104         PCRE with an internal linkage size of 3 or 4 (see the  README  file  in         PCRE  with  an  internal linkage size of 3 or 4 (see the README file in
105         the  source  distribution and the pcrebuild documentation for details).         the source distribution and the pcrebuild documentation  for  details).
106         In these cases the limit is substantially larger.  However,  the  speed         In  these  cases the limit is substantially larger.  However, the speed
107         of execution will be slower.         of execution is slower.
108    
109         All  values in repeating quantifiers must be less than 65536. The maxi-         All values in repeating quantifiers must be less than 65536. The  maxi-
110         mum compiled length of subpattern with  an  explicit  repeat  count  is         mum  compiled  length  of  subpattern  with an explicit repeat count is
111         30000 bytes. The maximum number of capturing subpatterns is 65535.         30000 bytes. The maximum number of capturing subpatterns is 65535.
112    
113         There  is  no limit to the number of non-capturing subpatterns, but the         There is no limit to the number of parenthesized subpatterns, but there
114         maximum depth of nesting of  all  kinds  of  parenthesized  subpattern,         can be no more than 65535 capturing subpatterns.
        including capturing subpatterns, assertions, and other types of subpat-  
        tern, is 200.  
115    
116         The maximum length of name for a named subpattern is 32, and the  maxi-         The maximum length of name for a named subpattern is 32 characters, and
117         mum number of named subpatterns is 10000.         the maximum number of named subpatterns is 10000.
118    
119         The  maximum  length of a subject string is the largest positive number         The maximum length of a subject string is the largest  positive  number
120         that an integer variable can hold. However, when using the  traditional         that  an integer variable can hold. However, when using the traditional
121         matching function, PCRE uses recursion to handle subpatterns and indef-         matching function, PCRE uses recursion to handle subpatterns and indef-
122         inite repetition.  This means that the available stack space may  limit         inite  repetition.  This means that the available stack space may limit
123         the size of a subject string that can be processed by certain patterns.         the size of a subject string that can be processed by certain patterns.
124         For a discussion of stack issues, see the pcrestack documentation.         For a discussion of stack issues, see the pcrestack documentation.
125    
126    
127  UTF-8 AND UNICODE PROPERTY SUPPORT  UTF-8 AND UNICODE PROPERTY SUPPORT
128    
129         From release 3.3, PCRE has  had  some  support  for  character  strings         From  release  3.3,  PCRE  has  had  some support for character strings
130         encoded  in the UTF-8 format. For release 4.0 this was greatly extended         encoded in the UTF-8 format. For release 4.0 this was greatly  extended
131         to cover most common requirements, and in release 5.0  additional  sup-         to  cover  most common requirements, and in release 5.0 additional sup-
132         port for Unicode general category properties was added.         port for Unicode general category properties was added.
133    
134         In  order  process  UTF-8 strings, you must build PCRE to include UTF-8         In order process UTF-8 strings, you must build PCRE  to  include  UTF-8
135         support in the code, and, in addition,  you  must  call  pcre_compile()         support  in  the  code,  and, in addition, you must call pcre_compile()
136         with  the PCRE_UTF8 option flag. When you do this, both the pattern and         with the PCRE_UTF8 option flag. When you do this, both the pattern  and
137         any subject strings that are matched against it are  treated  as  UTF-8         any  subject  strings  that are matched against it are treated as UTF-8
138         strings instead of just strings of bytes.         strings instead of just strings of bytes.
139    
140         If  you compile PCRE with UTF-8 support, but do not use it at run time,         If you compile PCRE with UTF-8 support, but do not use it at run  time,
141         the library will be a bit bigger, but the additional run time  overhead         the  library will be a bit bigger, but the additional run time overhead
142         is  limited  to testing the PCRE_UTF8 flag in several places, so should         is limited to testing the PCRE_UTF8 flag occasionally, so should not be
143         not be very large.         very big.
144    
145         If PCRE is built with Unicode character property support (which implies         If PCRE is built with Unicode character property support (which implies
146         UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-         UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
147         ported.  The available properties that can be tested are limited to the         ported.  The available properties that can be tested are limited to the
148         general  category  properties such as Lu for an upper case letter or Nd         general category properties such as Lu for an upper case letter  or  Nd
149         for a decimal number, the Unicode script names such as Arabic  or  Han,         for  a  decimal number, the Unicode script names such as Arabic or Han,
150         and  the  derived  properties  Any  and L&. A full list is given in the         and the derived properties Any and L&. A full  list  is  given  in  the
151         pcrepattern documentation. Only the short names for properties are sup-         pcrepattern documentation. Only the short names for properties are sup-
152         ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-         ported. For example, \p{L} matches a letter. Its Perl synonym,  \p{Let-
153         ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may         ter},  is  not  supported.   Furthermore,  in Perl, many properties may
154         optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE         optionally be prefixed by "Is", for compatibility with Perl  5.6.  PCRE
155         does not support this.         does not support this.
156    
157         The following comments apply when PCRE is running in UTF-8 mode:         The following comments apply when PCRE is running in UTF-8 mode:
158    
159         1. When you set the PCRE_UTF8 flag, the strings passed as patterns  and         1.  When you set the PCRE_UTF8 flag, the strings passed as patterns and
160         subjects  are  checked for validity on entry to the relevant functions.         subjects are checked for validity on entry to the  relevant  functions.
161         If an invalid UTF-8 string is passed, an error return is given. In some         If an invalid UTF-8 string is passed, an error return is given. In some
162         situations,  you  may  already  know  that  your strings are valid, and         situations, you may already know  that  your  strings  are  valid,  and
163         therefore want to skip these checks in order to improve performance. If         therefore want to skip these checks in order to improve performance. If
164         you  set  the  PCRE_NO_UTF8_CHECK  flag at compile time or at run time,         you set the PCRE_NO_UTF8_CHECK flag at compile time  or  at  run  time,
165         PCRE assumes that the pattern or subject  it  is  given  (respectively)         PCRE  assumes  that  the  pattern or subject it is given (respectively)
166         contains  only valid UTF-8 codes. In this case, it does not diagnose an         contains only valid UTF-8 codes. In this case, it does not diagnose  an
167         invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE  when         invalid  UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
168         PCRE_NO_UTF8_CHECK  is set, the results are undefined. Your program may         PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  may
169         crash.         crash.
170    
171         2. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a         2.  An  unbraced  hexadecimal  escape sequence (such as \xb3) matches a
172         two-byte UTF-8 character if the value is greater than 127.         two-byte UTF-8 character if the value is greater than 127.
173    
174         3.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8         3. Octal numbers up to \777 are recognized, and  match  two-byte  UTF-8
175         characters for values greater than \177.         characters for values greater than \177.
176    
177         4. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-         4.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
178         vidual bytes, for example: \x{100}{3}.         vidual bytes, for example: \x{100}{3}.
179    
180         5.  The dot metacharacter matches one UTF-8 character instead of a sin-         5. The dot metacharacter matches one UTF-8 character instead of a  sin-
181         gle byte.         gle byte.
182    
183         6. The escape sequence \C can be used to match a single byte  in  UTF-8         6.  The  escape sequence \C can be used to match a single byte in UTF-8
184         mode,  but  its  use can lead to some strange effects. This facility is         mode, but its use can lead to some strange effects.  This  facility  is
185         not available in the alternative matching function, pcre_dfa_exec().         not available in the alternative matching function, pcre_dfa_exec().
186    
187         7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly         7.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
188         test  characters of any code value, but the characters that PCRE recog-         test characters of any code value, but the characters that PCRE  recog-
189         nizes as digits, spaces, or word characters  remain  the  same  set  as         nizes  as  digits,  spaces,  or  word characters remain the same set as
190         before, all with values less than 256. This remains true even when PCRE         before, all with values less than 256. This remains true even when PCRE
191         includes Unicode property support, because to do otherwise  would  slow         includes  Unicode  property support, because to do otherwise would slow
192         down  PCRE in many common cases. If you really want to test for a wider         down PCRE in many common cases. If you really want to test for a  wider
193         sense of, say, "digit", you must use Unicode  property  tests  such  as         sense  of,  say,  "digit",  you must use Unicode property tests such as
194         \p{Nd}.         \p{Nd}.
195    
196         8.  Similarly,  characters that match the POSIX named character classes         8. Similarly, characters that match the POSIX named  character  classes
197         are all low-valued characters.         are all low-valued characters.
198    
199         9. Case-insensitive matching applies only to  characters  whose  values         9.  Case-insensitive  matching  applies only to characters whose values
200         are  less than 128, unless PCRE is built with Unicode property support.         are less than 128, unless PCRE is built with Unicode property  support.
201         Even when Unicode property support is available, PCRE  still  uses  its         Even  when  Unicode  property support is available, PCRE still uses its
202         own  character  tables when checking the case of low-valued characters,         own character tables when checking the case of  low-valued  characters,
203         so as not to degrade performance.  The Unicode property information  is         so  as not to degrade performance.  The Unicode property information is
204         used only for characters with higher values. Even when Unicode property         used only for characters with higher values. Even when Unicode property
205         support is available, PCRE supports case-insensitive matching only when         support is available, PCRE supports case-insensitive matching only when
206         there  is  a  one-to-one  mapping between a letter's cases. There are a         there is a one-to-one mapping between a letter's  cases.  There  are  a
207         small number of many-to-one mappings in Unicode;  these  are  not  sup-         small  number  of  many-to-one  mappings in Unicode; these are not sup-
208         ported by PCRE.         ported by PCRE.
209    
210    
211  AUTHOR  AUTHOR
212    
213         Philip Hazel         Philip Hazel
214         University Computing Service,         University Computing Service
215         Cambridge CB2 3QG, England.         Cambridge CB2 3QH, England.
216    
217         Putting  an actual email address here seems to have been a spam magnet,         Putting an actual email address here seems to have been a spam  magnet,
218         so I've taken it away. If you want to email me, use my initial and sur-         so I've taken it away. If you want to email me, use my initial and sur-
219         name, separated by a dot, at the domain ucs.cam.ac.uk.         name, separated by a dot, at the domain ucs.cam.ac.uk.
220    
221  Last updated: 05 June 2006  
222  Copyright (c) 1997-2006 University of Cambridge.  REVISION
223    
224           Last updated: 06 March 2007
225           Copyright (c) 1997-2007 University of Cambridge.
226  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
227    
228    
# Line 305  CODE VALUE OF NEWLINE Line 311  CODE VALUE OF NEWLINE
311    
312           --enable-newline-is-crlf           --enable-newline-is-crlf
313    
314         to  the  configure command. Whatever line ending convention is selected         to the configure command. There is a fourth option, specified by
315         when PCRE is built can be overridden when  the  library  functions  are  
316         called.  At  build time it is conventional to use the standard for your           --enable-newline-is-any
317         operating system.  
318           which causes PCRE to recognize any Unicode newline sequence.
319    
320           Whatever  line  ending convention is selected when PCRE is built can be
321           overridden when the library functions are called. At build time  it  is
322           conventional to use the standard for your operating system.
323    
324    
325  BUILDING SHARED AND STATIC LIBRARIES  BUILDING SHARED AND STATIC LIBRARIES
326    
327         The PCRE building process uses libtool to build both shared and  static         The  PCRE building process uses libtool to build both shared and static
328         Unix  libraries by default. You can suppress one of these by adding one         Unix libraries by default. You can suppress one of these by adding  one
329         of         of
330    
331           --disable-shared           --disable-shared
# Line 326  BUILDING SHARED AND STATIC LIBRARIES Line 337  BUILDING SHARED AND STATIC LIBRARIES
337  POSIX MALLOC USAGE  POSIX MALLOC USAGE
338    
339         When PCRE is called through the POSIX interface (see the pcreposix doc-         When PCRE is called through the POSIX interface (see the pcreposix doc-
340         umentation),  additional  working  storage  is required for holding the         umentation), additional working storage is  required  for  holding  the
341         pointers to capturing substrings, because PCRE requires three  integers         pointers  to capturing substrings, because PCRE requires three integers
342         per  substring,  whereas  the POSIX interface provides only two. If the         per substring, whereas the POSIX interface provides only  two.  If  the
343         number of expected substrings is small, the wrapper function uses space         number of expected substrings is small, the wrapper function uses space
344         on the stack, because this is faster than using malloc() for each call.         on the stack, because this is faster than using malloc() for each call.
345         The default threshold above which the stack is no longer used is 10; it         The default threshold above which the stack is no longer used is 10; it
# Line 341  POSIX MALLOC USAGE Line 352  POSIX MALLOC USAGE
352    
353  HANDLING VERY LARGE PATTERNS  HANDLING VERY LARGE PATTERNS
354    
355         Within  a  compiled  pattern,  offset values are used to point from one         Within a compiled pattern, offset values are used  to  point  from  one
356         part to another (for example, from an opening parenthesis to an  alter-         part  to another (for example, from an opening parenthesis to an alter-
357         nation  metacharacter).  By default, two-byte values are used for these         nation metacharacter). By default, two-byte values are used  for  these
358         offsets, leading to a maximum size for a  compiled  pattern  of  around         offsets,  leading  to  a  maximum size for a compiled pattern of around
359         64K.  This  is sufficient to handle all but the most gigantic patterns.         64K. This is sufficient to handle all but the most  gigantic  patterns.
360         Nevertheless, some people do want to process enormous patterns,  so  it         Nevertheless,  some  people do want to process enormous patterns, so it
361         is  possible  to compile PCRE to use three-byte or four-byte offsets by         is possible to compile PCRE to use three-byte or four-byte  offsets  by
362         adding a setting such as         adding a setting such as
363    
364           --with-link-size=3           --with-link-size=3
365    
366         to the configure command. The value given must be 2,  3,  or  4.  Using         to  the  configure  command.  The value given must be 2, 3, or 4. Using
367         longer  offsets slows down the operation of PCRE because it has to load         longer offsets slows down the operation of PCRE because it has to  load
368         additional bytes when handling them.         additional bytes when handling them.
369    
370         If you build PCRE with an increased link size, test 2 (and  test  5  if         If  you  build  PCRE with an increased link size, test 2 (and test 5 if
371         you  are using UTF-8) will fail. Part of the output of these tests is a         you are using UTF-8) will fail. Part of the output of these tests is  a
372         representation of the compiled pattern, and this changes with the  link         representation  of the compiled pattern, and this changes with the link
373         size.         size.
374    
375    
376  AVOIDING EXCESSIVE STACK USAGE  AVOIDING EXCESSIVE STACK USAGE
377    
378         When matching with the pcre_exec() function, PCRE implements backtrack-         When matching with the pcre_exec() function, PCRE implements backtrack-
379         ing by making recursive calls to an internal function  called  match().         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-         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         verely  limit  PCRE's operation. (The Unix environment does not usually
382         suffer from this problem, but it may sometimes be necessary to increase         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-         the  maximum  stack size.  There is a discussion in the pcrestack docu-
384         mentation.)  An alternative approach to recursion that uses memory from         mentation.) An alternative approach to recursion that uses memory  from
385         the heap to remember data, instead of using recursive  function  calls,         the  heap  to remember data, instead of using recursive function calls,
386         has  been  implemented to work round the problem of limited stack size.         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         If you want to build a version of PCRE that works this way, add
388    
389           --disable-stack-for-recursion           --disable-stack-for-recursion
390    
391         to the configure command. With this configuration, PCRE  will  use  the         to  the  configure  command. With this configuration, PCRE will use the
392         pcre_stack_malloc  and pcre_stack_free variables to call memory manage-         pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
393         ment functions. Separate functions are provided because  the  usage  is         ment  functions.  Separate  functions are provided because the usage is
394         very  predictable:  the  block sizes requested are always the same, and         very predictable: the block sizes requested are always  the  same,  and
395         the blocks are always freed in reverse order. A calling  program  might         the  blocks  are always freed in reverse order. A calling program might
396         be  able  to implement optimized functions that perform better than the         be able to implement optimized functions that perform better  than  the
397         standard malloc() and  free()  functions.  PCRE  runs  noticeably  more         standard  malloc()  and  free()  functions.  PCRE  runs noticeably more
398         slowly when built in this way. This option affects only the pcre_exec()         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.         function; it is not relevant for the the pcre_dfa_exec() function.
400    
401    
402  LIMITING PCRE RESOURCE USAGE  LIMITING PCRE RESOURCE USAGE
403    
404         Internally, PCRE has a function called match(), which it calls  repeat-         Internally,  PCRE has a function called match(), which it calls repeat-
405         edly   (sometimes   recursively)  when  matching  a  pattern  with  the         edly  (sometimes  recursively)  when  matching  a  pattern   with   the
406         pcre_exec() function. By controlling the maximum number of  times  this         pcre_exec()  function.  By controlling the maximum number of times this
407         function  may be called during a single matching operation, a limit can         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         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-         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         tation.  The default is 10 million, but this can be changed by adding a
411         setting such as         setting such as
412    
413           --with-match-limit=500000           --with-match-limit=500000
414    
415         to   the   configure  command.  This  setting  has  no  effect  on  the         to  the  configure  command.  This  setting  has  no  effect   on   the
416         pcre_dfa_exec() matching function.         pcre_dfa_exec() matching function.
417    
418         In some environments it is desirable to limit the  depth  of  recursive         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         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-         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;         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         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         imposes no additional constraints. However, you can set a  lower  limit
424         by adding, for example,         by adding, for example,
425    
426           --with-match-limit-recursion=10000           --with-match-limit-recursion=10000
427    
428         to the configure command. This value can  also  be  overridden  at  run         to  the  configure  command.  This  value can also be overridden at run
429         time.         time.
430    
431    
432  USING EBCDIC CODE  USING EBCDIC CODE
433    
434         PCRE  assumes  by  default that it will run in an environment where the         PCRE assumes by default that it will run in an  environment  where  the
435         character code is ASCII (or Unicode, which is  a  superset  of  ASCII).         character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
436         PCRE  can,  however,  be  compiled  to  run in an EBCDIC environment by         PCRE can, however, be compiled to  run  in  an  EBCDIC  environment  by
437         adding         adding
438    
439           --enable-ebcdic           --enable-ebcdic
440    
441         to the configure command.         to the configure command.
442    
443  Last updated: 06 June 2006  
444  Copyright (c) 1997-2006 University of Cambridge.  SEE ALSO
445    
446           pcreapi(3), pcre_config(3).
447    
448    
449    AUTHOR
450    
451           Philip Hazel
452           University Computing Service
453           Cambridge CB2 3QH, England.
454    
455    
456    REVISION
457    
458           Last updated: 06 March 2007
459           Copyright (c) 1997-2007 University of Cambridge.
460  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
461    
462    
# Line 466  PCRE MATCHING ALGORITHMS Line 492  PCRE MATCHING ALGORITHMS
492           <something> <something else> <something further>           <something> <something else> <something further>
493    
494         there are three possible answers. The standard algorithm finds only one         there are three possible answers. The standard algorithm finds only one
495         of them, whereas the DFA algorithm finds all three.         of them, whereas the alternative algorithm finds all three.
496    
497    
498  REGULAR EXPRESSIONS AS TREES  REGULAR EXPRESSIONS AS TREES
# Line 507  THE STANDARD MATCHING ALGORITHM Line 533  THE STANDARD MATCHING ALGORITHM
533         This provides support for capturing parentheses and back references.         This provides support for capturing parentheses and back references.
534    
535    
536  THE DFA MATCHING ALGORITHM  THE ALTERNATIVE MATCHING ALGORITHM
537    
538         DFA stands for "deterministic finite automaton", but you do not need to         This algorithm conducts a breadth-first search of  the  tree.  Starting
539         understand the origins of that name. This algorithm conducts a breadth-         from  the  first  matching  point  in the subject, it scans the subject
540         first search of the tree. Starting from the first matching point in the         string from left to right, once, character by character, and as it does
541         subject,  it scans the subject string from left to right, once, charac-         this,  it remembers all the paths through the tree that represent valid
542         ter by character, and as it does  this,  it  remembers  all  the  paths         matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
543         through the tree that represent valid matches.         though  it is not implemented as a traditional finite state machine (it
544           keeps multiple states active simultaneously).
545         The  scan  continues until either the end of the subject is reached, or  
546         there are no more unterminated paths. At this point,  terminated  paths         The scan continues until either the end of the subject is  reached,  or
547         represent  the different matching possibilities (if there are none, the         there  are  no more unterminated paths. At this point, terminated paths
548         match has failed).  Thus, if there is more  than  one  possible  match,         represent the different matching possibilities (if there are none,  the
549           match  has  failed).   Thus,  if there is more than one possible match,
550         this algorithm finds all of them, and in particular, it finds the long-         this algorithm finds all of them, and in particular, it finds the long-
551         est. In PCRE, there is an option to stop the algorithm after the  first         est.  In PCRE, there is an option to stop the algorithm after the first
552         match (which is necessarily the shortest) has been found.         match (which is necessarily the shortest) has been found.
553    
554         Note that all the matches that are found start at the same point in the         Note that all the matches that are found start at the same point in the
# Line 529  THE DFA MATCHING ALGORITHM Line 556  THE DFA MATCHING ALGORITHM
556    
557           cat(er(pillar)?)           cat(er(pillar)?)
558    
559         is matched against the string "the caterpillar catchment",  the  result         is  matched  against the string "the caterpillar catchment", the result
560         will  be the three strings "cat", "cater", and "caterpillar" that start         will be the three strings "cat", "cater", and "caterpillar" that  start
561         at the fourth character of the subject. The algorithm does not automat-         at the fourth character of the subject. The algorithm does not automat-
562         ically move on to find matches that start at later positions.         ically move on to find matches that start at later positions.
563    
564         There are a number of features of PCRE regular expressions that are not         There are a number of features of PCRE regular expressions that are not
565         supported by the DFA matching algorithm. They are as follows:         supported by the alternative matching algorithm. They are as follows:
566    
567         1. Because the algorithm finds all  possible  matches,  the  greedy  or         1.  Because  the  algorithm  finds  all possible matches, the greedy or
568         ungreedy  nature  of repetition quantifiers is not relevant. Greedy and         ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
569         ungreedy quantifiers are treated in exactly the same way.         ungreedy quantifiers are treated in exactly the same way. However, pos-
570           sessive quantifiers can make a difference when what follows could  also
571           match what is quantified, for example in a pattern like this:
572    
573             ^a++\w!
574    
575           This  pattern matches "aaab!" but not "aaa!", which would be matched by
576           a non-possessive quantifier. Similarly, if an atomic group is  present,
577           it  is matched as if it were a standalone pattern at the current point,
578           and the longest match is then "locked in" for the rest of  the  overall
579           pattern.
580    
581         2. When dealing with multiple paths through the tree simultaneously, it         2. When dealing with multiple paths through the tree simultaneously, it
582         is  not  straightforward  to  keep track of captured substrings for the         is not straightforward to keep track of  captured  substrings  for  the
583         different matching possibilities, and  PCRE's  implementation  of  this         different  matching  possibilities,  and  PCRE's implementation of this
584         algorithm does not attempt to do this. This means that no captured sub-         algorithm does not attempt to do this. This means that no captured sub-
585         strings are available.         strings are available.
586    
587         3. Because no substrings are captured, back references within the  pat-         3.  Because no substrings are captured, back references within the pat-
588         tern are not supported, and cause errors if encountered.         tern are not supported, and cause errors if encountered.
589    
590         4.  For  the same reason, conditional expressions that use a backrefer-         4. For the same reason, conditional expressions that use  a  backrefer-
591         ence as the condition are not supported.         ence  as  the  condition or test for a specific group recursion are not
592           supported.
593    
594         5. Callouts are supported, but the value of the  capture_top  field  is         5. Callouts are supported, but the value of the  capture_top  field  is
595         always 1, and the value of the capture_last field is always -1.         always 1, and the value of the capture_last field is always -1.
596    
597         6.  The \C escape sequence, which (in the standard algorithm) matches a         6.  The \C escape sequence, which (in the standard algorithm) matches a
598         single byte, even in UTF-8 mode, is not supported because the DFA algo-         single byte, even in UTF-8 mode, is not supported because the  alterna-
599         rithm moves through the subject string one character at a time, for all         tive  algorithm  moves  through  the  subject string one character at a
600         active paths through the tree.         time, for all active paths through the tree.
601    
602    
603  ADVANTAGES OF THE DFA ALGORITHM  ADVANTAGES OF THE ALTERNATIVE ALGORITHM
604    
605         Using the DFA matching algorithm provides the following advantages:         Using the alternative matching algorithm provides the following  advan-
606           tages:
607    
608         1. All possible matches (at a single point in the subject) are automat-         1. All possible matches (at a single point in the subject) are automat-
609         ically  found,  and  in particular, the longest match is found. To find         ically found, and in particular, the longest match is  found.  To  find
610         more than one match using the standard algorithm, you have to do kludgy         more than one match using the standard algorithm, you have to do kludgy
611         things with callouts.         things with callouts.
612    
613         2.  There is much better support for partial matching. The restrictions         2. There is much better support for partial matching. The  restrictions
614         on the content of the pattern that apply when using the standard  algo-         on  the content of the pattern that apply when using the standard algo-
615         rithm  for partial matching do not apply to the DFA algorithm. For non-         rithm for partial matching do not apply to the  alternative  algorithm.
616         anchored patterns, the starting position of a partial match  is  avail-         For  non-anchored patterns, the starting position of a partial match is
617         able.         available.
618    
619         3.  Because  the  DFA algorithm scans the subject string just once, and         3. Because the alternative algorithm  scans  the  subject  string  just
620         never needs to backtrack, it is possible  to  pass  very  long  subject         once,  and  never  needs to backtrack, it is possible to pass very long
621         strings  to  the matching function in several pieces, checking for par-         subject strings to the matching function in  several  pieces,  checking
622         tial matching each time.         for partial matching each time.
623    
624    
625  DISADVANTAGES OF THE DFA ALGORITHM  DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
626    
627         The DFA algorithm suffers from a number of disadvantages:         The alternative algorithm suffers from a number of disadvantages:
628    
629         1. It is substantially slower than  the  standard  algorithm.  This  is         1.  It  is  substantially  slower  than the standard algorithm. This is
630         partly  because  it has to search for all possible matches, but is also         partly because it has to search for all possible matches, but  is  also
631         because it is less susceptible to optimization.         because it is less susceptible to optimization.
632    
633         2. Capturing parentheses and back references are not supported.         2. Capturing parentheses and back references are not supported.
634    
635         3. The "atomic group" feature of PCRE regular expressions is supported,         3. Although atomic groups are supported, their use does not provide the
636         but  does not provide the advantage that it does for the standard algo-         performance advantage that it does for the standard algorithm.
637         rithm.  
638    
639    AUTHOR
640    
641  Last updated: 06 June 2006         Philip Hazel
642  Copyright (c) 1997-2006 University of Cambridge.         University Computing Service
643           Cambridge CB2 3QH, England.
644    
645    
646    REVISION
647    
648           Last updated: 06 March 2007
649           Copyright (c) 1997-2007 University of Cambridge.
650  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
651    
652    
# Line 692  PCRE NATIVE API Line 740  PCRE NATIVE API
740  PCRE API OVERVIEW  PCRE API OVERVIEW
741    
742         PCRE has its own native API, which is described in this document. There         PCRE has its own native API, which is described in this document. There
743         is also a set of wrapper functions that correspond to the POSIX regular         are also some wrapper functions that correspond to  the  POSIX  regular
744         expression  API.  These  are  described in the pcreposix documentation.         expression  API.  These  are  described in the pcreposix documentation.
745         Both of these APIs define a set of C function calls. A C++  wrapper  is         Both of these APIs define a set of C function calls. A C++  wrapper  is
746         distributed with PCRE. It is documented in the pcrecpp page.         distributed with PCRE. It is documented in the pcrecpp page.
# Line 715  PCRE API OVERVIEW Line 763  PCRE API OVERVIEW
763         A second matching function, pcre_dfa_exec(), which is not Perl-compati-         A second matching function, pcre_dfa_exec(), which is not Perl-compati-
764         ble,  is  also provided. This uses a different algorithm for the match-         ble,  is  also provided. This uses a different algorithm for the match-
765         ing. The alternative algorithm finds all possible matches (at  a  given         ing. The alternative algorithm finds all possible matches (at  a  given
766         point in the subject). However, this algorithm does not return captured         point  in  the subject), and scans the subject just once. However, this
767         substrings. A description of the  two  matching  algorithms  and  their         algorithm does not return captured substrings. A description of the two
768         advantages  and  disadvantages  is given in the pcrematching documenta-         matching  algorithms and their advantages and disadvantages is given in
769         tion.         the pcrematching documentation.
770    
771         In addition to the main compiling and  matching  functions,  there  are         In addition to the main compiling and  matching  functions,  there  are
772         convenience functions for extracting captured substrings from a subject         convenience functions for extracting captured substrings from a subject
# Line 779  PCRE API OVERVIEW Line 827  PCRE API OVERVIEW
827    
828    
829  NEWLINES  NEWLINES
830         PCRE supports three different conventions for indicating line breaks in  
831         strings: a single CR character, a single LF character, or the two-char-         PCRE  supports four different conventions for indicating line breaks in
832         acter  sequence  CRLF.  All  three  are used as "standard" by different         strings: a single CR (carriage return) character, a  single  LF  (line-
833         operating systems.  When PCRE is built, a default can be specified. The         feed)  character,  the two-character sequence CRLF, or any Unicode new-
834         default  default  is  LF, which is the Unix standard. When PCRE is run,         line sequence.  The Unicode newline sequences are the three  just  men-
835         the default can be overridden, either when a pattern  is  compiled,  or         tioned, plus the single characters VT (vertical tab, U+000B), FF (form-
836         when it is matched.         feed, U+000C), NEL (next line, U+0085), LS  (line  separator,  U+2028),
837           and PS (paragraph separator, U+2029).
838    
839           Each  of  the first three conventions is used by at least one operating
840           system as its standard newline sequence. When PCRE is built, a  default
841           can  be  specified.  The default default is LF, which is the Unix stan-
842           dard. When PCRE is run, the default can be overridden,  either  when  a
843           pattern is compiled, or when it is matched.
844    
845         In the PCRE documentation the word "newline" is used to mean "the char-         In the PCRE documentation the word "newline" is used to mean "the char-
846         acter or pair of characters that indicate a line break".         acter or pair of characters that indicate a line break". The choice  of
847           newline  convention  affects  the  handling of the dot, circumflex, and
848           dollar metacharacters, the handling of #-comments in /x mode, and, when
849           CRLF  is a recognized line ending sequence, the match position advance-
850           ment for a non-anchored pattern. The choice of newline convention  does
851           not affect the interpretation of the \n or \r escape sequences.
852    
853    
854  MULTITHREADING  MULTITHREADING
855    
856         The PCRE functions can be used in  multi-threading  applications,  with         The  PCRE  functions  can be used in multi-threading applications, with
857         the  proviso  that  the  memory  management  functions  pointed  to  by         the  proviso  that  the  memory  management  functions  pointed  to  by
858         pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the         pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
859         callout function pointed to by pcre_callout, are shared by all threads.         callout function pointed to by pcre_callout, are shared by all threads.
860    
861         The compiled form of a regular expression is not altered during  match-         The  compiled form of a regular expression is not altered during match-
862         ing, so the same compiled pattern can safely be used by several threads         ing, so the same compiled pattern can safely be used by several threads
863         at once.         at once.
864    
# Line 806  MULTITHREADING Line 866  MULTITHREADING
866  SAVING PRECOMPILED PATTERNS FOR LATER USE  SAVING PRECOMPILED PATTERNS FOR LATER USE
867    
868         The compiled form of a regular expression can be saved and re-used at a         The compiled form of a regular expression can be saved and re-used at a
869         later  time,  possibly by a different program, and even on a host other         later time, possibly by a different program, and even on a  host  other
870         than the one on which  it  was  compiled.  Details  are  given  in  the         than  the  one  on  which  it  was  compiled.  Details are given in the
871         pcreprecompile documentation.         pcreprecompile documentation.
872    
873    
# Line 815  CHECKING BUILD-TIME OPTIONS Line 875  CHECKING BUILD-TIME OPTIONS
875    
876         int pcre_config(int what, void *where);         int pcre_config(int what, void *where);
877    
878         The  function pcre_config() makes it possible for a PCRE client to dis-         The function pcre_config() makes it possible for a PCRE client to  dis-
879         cover which optional features have been compiled into the PCRE library.         cover which optional features have been compiled into the PCRE library.
880         The  pcrebuild documentation has more details about these optional fea-         The pcrebuild documentation has more details about these optional  fea-
881         tures.         tures.
882    
883         The first argument for pcre_config() is an  integer,  specifying  which         The  first  argument  for pcre_config() is an integer, specifying which
884         information is required; the second argument is a pointer to a variable         information is required; the second argument is a pointer to a variable
885         into which the information is  placed.  The  following  information  is         into  which  the  information  is  placed. The following information is
886         available:         available:
887    
888           PCRE_CONFIG_UTF8           PCRE_CONFIG_UTF8
889    
890         The  output is an integer that is set to one if UTF-8 support is avail-         The output is an integer that is set to one if UTF-8 support is  avail-
891         able; otherwise it is set to zero.         able; otherwise it is set to zero.
892    
893           PCRE_CONFIG_UNICODE_PROPERTIES           PCRE_CONFIG_UNICODE_PROPERTIES
894    
895         The output is an integer that is set to  one  if  support  for  Unicode         The  output  is  an  integer  that is set to one if support for Unicode
896         character properties is available; otherwise it is set to zero.         character properties is available; otherwise it is set to zero.
897    
898           PCRE_CONFIG_NEWLINE           PCRE_CONFIG_NEWLINE
899    
900         The  output  is  an integer whose value specifies the default character         The output is an integer whose value specifies  the  default  character
901         sequence that is recognized as meaning "newline". The three values that         sequence  that is recognized as meaning "newline". The four values that
902         are supported are: 10 for LF, 13 for CR, and 3338 for CRLF. The default         are supported are: 10 for LF, 13 for CR, 3338 for CRLF, and -1 for ANY.
903         should normally be the standard sequence for your operating system.         The default should normally be the standard sequence for your operating
904           system.
905    
906           PCRE_CONFIG_LINK_SIZE           PCRE_CONFIG_LINK_SIZE
907    
# Line 909  COMPILING A PATTERN Line 970  COMPILING A PATTERN
970         fully relocatable, because it may contain a copy of the tableptr  argu-         fully relocatable, because it may contain a copy of the tableptr  argu-
971         ment, which is an address (see below).         ment, which is an address (see below).
972    
973         The options argument contains independent bits that affect the compila-         The options argument contains various bit settings that affect the com-
974         tion. It should be zero if  no  options  are  required.  The  available         pilation. It should be zero if no options are required.  The  available
975         options  are  described  below. Some of them, in particular, those that         options  are  described  below. Some of them, in particular, those that
976         are compatible with Perl, can also be set and  unset  from  within  the         are compatible with Perl, can also be set and  unset  from  within  the
977         pattern  (see  the  detailed  description in the pcrepattern documenta-         pattern  (see  the  detailed  description in the pcrepattern documenta-
# Line 1000  COMPILING A PATTERN Line 1061  COMPILING A PATTERN
1061         not  match  when  the  current position is at a newline. This option is         not  match  when  the  current position is at a newline. This option is
1062         equivalent to Perl's /s option, and it can be changed within a  pattern         equivalent to Perl's /s option, and it can be changed within a  pattern
1063         by  a (?s) option setting. A negative class such as [^a] always matches         by  a (?s) option setting. A negative class such as [^a] always matches
1064         newlines, independent of the setting of this option.         newline characters, independent of the setting of this option.
1065    
1066           PCRE_DUPNAMES           PCRE_DUPNAMES
1067    
# Line 1064  COMPILING A PATTERN Line 1125  COMPILING A PATTERN
1125           PCRE_NEWLINE_CR           PCRE_NEWLINE_CR
1126           PCRE_NEWLINE_LF           PCRE_NEWLINE_LF
1127           PCRE_NEWLINE_CRLF           PCRE_NEWLINE_CRLF
1128             PCRE_NEWLINE_ANY
1129    
1130         These  options  override the default newline definition that was chosen         These  options  override the default newline definition that was chosen
1131         when PCRE was built. Setting the first or the second specifies  that  a         when PCRE was built. Setting the first or the second specifies  that  a
1132         newline  is  indicated  by a single character (CR or LF, respectively).         newline  is  indicated  by a single character (CR or LF, respectively).
1133         Setting both of them specifies that a newline is indicated by the  two-         Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
1134         character  CRLF sequence. For convenience, PCRE_NEWLINE_CRLF is defined         two-character  CRLF  sequence.  Setting PCRE_NEWLINE_ANY specifies that
1135         to contain both bits. The only time that a line break is relevant  when         any Unicode newline sequence should be recognized. The Unicode  newline
1136         compiling a pattern is if PCRE_EXTENDED is set, and an unescaped # out-         sequences  are  the three just mentioned, plus the single characters VT
1137         side a character class is encountered. This indicates  a  comment  that         (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085),
1138         lasts until after the next newline.         LS  (line separator, U+2028), and PS (paragraph separator, U+2029). The
1139           last two are recognized only in UTF-8 mode.
1140    
1141           The newline setting in the  options  word  uses  three  bits  that  are
1142           treated  as  a  number, giving eight possibilities. Currently only five
1143           are used (default plus the four values above). This means that  if  you
1144           set  more  than  one  newline option, the combination may or may not be
1145           sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is  equiva-
1146           lent  to PCRE_NEWLINE_CRLF, but other combinations yield unused numbers
1147           and cause an error.
1148    
1149           The only time that a line break is specially recognized when  compiling
1150           a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
1151           character class is encountered. This indicates  a  comment  that  lasts
1152           until  after the next line break sequence. In other circumstances, line
1153           break  sequences  are  treated  as  literal  data,   except   that   in
1154           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1155           and are therefore ignored.
1156    
1157         The newline option set at compile time becomes the default that is used         The newline option that is set at compile time becomes the default that
1158         for pcre_exec() and pcre_dfa_exec(), but it can be overridden.         is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1159    
1160           PCRE_NO_AUTO_CAPTURE           PCRE_NO_AUTO_CAPTURE
1161    
# Line 1119  COMPILATION ERROR CODES Line 1198  COMPILATION ERROR CODES
1198    
1199         The following table lists the error  codes  than  may  be  returned  by         The following table lists the error  codes  than  may  be  returned  by
1200         pcre_compile2(),  along with the error messages that may be returned by         pcre_compile2(),  along with the error messages that may be returned by
1201         both compiling functions.         both compiling functions. As PCRE has developed, some error codes  have
1202           fallen out of use. To avoid confusion, they have not been re-used.
1203    
1204            0  no error            0  no error
1205            1  \ at end of pattern            1  \ at end of pattern
# Line 1131  COMPILATION ERROR CODES Line 1211  COMPILATION ERROR CODES
1211            7  invalid escape sequence in character class            7  invalid escape sequence in character class
1212            8  range out of order in character class            8  range out of order in character class
1213            9  nothing to repeat            9  nothing to repeat
1214           10  operand of unlimited repeat could match the empty string           10  [this code is not in use]
1215           11  internal error: unexpected repeat           11  internal error: unexpected repeat
1216           12  unrecognized character after (?           12  unrecognized character after (?
1217           13  POSIX named classes are supported only within a class           13  POSIX named classes are supported only within a class
# Line 1140  COMPILATION ERROR CODES Line 1220  COMPILATION ERROR CODES
1220           16  erroffset passed as NULL           16  erroffset passed as NULL
1221           17  unknown option bit(s) set           17  unknown option bit(s) set
1222           18  missing ) after comment           18  missing ) after comment
1223           19  parentheses nested too deeply           19  [this code is not in use]
1224           20  regular expression too large           20  regular expression too large
1225           21  failed to get memory           21  failed to get memory
1226           22  unmatched parentheses           22  unmatched parentheses
# Line 1154  COMPILATION ERROR CODES Line 1234  COMPILATION ERROR CODES
1234           30  unknown POSIX class name           30  unknown POSIX class name
1235           31  POSIX collating elements are not supported           31  POSIX collating elements are not supported
1236           32  this version of PCRE is not compiled with PCRE_UTF8 support           32  this version of PCRE is not compiled with PCRE_UTF8 support
1237           33  spare error           33  [this code is not in use]
1238           34  character value in \x{...} sequence is too large           34  character value in \x{...} sequence is too large
1239           35  invalid condition (?(0)           35  invalid condition (?(0)
1240           36  \C not allowed in lookbehind assertion           36  \C not allowed in lookbehind assertion
# Line 1163  COMPILATION ERROR CODES Line 1243  COMPILATION ERROR CODES
1243           39  closing ) for (?C expected           39  closing ) for (?C expected
1244           40  recursive call could loop indefinitely           40  recursive call could loop indefinitely
1245           41  unrecognized character after (?P           41  unrecognized character after (?P
1246           42  syntax error after (?P           42  syntax error in subpattern name (missing terminator)
1247           43  two named subpatterns have the same name           43  two named subpatterns have the same name
1248           44  invalid UTF-8 string           44  invalid UTF-8 string
1249           45  support for \P, \p, and \X has not been compiled           45  support for \P, \p, and \X has not been compiled
# Line 1173  COMPILATION ERROR CODES Line 1253  COMPILATION ERROR CODES
1253           49  too many named subpatterns (maximum 10,000)           49  too many named subpatterns (maximum 10,000)
1254           50  repeated subpattern is too long           50  repeated subpattern is too long
1255           51  octal value is greater than \377 (not in UTF-8 mode)           51  octal value is greater than \377 (not in UTF-8 mode)
1256             52  internal error: overran compiling workspace
1257             53   internal  error:  previously-checked  referenced  subpattern not
1258           found
1259             54  DEFINE group contains more than one branch
1260             55  repeating a DEFINE group is not allowed
1261             56  inconsistent NEWLINE options"
1262    
1263    
1264  STUDYING A PATTERN  STUDYING A PATTERN
# Line 1331  INFORMATION ABOUT A PATTERN Line 1417  INFORMATION ABOUT A PATTERN
1417         is still recognized for backwards compatibility.)         is still recognized for backwards compatibility.)
1418    
1419         If there is a fixed first byte, for example, from  a  pattern  such  as         If there is a fixed first byte, for example, from  a  pattern  such  as
1420         (cat|cow|coyote). Otherwise, if either         (cat|cow|coyote), its value is returned. Otherwise, if either
1421    
1422         (a)  the pattern was compiled with the PCRE_MULTILINE option, and every         (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1423         branch starts with "^", or         branch starts with "^", or
# Line 1388  INFORMATION ABOUT A PATTERN Line 1474  INFORMATION ABOUT A PATTERN
1474         PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is         PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
1475         ignored):         ignored):
1476    
1477           (?P<date> (?P<year>(\d\d)?\d\d) -           (?<date> (?<year>(\d\d)?\d\d) -
1478           (?P<month>\d\d) - (?P<day>\d\d) )           (?<month>\d\d) - (?<day>\d\d) )
1479    
1480         There are four named subpatterns, so the table has  four  entries,  and         There are four named subpatterns, so the table has  four  entries,  and
1481         each  entry  in the table is eight bytes long. The table is as follows,         each  entry  in the table is eight bytes long. The table is as follows,
# Line 1616  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1702  MATCHING A PATTERN: THE TRADITIONAL FUNC
1702           PCRE_NEWLINE_CR           PCRE_NEWLINE_CR
1703           PCRE_NEWLINE_LF           PCRE_NEWLINE_LF
1704           PCRE_NEWLINE_CRLF           PCRE_NEWLINE_CRLF
1705             PCRE_NEWLINE_ANY
1706    
1707         These options override  the  newline  definition  that  was  chosen  or         These options override  the  newline  definition  that  was  chosen  or
1708         defaulted  when the pattern was compiled. For details, see the descrip-         defaulted  when the pattern was compiled. For details, see the descrip-
1709         tion pcre_compile() above. During matching, the newline choice  affects         tion of pcre_compile()  above.  During  matching,  the  newline  choice
1710         the behaviour of the dot, circumflex, and dollar metacharacters.         affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
1711           ters. It may also alter the way the match position is advanced after  a
1712           match  failure  for  an  unanchored  pattern. When PCRE_NEWLINE_CRLF or
1713           PCRE_NEWLINE_ANY is set, and a match attempt  fails  when  the  current
1714           position  is  at a CRLF sequence, the match position is advanced by two
1715           characters instead of one, in other words, to after the CRLF.
1716    
1717           PCRE_NOTBOL           PCRE_NOTBOL
1718    
1719         This option specifies that first character of the subject string is not         This option specifies that first character of the subject string is not
1720         the beginning of a line, so the  circumflex  metacharacter  should  not         the  beginning  of  a  line, so the circumflex metacharacter should not
1721         match  before it. Setting this without PCRE_MULTILINE (at compile time)         match before it. Setting this without PCRE_MULTILINE (at compile  time)
1722         causes circumflex never to match. This option affects only  the  behav-         causes  circumflex  never to match. This option affects only the behav-
1723         iour of the circumflex metacharacter. It does not affect \A.         iour of the circumflex metacharacter. It does not affect \A.
1724    
1725           PCRE_NOTEOL           PCRE_NOTEOL
1726    
1727         This option specifies that the end of the subject string is not the end         This option specifies that the end of the subject string is not the end
1728         of a line, so the dollar metacharacter should not match it nor  (except         of  a line, so the dollar metacharacter should not match it nor (except
1729         in  multiline mode) a newline immediately before it. Setting this with-         in multiline mode) a newline immediately before it. Setting this  with-
1730         out PCRE_MULTILINE (at compile time) causes dollar never to match. This         out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1731         option  affects only the behaviour of the dollar metacharacter. It does         option affects only the behaviour of the dollar metacharacter. It  does
1732         not affect \Z or \z.         not affect \Z or \z.
1733    
1734           PCRE_NOTEMPTY           PCRE_NOTEMPTY
1735    
1736         An empty string is not considered to be a valid match if this option is         An empty string is not considered to be a valid match if this option is
1737         set.  If  there are alternatives in the pattern, they are tried. If all         set. If there are alternatives in the pattern, they are tried.  If  all
1738         the alternatives match the empty string, the entire  match  fails.  For         the  alternatives  match  the empty string, the entire match fails. For
1739         example, if the pattern         example, if the pattern
1740    
1741           a?b?           a?b?
1742    
1743         is  applied  to  a string not beginning with "a" or "b", it matches the         is applied to a string not beginning with "a" or "b",  it  matches  the
1744         empty string at the start of the subject. With PCRE_NOTEMPTY set,  this         empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1745         match is not valid, so PCRE searches further into the string for occur-         match is not valid, so PCRE searches further into the string for occur-
1746         rences of "a" or "b".         rences of "a" or "b".
1747    
1748         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1749         cial  case  of  a  pattern match of the empty string within its split()         cial case of a pattern match of the empty  string  within  its  split()
1750         function, and when using the /g modifier. It  is  possible  to  emulate         function,  and  when  using  the /g modifier. It is possible to emulate
1751         Perl's behaviour after matching a null string by first trying the match         Perl's behaviour after matching a null string by first trying the match
1752         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1753         if  that  fails by advancing the starting offset (see below) and trying         if that fails by advancing the starting offset (see below)  and  trying
1754         an ordinary match again. There is some code that demonstrates how to do         an ordinary match again. There is some code that demonstrates how to do
1755         this in the pcredemo.c sample program.         this in the pcredemo.c sample program.
1756    
1757           PCRE_NO_UTF8_CHECK           PCRE_NO_UTF8_CHECK
1758    
1759         When PCRE_UTF8 is set at compile time, the validity of the subject as a         When PCRE_UTF8 is set at compile time, the validity of the subject as a
1760         UTF-8 string is automatically checked when pcre_exec() is  subsequently         UTF-8  string is automatically checked when pcre_exec() is subsequently
1761         called.   The  value  of  startoffset is also checked to ensure that it         called.  The value of startoffset is also checked  to  ensure  that  it
1762         points to the start of a UTF-8 character. If an invalid UTF-8  sequence         points  to the start of a UTF-8 character. If an invalid UTF-8 sequence
1763         of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If         of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1764         startoffset contains an  invalid  value,  PCRE_ERROR_BADUTF8_OFFSET  is         startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is
1765         returned.         returned.
1766    
1767         If  you  already  know that your subject is valid, and you want to skip         If you already know that your subject is valid, and you  want  to  skip
1768         these   checks   for   performance   reasons,   you   can    set    the         these    checks    for   performance   reasons,   you   can   set   the
1769         PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to         PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
1770         do this for the second and subsequent calls to pcre_exec() if  you  are         do  this  for the second and subsequent calls to pcre_exec() if you are
1771         making  repeated  calls  to  find  all  the matches in a single subject         making repeated calls to find all  the  matches  in  a  single  subject
1772         string. However, you should be  sure  that  the  value  of  startoffset         string.  However,  you  should  be  sure  that the value of startoffset
1773         points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is         points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
1774         set, the effect of passing an invalid UTF-8 string as a subject,  or  a         set,  the  effect of passing an invalid UTF-8 string as a subject, or a
1775         value  of startoffset that does not point to the start of a UTF-8 char-         value of startoffset that does not point to the start of a UTF-8  char-
1776         acter, is undefined. Your program may crash.         acter, is undefined. Your program may crash.
1777    
1778           PCRE_PARTIAL           PCRE_PARTIAL
1779    
1780         This option turns on the  partial  matching  feature.  If  the  subject         This  option  turns  on  the  partial  matching feature. If the subject
1781         string  fails to match the pattern, but at some point during the match-         string fails to match the pattern, but at some point during the  match-
1782         ing process the end of the subject was reached (that  is,  the  subject         ing  process  the  end of the subject was reached (that is, the subject
1783         partially  matches  the  pattern and the failure to match occurred only         partially matches the pattern and the failure to  match  occurred  only
1784         because there were not enough subject characters), pcre_exec()  returns         because  there were not enough subject characters), pcre_exec() returns
1785         PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is         PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
1786         used, there are restrictions on what may appear in the  pattern.  These         used,  there  are restrictions on what may appear in the pattern. These
1787         are discussed in the pcrepartial documentation.         are discussed in the pcrepartial documentation.
1788    
1789     The string to be matched by pcre_exec()     The string to be matched by pcre_exec()
1790    
1791         The  subject string is passed to pcre_exec() as a pointer in subject, a         The subject string is passed to pcre_exec() as a pointer in subject,  a
1792         length in length, and a starting byte offset in startoffset.  In  UTF-8         length  in  length, and a starting byte offset in startoffset. In UTF-8
1793         mode,  the  byte  offset  must point to the start of a UTF-8 character.         mode, the byte offset must point to the start  of  a  UTF-8  character.
1794         Unlike the pattern string, the subject may contain binary  zero  bytes.         Unlike  the  pattern string, the subject may contain binary zero bytes.
1795         When  the starting offset is zero, the search for a match starts at the         When the starting offset is zero, the search for a match starts at  the
1796         beginning of the subject, and this is by far the most common case.         beginning of the subject, and this is by far the most common case.
1797    
1798         A non-zero starting offset is useful when searching for  another  match         A  non-zero  starting offset is useful when searching for another match
1799         in  the same subject by calling pcre_exec() again after a previous suc-         in the same subject by calling pcre_exec() again after a previous  suc-
1800         cess.  Setting startoffset differs from just passing over  a  shortened         cess.   Setting  startoffset differs from just passing over a shortened
1801         string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins         string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
1802         with any kind of lookbehind. For example, consider the pattern         with any kind of lookbehind. For example, consider the pattern
1803    
1804           \Biss\B           \Biss\B
1805    
1806         which finds occurrences of "iss" in the middle of  words.  (\B  matches         which  finds  occurrences  of "iss" in the middle of words. (\B matches
1807         only  if  the  current position in the subject is not a word boundary.)         only if the current position in the subject is not  a  word  boundary.)
1808         When applied to the string "Mississipi" the first call  to  pcre_exec()         When  applied  to the string "Mississipi" the first call to pcre_exec()
1809         finds  the  first  occurrence. If pcre_exec() is called again with just         finds the first occurrence. If pcre_exec() is called  again  with  just
1810         the remainder of the subject,  namely  "issipi",  it  does  not  match,         the  remainder  of  the  subject,  namely  "issipi", it does not match,
1811         because \B is always false at the start of the subject, which is deemed         because \B is always false at the start of the subject, which is deemed
1812         to be a word boundary. However, if pcre_exec()  is  passed  the  entire         to  be  a  word  boundary. However, if pcre_exec() is passed the entire
1813         string again, but with startoffset set to 4, it finds the second occur-         string again, but with startoffset set to 4, it finds the second occur-
1814         rence of "iss" because it is able to look behind the starting point  to         rence  of "iss" because it is able to look behind the starting point to
1815         discover that it is preceded by a letter.         discover that it is preceded by a letter.
1816    
1817         If  a  non-zero starting offset is passed when the pattern is anchored,         If a non-zero starting offset is passed when the pattern  is  anchored,
1818         one attempt to match at the given offset is made. This can only succeed         one attempt to match at the given offset is made. This can only succeed
1819         if  the  pattern  does  not require the match to be at the start of the         if the pattern does not require the match to be at  the  start  of  the
1820         subject.         subject.
1821    
1822     How pcre_exec() returns captured substrings     How pcre_exec() returns captured substrings
1823    
1824         In general, a pattern matches a certain portion of the subject, and  in         In  general, a pattern matches a certain portion of the subject, and in
1825         addition,  further  substrings  from  the  subject may be picked out by         addition, further substrings from the subject  may  be  picked  out  by
1826         parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,         parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
1827         this  is  called "capturing" in what follows, and the phrase "capturing         this is called "capturing" in what follows, and the  phrase  "capturing
1828         subpattern" is used for a fragment of a pattern that picks out  a  sub-         subpattern"  is  used for a fragment of a pattern that picks out a sub-
1829         string.  PCRE  supports several other kinds of parenthesized subpattern         string. PCRE supports several other kinds of  parenthesized  subpattern
1830         that do not cause substrings to be captured.         that do not cause substrings to be captured.
1831    
1832         Captured substrings are returned to the caller via a vector of  integer         Captured  substrings are returned to the caller via a vector of integer
1833         offsets  whose  address is passed in ovector. The number of elements in         offsets whose address is passed in ovector. The number of  elements  in
1834         the vector is passed in ovecsize, which must be a non-negative  number.         the  vector is passed in ovecsize, which must be a non-negative number.
1835         Note: this argument is NOT the size of ovector in bytes.         Note: this argument is NOT the size of ovector in bytes.
1836    
1837         The  first  two-thirds of the vector is used to pass back captured sub-         The first two-thirds of the vector is used to pass back  captured  sub-
1838         strings, each substring using a pair of integers. The  remaining  third         strings,  each  substring using a pair of integers. The remaining third
1839         of  the  vector is used as workspace by pcre_exec() while matching cap-         of the vector is used as workspace by pcre_exec() while  matching  cap-
1840         turing subpatterns, and is not available for passing back  information.         turing  subpatterns, and is not available for passing back information.
1841         The  length passed in ovecsize should always be a multiple of three. If         The length passed in ovecsize should always be a multiple of three.  If
1842         it is not, it is rounded down.         it is not, it is rounded down.
1843    
1844         When a match is successful, information about  captured  substrings  is         When  a  match  is successful, information about captured substrings is
1845         returned  in  pairs  of integers, starting at the beginning of ovector,         returned in pairs of integers, starting at the  beginning  of  ovector,
1846         and continuing up to two-thirds of its length at the  most.  The  first         and  continuing  up  to two-thirds of its length at the most. The first
1847         element of a pair is set to the offset of the first character in a sub-         element of a pair is set to the offset of the first character in a sub-
1848         string, and the second is set to the  offset  of  the  first  character         string,  and  the  second  is  set to the offset of the first character
1849         after  the  end  of  a  substring. The first pair, ovector[0] and ovec-         after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
1850         tor[1], identify the portion of  the  subject  string  matched  by  the         tor[1],  identify  the  portion  of  the  subject string matched by the
1851         entire  pattern.  The next pair is used for the first capturing subpat-         entire pattern. The next pair is used for the first  capturing  subpat-
1852         tern, and so on. The value returned by pcre_exec() is one more than the         tern, and so on. The value returned by pcre_exec() is one more than the
1853         highest numbered pair that has been set. For example, if two substrings         highest numbered pair that has been set. For example, if two substrings
1854         have been captured, the returned value is 3. If there are no  capturing         have  been captured, the returned value is 3. If there are no capturing
1855         subpatterns,  the return value from a successful match is 1, indicating         subpatterns, the return value from a successful match is 1,  indicating
1856         that just the first pair of offsets has been set.         that just the first pair of offsets has been set.
1857    
1858         If a capturing subpattern is matched repeatedly, it is the last portion         If a capturing subpattern is matched repeatedly, it is the last portion
1859         of the string that it matched that is returned.         of the string that it matched that is returned.
1860    
1861         If  the vector is too small to hold all the captured substring offsets,         If the vector is too small to hold all the captured substring  offsets,
1862         it is used as far as possible (up to two-thirds of its length), and the         it is used as far as possible (up to two-thirds of its length), and the
1863         function  returns a value of zero. In particular, if the substring off-         function returns a value of zero. In particular, if the substring  off-
1864         sets are not of interest, pcre_exec() may be called with ovector passed         sets are not of interest, pcre_exec() may be called with ovector passed
1865         as  NULL  and  ovecsize  as zero. However, if the pattern contains back         as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1866         references and the ovector is not big enough to  remember  the  related         references  and  the  ovector is not big enough to remember the related
1867         substrings,  PCRE has to get additional memory for use during matching.         substrings, PCRE has to get additional memory for use during  matching.
1868         Thus it is usually advisable to supply an ovector.         Thus it is usually advisable to supply an ovector.
1869    
1870         The pcre_info() function can be used to find  out  how  many  capturing         The  pcre_info()  function  can  be used to find out how many capturing
1871         subpatterns  there  are  in  a  compiled pattern. The smallest size for         subpatterns there are in a compiled  pattern.  The  smallest  size  for
1872         ovector that will allow for n captured substrings, in addition  to  the         ovector  that  will allow for n captured substrings, in addition to the
1873         offsets of the substring matched by the whole pattern, is (n+1)*3.         offsets of the substring matched by the whole pattern, is (n+1)*3.
1874    
1875         It  is  possible for capturing subpattern number n+1 to match some part         It is possible for capturing subpattern number n+1 to match  some  part
1876         of the subject when subpattern n has not been used at all. For example,         of the subject when subpattern n has not been used at all. For example,
1877         if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the         if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
1878         return from the function is 4, and subpatterns 1 and 3 are matched, but         return from the function is 4, and subpatterns 1 and 3 are matched, but
1879         2  is  not.  When  this happens, both values in the offset pairs corre-         2 is not. When this happens, both values in  the  offset  pairs  corre-
1880         sponding to unused subpatterns are set to -1.         sponding to unused subpatterns are set to -1.
1881    
1882         Offset values that correspond to unused subpatterns at the end  of  the         Offset  values  that correspond to unused subpatterns at the end of the
1883         expression  are  also  set  to  -1. For example, if the string "abc" is         expression are also set to -1. For example,  if  the  string  "abc"  is
1884         matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not         matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1885         matched.  The  return  from the function is 2, because the highest used         matched. The return from the function is 2, because  the  highest  used
1886         capturing subpattern number is 1. However, you can refer to the offsets         capturing subpattern number is 1. However, you can refer to the offsets
1887         for  the  second  and third capturing subpatterns if you wish (assuming         for the second and third capturing subpatterns if  you  wish  (assuming
1888         the vector is large enough, of course).         the vector is large enough, of course).
1889    
1890         Some convenience functions are provided  for  extracting  the  captured         Some  convenience  functions  are  provided for extracting the captured
1891         substrings as separate strings. These are described below.         substrings as separate strings. These are described below.
1892    
1893     Error return values from pcre_exec()     Error return values from pcre_exec()
1894    
1895         If  pcre_exec()  fails, it returns a negative number. The following are         If pcre_exec() fails, it returns a negative number. The  following  are
1896         defined in the header file:         defined in the header file:
1897    
1898           PCRE_ERROR_NOMATCH        (-1)           PCRE_ERROR_NOMATCH        (-1)
# Line 1809  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1901  MATCHING A PATTERN: THE TRADITIONAL FUNC
1901    
1902           PCRE_ERROR_NULL           (-2)           PCRE_ERROR_NULL           (-2)
1903    
1904         Either code or subject was passed as NULL,  or  ovector  was  NULL  and         Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1905         ovecsize was not zero.         ovecsize was not zero.
1906    
1907           PCRE_ERROR_BADOPTION      (-3)           PCRE_ERROR_BADOPTION      (-3)
# Line 1818  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1910  MATCHING A PATTERN: THE TRADITIONAL FUNC
1910    
1911           PCRE_ERROR_BADMAGIC       (-4)           PCRE_ERROR_BADMAGIC       (-4)
1912    
1913         PCRE  stores a 4-byte "magic number" at the start of the compiled code,         PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1914         to catch the case when it is passed a junk pointer and to detect when a         to catch the case when it is passed a junk pointer and to detect when a
1915         pattern that was compiled in an environment of one endianness is run in         pattern that was compiled in an environment of one endianness is run in
1916         an environment with the other endianness. This is the error  that  PCRE         an  environment  with the other endianness. This is the error that PCRE
1917         gives when the magic number is not present.         gives when the magic number is not present.
1918    
1919           PCRE_ERROR_UNKNOWN_NODE   (-5)           PCRE_ERROR_UNKNOWN_OPCODE (-5)
1920    
1921         While running the pattern match, an unknown item was encountered in the         While running the pattern match, an unknown item was encountered in the
1922         compiled pattern. This error could be caused by a bug  in  PCRE  or  by         compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1923         overwriting of the compiled pattern.         overwriting of the compiled pattern.
1924    
1925           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
1926    
1927         If  a  pattern contains back references, but the ovector that is passed         If a pattern contains back references, but the ovector that  is  passed
1928         to pcre_exec() is not big enough to remember the referenced substrings,         to pcre_exec() is not big enough to remember the referenced substrings,
1929         PCRE  gets  a  block of memory at the start of matching to use for this         PCRE gets a block of memory at the start of matching to  use  for  this
1930         purpose. If the call via pcre_malloc() fails, this error is given.  The         purpose.  If the call via pcre_malloc() fails, this error is given. The
1931         memory is automatically freed at the end of matching.         memory is automatically freed at the end of matching.
1932    
1933           PCRE_ERROR_NOSUBSTRING    (-7)           PCRE_ERROR_NOSUBSTRING    (-7)
1934    
1935         This  error is used by the pcre_copy_substring(), pcre_get_substring(),         This error is used by the pcre_copy_substring(),  pcre_get_substring(),
1936         and  pcre_get_substring_list()  functions  (see  below).  It  is  never         and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1937         returned by pcre_exec().         returned by pcre_exec().
1938    
1939           PCRE_ERROR_MATCHLIMIT     (-8)           PCRE_ERROR_MATCHLIMIT     (-8)
1940    
1941         The  backtracking  limit,  as  specified  by the match_limit field in a         The backtracking limit, as specified by  the  match_limit  field  in  a
1942         pcre_extra structure (or defaulted) was reached.  See  the  description         pcre_extra  structure  (or  defaulted) was reached. See the description
1943         above.         above.
1944    
          PCRE_ERROR_RECURSIONLIMIT (-21)  
   
        The internal recursion limit, as specified by the match_limit_recursion  
        field in a pcre_extra structure (or defaulted)  was  reached.  See  the  
        description above.  
   
1945           PCRE_ERROR_CALLOUT        (-9)           PCRE_ERROR_CALLOUT        (-9)
1946    
1947         This error is never generated by pcre_exec() itself. It is provided for         This error is never generated by pcre_exec() itself. It is provided for
1948         use by callout functions that want to yield a distinctive  error  code.         use  by  callout functions that want to yield a distinctive error code.
1949         See the pcrecallout documentation for details.         See the pcrecallout documentation for details.
1950    
1951           PCRE_ERROR_BADUTF8        (-10)           PCRE_ERROR_BADUTF8        (-10)
1952    
1953         A  string  that contains an invalid UTF-8 byte sequence was passed as a         A string that contains an invalid UTF-8 byte sequence was passed  as  a
1954         subject.         subject.
1955    
1956           PCRE_ERROR_BADUTF8_OFFSET (-11)           PCRE_ERROR_BADUTF8_OFFSET (-11)
1957    
1958         The UTF-8 byte sequence that was passed as a subject was valid, but the         The UTF-8 byte sequence that was passed as a subject was valid, but the
1959         value  of startoffset did not point to the beginning of a UTF-8 charac-         value of startoffset did not point to the beginning of a UTF-8  charac-
1960         ter.         ter.
1961    
1962           PCRE_ERROR_PARTIAL        (-12)           PCRE_ERROR_PARTIAL        (-12)
1963    
1964         The subject string did not match, but it did match partially.  See  the         The  subject  string did not match, but it did match partially. See the
1965         pcrepartial documentation for details of partial matching.         pcrepartial documentation for details of partial matching.
1966    
1967           PCRE_ERROR_BADPARTIAL     (-13)           PCRE_ERROR_BADPARTIAL     (-13)
1968    
1969         The  PCRE_PARTIAL  option  was  used with a compiled pattern containing         The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
1970         items that are not supported for partial matching. See the  pcrepartial         items  that are not supported for partial matching. See the pcrepartial
1971         documentation for details of partial matching.         documentation for details of partial matching.
1972    
1973           PCRE_ERROR_INTERNAL       (-14)           PCRE_ERROR_INTERNAL       (-14)
1974    
1975         An  unexpected  internal error has occurred. This error could be caused         An unexpected internal error has occurred. This error could  be  caused
1976         by a bug in PCRE or by overwriting of the compiled pattern.         by a bug in PCRE or by overwriting of the compiled pattern.
1977    
1978           PCRE_ERROR_BADCOUNT       (-15)           PCRE_ERROR_BADCOUNT       (-15)
1979    
1980         This error is given if the value of the ovecsize argument is  negative.         This  error is given if the value of the ovecsize argument is negative.
1981    
1982             PCRE_ERROR_RECURSIONLIMIT (-21)
1983    
1984           The internal recursion limit, as specified by the match_limit_recursion
1985           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
1986           description above.
1987    
1988             PCRE_ERROR_NULLWSLIMIT    (-22)
1989    
1990           When a group that can match an empty  substring  is  repeated  with  an
1991           unbounded  upper  limit, the subject position at the start of the group
1992           must be remembered, so that a test for an empty string can be made when
1993           the  end  of the group is reached. Some workspace is required for this;
1994           if it runs out, this error is given.
1995    
1996             PCRE_ERROR_BADNEWLINE     (-23)
1997    
1998           An invalid combination of PCRE_NEWLINE_xxx options was given.
1999    
2000           Error numbers -16 to -20 are not used by pcre_exec().
2001    
2002    
2003  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
# Line 1907  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER Line 2013  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2013         int pcre_get_substring_list(const char *subject,         int pcre_get_substring_list(const char *subject,
2014              int *ovector, int stringcount, const char ***listptr);              int *ovector, int stringcount, const char ***listptr);
2015    
2016         Captured  substrings  can  be  accessed  directly  by using the offsets         Captured substrings can be  accessed  directly  by  using  the  offsets
2017         returned by pcre_exec() in  ovector.  For  convenience,  the  functions         returned  by  pcre_exec()  in  ovector.  For convenience, the functions
2018         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2019         string_list() are provided for extracting captured substrings  as  new,         string_list()  are  provided for extracting captured substrings as new,
2020         separate,  zero-terminated strings. These functions identify substrings         separate, zero-terminated strings. These functions identify  substrings
2021         by number. The next section describes functions  for  extracting  named         by  number.  The  next section describes functions for extracting named
2022         substrings.         substrings.
2023    
2024         A  substring that contains a binary zero is correctly extracted and has         A substring that contains a binary zero is correctly extracted and  has
2025         a further zero added on the end, but the result is not, of course, a  C         a  further zero added on the end, but the result is not, of course, a C
2026         string.   However,  you  can  process such a string by referring to the         string.  However, you can process such a string  by  referring  to  the
2027         length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-         length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
2028         string().  Unfortunately, the interface to pcre_get_substring_list() is         string().  Unfortunately, the interface to pcre_get_substring_list() is
2029         not adequate for handling strings containing binary zeros, because  the         not  adequate for handling strings containing binary zeros, because the
2030         end of the final string is not independently indicated.         end of the final string is not independently indicated.
2031    
2032         The  first  three  arguments  are the same for all three of these func-         The first three arguments are the same for all  three  of  these  func-
2033         tions: subject is the subject string that has  just  been  successfully         tions:  subject  is  the subject string that has just been successfully
2034         matched, ovector is a pointer to the vector of integer offsets that was         matched, ovector is a pointer to the vector of integer offsets that was
2035         passed to pcre_exec(), and stringcount is the number of substrings that         passed to pcre_exec(), and stringcount is the number of substrings that
2036         were  captured  by  the match, including the substring that matched the         were captured by the match, including the substring  that  matched  the
2037         entire regular expression. This is the value returned by pcre_exec() if         entire regular expression. This is the value returned by pcre_exec() if
2038         it  is greater than zero. If pcre_exec() returned zero, indicating that         it is greater than zero. If pcre_exec() returned zero, indicating  that
2039         it ran out of space in ovector, the value passed as stringcount  should         it  ran out of space in ovector, the value passed as stringcount should
2040         be the number of elements in the vector divided by three.         be the number of elements in the vector divided by three.
2041    
2042         The  functions pcre_copy_substring() and pcre_get_substring() extract a         The functions pcre_copy_substring() and pcre_get_substring() extract  a
2043         single substring, whose number is given as  stringnumber.  A  value  of         single  substring,  whose  number  is given as stringnumber. A value of
2044         zero  extracts  the  substring that matched the entire pattern, whereas         zero extracts the substring that matched the  entire  pattern,  whereas
2045         higher values  extract  the  captured  substrings.  For  pcre_copy_sub-         higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
2046         string(),  the  string  is  placed  in buffer, whose length is given by         string(), the string is placed in buffer,  whose  length  is  given  by
2047         buffersize, while for pcre_get_substring() a new  block  of  memory  is         buffersize,  while  for  pcre_get_substring()  a new block of memory is
2048         obtained  via  pcre_malloc,  and its address is returned via stringptr.         obtained via pcre_malloc, and its address is  returned  via  stringptr.
2049         The yield of the function is the length of the  string,  not  including         The  yield  of  the function is the length of the string, not including
2050         the terminating zero, or one of         the terminating zero, or one of these error codes:
2051    
2052           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2053    
2054         The  buffer  was too small for pcre_copy_substring(), or the attempt to         The buffer was too small for pcre_copy_substring(), or the  attempt  to
2055         get memory failed for pcre_get_substring().         get memory failed for pcre_get_substring().
2056    
2057           PCRE_ERROR_NOSUBSTRING    (-7)           PCRE_ERROR_NOSUBSTRING    (-7)
2058    
2059         There is no substring whose number is stringnumber.         There is no substring whose number is stringnumber.
2060    
2061         The pcre_get_substring_list()  function  extracts  all  available  sub-         The  pcre_get_substring_list()  function  extracts  all  available sub-
2062         strings  and  builds  a list of pointers to them. All this is done in a         strings and builds a list of pointers to them. All this is  done  in  a
2063         single block of memory that is obtained via pcre_malloc. The address of         single block of memory that is obtained via pcre_malloc. The address of
2064         the  memory  block  is returned via listptr, which is also the start of         the memory block is returned via listptr, which is also  the  start  of
2065         the list of string pointers. The end of the list is marked  by  a  NULL         the  list  of  string pointers. The end of the list is marked by a NULL
2066         pointer. The yield of the function is zero if all went well, or         pointer. The yield of the function is zero if all  went  well,  or  the
2067           error code
2068    
2069           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2070    
# Line 1999  EXTRACTING CAPTURED SUBSTRINGS BY NAME Line 2106  EXTRACTING CAPTURED SUBSTRINGS BY NAME
2106         To extract a substring by name, you first have to find associated  num-         To extract a substring by name, you first have to find associated  num-
2107         ber.  For example, for this pattern         ber.  For example, for this pattern
2108    
2109           (a+)b(?P<xxx>\d+)...           (a+)b(?<xxx>\d+)...
2110    
2111         the number of the subpattern called "xxx" is 2. If the name is known to         the number of the subpattern called "xxx" is 2. If the name is known to
2112         be unique (PCRE_DUPNAMES was not set), you can find the number from the         be unique (PCRE_DUPNAMES was not set), you can find the number from the
# Line 2050  DUPLICATE SUBPATTERN NAMES Line 2157  DUPLICATE SUBPATTERN NAMES
2157         third  and  fourth  are  pointers to variables which are updated by the         third  and  fourth  are  pointers to variables which are updated by the
2158         function. After it has run, they point to the first and last entries in         function. After it has run, they point to the first and last entries in
2159         the  name-to-number  table  for  the  given  name.  The function itself         the  name-to-number  table  for  the  given  name.  The function itself
2160         returns the length of each entry, or  PCRE_ERROR_NOSUBSTRING  if  there         returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
2161         are  none.  The  format  of the table is described above in the section         there  are none. The format of the table is described above in the sec-
2162         entitled Information about a pattern. Given all  the  relevant  entries         tion entitled Information about a  pattern.   Given  all  the  relevant
2163         for the name, you can extract each of their numbers, and hence the cap-         entries  for the name, you can extract each of their numbers, and hence
2164         tured data, if any.         the captured data, if any.
2165    
2166    
2167  FINDING ALL POSSIBLE MATCHES  FINDING ALL POSSIBLE MATCHES
# Line 2083  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2190  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2190              int *workspace, int wscount);              int *workspace, int wscount);
2191    
2192         The  function  pcre_dfa_exec()  is  called  to  match  a subject string         The  function  pcre_dfa_exec()  is  called  to  match  a subject string
2193         against a compiled pattern, using a "DFA" matching algorithm. This  has         against a compiled pattern, using a matching algorithm that  scans  the
2194         different  characteristics to the normal algorithm, and is not compati-         subject  string  just  once, and does not backtrack. This has different
2195         ble with Perl. Some of the features of PCRE patterns are not supported.         characteristics to the normal algorithm, and  is  not  compatible  with
2196         Nevertheless, there are times when this kind of matching can be useful.         Perl.  Some  of the features of PCRE patterns are not supported. Never-
2197         For a discussion of the two matching algorithms, see  the  pcrematching         theless, there are times when this kind of matching can be useful.  For
2198         documentation.         a discussion of the two matching algorithms, see the pcrematching docu-
2199           mentation.
2200    
2201         The  arguments  for  the  pcre_dfa_exec()  function are the same as for         The arguments for the pcre_dfa_exec() function  are  the  same  as  for
2202         pcre_exec(), plus two extras. The ovector argument is used in a differ-         pcre_exec(), plus two extras. The ovector argument is used in a differ-
2203         ent  way,  and  this is described below. The other common arguments are         ent way, and this is described below. The other  common  arguments  are
2204         used in the same way as for pcre_exec(), so their  description  is  not         used  in  the  same way as for pcre_exec(), so their description is not
2205         repeated here.         repeated here.
2206    
2207         The  two  additional  arguments provide workspace for the function. The         The two additional arguments provide workspace for  the  function.  The
2208         workspace vector should contain at least 20 elements. It  is  used  for         workspace  vector  should  contain at least 20 elements. It is used for
2209         keeping  track  of  multiple  paths  through  the  pattern  tree.  More         keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2210         workspace will be needed for patterns and subjects where  there  are  a         workspace  will  be  needed for patterns and subjects where there are a
2211         lot of potential matches.         lot of potential matches.
2212    
2213         Here is an example of a simple call to pcre_dfa_exec():         Here is an example of a simple call to pcre_dfa_exec():
# Line 2121  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2229  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2229    
2230     Option bits for pcre_dfa_exec()     Option bits for pcre_dfa_exec()
2231    
2232         The  unused  bits  of  the options argument for pcre_dfa_exec() must be         The unused bits of the options argument  for  pcre_dfa_exec()  must  be
2233         zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-         zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
2234         LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,         LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,
2235         PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last         PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2236         three of these are the same as for pcre_exec(), so their description is         three of these are the same as for pcre_exec(), so their description is
2237         not repeated here.         not repeated here.
2238    
2239           PCRE_PARTIAL           PCRE_PARTIAL
2240    
2241         This has the same general effect as it does for  pcre_exec(),  but  the         This  has  the  same general effect as it does for pcre_exec(), but the
2242         details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for         details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
2243         pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into         pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
2244         PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have         PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
2245         been no complete matches, but there is still at least one matching pos-         been no complete matches, but there is still at least one matching pos-
2246         sibility.  The portion of the string that provided the partial match is         sibility. The portion of the string that provided the partial match  is
2247         set as the first matching string.         set as the first matching string.
2248    
2249           PCRE_DFA_SHORTEST           PCRE_DFA_SHORTEST
2250    
2251         Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to         Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
2252         stop  as  soon  as  it  has found one match. Because of the way the DFA         stop as soon as it has found one match. Because of the way the alterna-
2253         algorithm works, this is necessarily the shortest possible match at the         tive  algorithm  works, this is necessarily the shortest possible match
2254         first possible matching point in the subject string.         at the first possible matching point in the subject string.
2255    
2256           PCRE_DFA_RESTART           PCRE_DFA_RESTART
2257    
2258         When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and         When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
2259         returns a partial match, it is possible to call it  again,  with  addi-         returns  a  partial  match, it is possible to call it again, with addi-
2260         tional  subject  characters,  and have it continue with the same match.         tional subject characters, and have it continue with  the  same  match.
2261         The PCRE_DFA_RESTART option requests this action; when it is  set,  the         The  PCRE_DFA_RESTART  option requests this action; when it is set, the
2262         workspace  and wscount options must reference the same vector as before         workspace and wscount options must reference the same vector as  before
2263         because data about the match so far is left in  them  after  a  partial         because  data  about  the  match so far is left in them after a partial
2264         match.  There  is  more  discussion of this facility in the pcrepartial         match. There is more discussion of this  facility  in  the  pcrepartial
2265         documentation.         documentation.
2266    
2267     Successful returns from pcre_dfa_exec()     Successful returns from pcre_dfa_exec()
2268    
2269         When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-         When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
2270         string in the subject. Note, however, that all the matches from one run         string in the subject. Note, however, that all the matches from one run
2271         of the function start at the same point in  the  subject.  The  shorter         of  the  function  start  at the same point in the subject. The shorter
2272         matches  are all initial substrings of the longer matches. For example,         matches are all initial substrings of the longer matches. For  example,
2273         if the pattern         if the pattern
2274    
2275           <.*>           <.*>
# Line 2176  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2284  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2284           <something> <something else>           <something> <something else>
2285           <something> <something else> <something further>           <something> <something else> <something further>
2286    
2287         On success, the yield of the function is a number  greater  than  zero,         On  success,  the  yield of the function is a number greater than zero,
2288         which  is  the  number of matched substrings. The substrings themselves         which is the number of matched substrings.  The  substrings  themselves
2289         are returned in ovector. Each string uses two elements;  the  first  is         are  returned  in  ovector. Each string uses two elements; the first is
2290         the  offset  to the start, and the second is the offset to the end. All         the offset to the start, and the second is the offset to  the  end.  In
2291         the strings have the same start offset. (Space could have been saved by         fact,  all  the  strings  have the same start offset. (Space could have
2292         giving  this only once, but it was decided to retain some compatibility         been saved by giving this only once, but it was decided to retain  some
2293         with the way pcre_exec() returns data, even though the meaning  of  the         compatibility  with  the  way pcre_exec() returns data, even though the
2294         strings is different.)         meaning of the strings is different.)
2295    
2296         The strings are returned in reverse order of length; that is, the long-         The strings are returned in reverse order of length; that is, the long-
2297         est matching string is given first. If there were too many  matches  to         est  matching  string is given first. If there were too many matches to
2298         fit  into ovector, the yield of the function is zero, and the vector is         fit into ovector, the yield of the function is zero, and the vector  is
2299         filled with the longest matches.         filled with the longest matches.
2300    
2301     Error returns from pcre_dfa_exec()     Error returns from pcre_dfa_exec()
2302    
2303         The pcre_dfa_exec() function returns a negative number when  it  fails.         The  pcre_dfa_exec()  function returns a negative number when it fails.
2304         Many  of  the  errors  are  the  same as for pcre_exec(), and these are         Many of the errors are the same  as  for  pcre_exec(),  and  these  are
2305         described above.  There are in addition the following errors  that  are         described  above.   There are in addition the following errors that are
2306         specific to pcre_dfa_exec():         specific to pcre_dfa_exec():
2307    
2308           PCRE_ERROR_DFA_UITEM      (-16)           PCRE_ERROR_DFA_UITEM      (-16)
2309    
2310         This  return is given if pcre_dfa_exec() encounters an item in the pat-         This return is given if pcre_dfa_exec() encounters an item in the  pat-
2311         tern that it does not support, for instance, the use of \C  or  a  back         tern  that  it  does not support, for instance, the use of \C or a back
2312         reference.         reference.
2313    
2314           PCRE_ERROR_DFA_UCOND      (-17)           PCRE_ERROR_DFA_UCOND      (-17)
2315    
2316         This  return is given if pcre_dfa_exec() encounters a condition item in         This return is given if pcre_dfa_exec()  encounters  a  condition  item
2317         a pattern that uses a back reference for the  condition.  This  is  not         that  uses  a back reference for the condition, or a test for recursion
2318         supported.         in a specific group. These are not supported.
2319    
2320           PCRE_ERROR_DFA_UMLIMIT    (-18)           PCRE_ERROR_DFA_UMLIMIT    (-18)
2321    
2322         This  return  is given if pcre_dfa_exec() is called with an extra block         This return is given if pcre_dfa_exec() is called with an  extra  block
2323         that contains a setting of the match_limit field. This is not supported         that contains a setting of the match_limit field. This is not supported
2324         (it is meaningless).         (it is meaningless).
2325    
2326           PCRE_ERROR_DFA_WSSIZE     (-19)           PCRE_ERROR_DFA_WSSIZE     (-19)
2327    
2328         This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the         This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
2329         workspace vector.         workspace vector.
2330    
2331           PCRE_ERROR_DFA_RECURSE    (-20)           PCRE_ERROR_DFA_RECURSE    (-20)
2332    
2333         When a recursive subpattern is processed, the matching  function  calls         When  a  recursive subpattern is processed, the matching function calls
2334         itself  recursively,  using  private vectors for ovector and workspace.         itself recursively, using private vectors for  ovector  and  workspace.
2335         This error is given if the output vector  is  not  large  enough.  This         This  error  is  given  if  the output vector is not large enough. This
2336         should be extremely rare, as a vector of size 1000 is used.         should be extremely rare, as a vector of size 1000 is used.
2337    
2338  Last updated: 08 June 2006  
2339  Copyright (c) 1997-2006 University of Cambridge.  SEE ALSO
2340    
2341           pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
2342           tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2343    
2344    
2345    AUTHOR
2346    
2347           Philip Hazel
2348           University Computing Service
2349           Cambridge CB2 3QH, England.
2350    
2351    
2352    REVISION
2353    
2354           Last updated: 06 March 2007
2355           Copyright (c) 1997-2007 University of Cambridge.
2356  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2357    
2358    
# Line 2386  RETURN VALUES Line 2510  RETURN VALUES
2510         reserved for use by callout functions; it will never be  used  by  PCRE         reserved for use by callout functions; it will never be  used  by  PCRE
2511         itself.         itself.
2512    
2513  Last updated: 28 February 2005  
2514  Copyright (c) 1997-2005 University of Cambridge.  AUTHOR
2515    
2516           Philip Hazel
2517           University Computing Service
2518           Cambridge CB2 3QH, England.
2519    
2520    
2521    REVISION
2522    
2523           Last updated: 06 March 2007
2524           Copyright (c) 1997-2007 University of Cambridge.
2525  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2526    
2527    
# Line 2401  NAME Line 2535  NAME
2535  DIFFERENCES BETWEEN PCRE AND PERL  DIFFERENCES BETWEEN PCRE AND PERL
2536    
2537         This  document describes the differences in the ways that PCRE and Perl         This  document describes the differences in the ways that PCRE and Perl
2538         handle regular expressions. The differences  described  here  are  with         handle regular expressions. The differences described here  are  mainly
2539         respect to Perl 5.8.         with  respect  to  Perl 5.8, though PCRE version 7.0 contains some fea-
2540           tures that are expected to be in the forthcoming Perl 5.10.
2541    
2542         1.  PCRE has only a subset of Perl's UTF-8 and Unicode support. Details         1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2543         of what it does have are given in the section on UTF-8 support  in  the         of  what  it does have are given in the section on UTF-8 support in the
2544         main pcre page.         main pcre page.
2545    
2546         2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl         2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2547         permits them, but they do not mean what you might think.  For  example,         permits  them,  but they do not mean what you might think. For example,
2548         (?!a){3} does not assert that the next three characters are not "a". It         (?!a){3} does not assert that the next three characters are not "a". It
2549         just asserts that the next character is not "a" three times.         just asserts that the next character is not "a" three times.
2550    
2551         3. Capturing subpatterns that occur inside  negative  lookahead  asser-         3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2552         tions  are  counted,  but their entries in the offsets vector are never         tions are counted, but their entries in the offsets  vector  are  never
2553         set. Perl sets its numerical variables from any such patterns that  are         set.  Perl sets its numerical variables from any such patterns that are
2554         matched before the assertion fails to match something (thereby succeed-         matched before the assertion fails to match something (thereby succeed-
2555         ing), but only if the negative lookahead assertion  contains  just  one         ing),  but  only  if the negative lookahead assertion contains just one
2556         branch.         branch.
2557    
2558         4.  Though  binary zero characters are supported in the subject string,         4. Though binary zero characters are supported in the  subject  string,
2559         they are not allowed in a pattern string because it is passed as a nor-         they are not allowed in a pattern string because it is passed as a nor-
2560         mal C string, terminated by zero. The escape sequence \0 can be used in         mal C string, terminated by zero. The escape sequence \0 can be used in
2561         the pattern to represent a binary zero.         the pattern to represent a binary zero.
2562    
2563         5. The following Perl escape sequences are not supported: \l,  \u,  \L,         5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2564         \U, and \N. In fact these are implemented by Perl's general string-han-         \U, and \N. In fact these are implemented by Perl's general string-han-
2565         dling and are not part of its pattern matching engine. If any of  these         dling  and are not part of its pattern matching engine. If any of these
2566         are encountered by PCRE, an error is generated.         are encountered by PCRE, an error is generated.
2567    
2568         6.  The Perl escape sequences \p, \P, and \X are supported only if PCRE         6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2569         is built with Unicode character property support. The  properties  that         is  built  with Unicode character property support. The properties that
2570         can  be tested with \p and \P are limited to the general category prop-         can be tested with \p and \P are limited to the general category  prop-
2571         erties such as Lu and Nd, script names such as Greek or  Han,  and  the         erties  such  as  Lu and Nd, script names such as Greek or Han, and the
2572         derived properties Any and L&.         derived properties Any and L&.
2573    
2574         7. PCRE does support the \Q...\E escape for quoting substrings. Charac-         7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2575         ters in between are treated as literals.  This  is  slightly  different         ters  in  between  are  treated as literals. This is slightly different
2576         from  Perl  in  that  $  and  @ are also handled as literals inside the         from Perl in that $ and @ are  also  handled  as  literals  inside  the
2577         quotes. In Perl, they cause variable interpolation (but of course  PCRE         quotes.  In Perl, they cause variable interpolation (but of course PCRE
2578         does not have variables). Note the following examples:         does not have variables). Note the following examples:
2579    
2580             Pattern            PCRE matches      Perl matches             Pattern            PCRE matches      Perl matches
# Line 2449  DIFFERENCES BETWEEN PCRE AND PERL Line 2584  DIFFERENCES BETWEEN PCRE AND PERL
2584             \Qabc\$xyz\E       abc\$xyz          abc\$xyz             \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2585             \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz             \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2586    
2587         The  \Q...\E  sequence  is recognized both inside and outside character         The \Q...\E sequence is recognized both inside  and  outside  character
2588         classes.         classes.
2589    
2590         8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})         8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2591         constructions.  However,  there is support for recursive patterns using         constructions. However, there is support for recursive  patterns.  This
2592         the non-Perl items (?R),  (?number),  and  (?P>name).  Also,  the  PCRE         is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2593         "callout"  feature allows an external function to be called during pat-         "callout" feature allows an external function to be called during  pat-
2594         tern matching. See the pcrecallout documentation for details.         tern matching. See the pcrecallout documentation for details.
2595    
2596         9. There are some differences that are concerned with the  settings  of         9.  Subpatterns  that  are  called  recursively or as "subroutines" are
2597         captured  strings  when  part  of  a  pattern is repeated. For example,         always treated as atomic groups in  PCRE.  This  is  like  Python,  but
2598         matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2         unlike Perl.
2599    
2600           10.  There are some differences that are concerned with the settings of
2601           captured strings when part of  a  pattern  is  repeated.  For  example,
2602           matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
2603         unset, but in PCRE it is set to "b".         unset, but in PCRE it is set to "b".
2604    
2605         10. PCRE provides some extensions to the Perl regular expression facil-         11. PCRE provides some extensions to the Perl regular expression facil-
2606         ities:         ities.   Perl  5.10  will  include new features that are not in earlier
2607           versions, some of which (such as named parentheses) have been  in  PCRE
2608           for some time. This list is with respect to Perl 5.10:
2609    
2610         (a) Although lookbehind assertions must  match  fixed  length  strings,         (a)  Although  lookbehind  assertions  must match fixed length strings,
2611         each alternative branch of a lookbehind assertion can match a different         each alternative branch of a lookbehind assertion can match a different
2612         length of string. Perl requires them all to have the same length.         length of string. Perl requires them all to have the same length.
2613    
2614         (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $         (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2615         meta-character matches only at the very end of the string.         meta-character matches only at the very end of the string.
2616    
2617         (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-         (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2618         cial meaning  is  faulted.  Otherwise,  like  Perl,  the  backslash  is         cial  meaning  is  faulted.  Otherwise,  like  Perl,  the  backslash is
2619         ignored. (Perl can be made to issue a warning.)         ignored. (Perl can be made to issue a warning.)
2620    
2621         (d)  If  PCRE_UNGREEDY is set, the greediness of the repetition quanti-         (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2622         fiers is inverted, that is, by default they are not greedy, but if fol-         fiers is inverted, that is, by default they are not greedy, but if fol-
2623         lowed by a question mark they are.         lowed by a question mark they are.
2624    
2625         (e) PCRE_ANCHORED can be used at matching time to force a pattern to be         (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2626         tried only at the first matching position in the subject string.         tried only at the first matching position in the subject string.
2627    
2628         (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and  PCRE_NO_AUTO_CAP-         (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2629         TURE options for pcre_exec() have no Perl equivalents.         TURE options for pcre_exec() have no Perl equivalents.
2630    
2631         (g)  The (?R), (?number), and (?P>name) constructs allows for recursive         (g) The callout facility is PCRE-specific.
        pattern matching (Perl can do  this  using  the  (?p{code})  construct,  
        which PCRE cannot support.)  
2632    
2633         (h)  PCRE supports named capturing substrings, using the Python syntax.         (h) The partial matching facility is PCRE-specific.
2634    
2635           (i) Patterns compiled by PCRE can be saved and re-used at a later time,
2636           even on different hosts that have the other endianness.
2637    
2638         (i) PCRE supports the possessive quantifier  "++"  syntax,  taken  from         (j)  The  alternative  matching function (pcre_dfa_exec()) matches in a
2639         Sun's Java package.         different way and is not Perl-compatible.
2640    
        (j) The (R) condition, for testing recursion, is a PCRE extension.  
2641    
2642         (k) The callout facility is PCRE-specific.  AUTHOR
2643    
2644         (l) The partial matching facility is PCRE-specific.         Philip Hazel
2645           University Computing Service
2646           Cambridge CB2 3QH, England.
2647    
        (m) Patterns compiled by PCRE can be saved and re-used at a later time,  
        even on different hosts that have the other endianness.  
2648    
2649         (n) The alternative matching function (pcre_dfa_exec())  matches  in  a  REVISION
        different way and is not Perl-compatible.  
2650    
2651  Last updated: 06 June 2006         Last updated: 06 March 2007
2652  Copyright (c) 1997-2006 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
2653  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2654    
2655    
# Line 2545  PCRE REGULAR EXPRESSION DETAILS Line 2685  PCRE REGULAR EXPRESSION DETAILS
2685         function, and how it differs from the normal function, are discussed in         function, and how it differs from the normal function, are discussed in
2686         the pcrematching page.         the pcrematching page.
2687    
2688    
2689    CHARACTERS AND METACHARACTERS
2690    
2691         A regular expression is a pattern that is  matched  against  a  subject         A regular expression is a pattern that is  matched  against  a  subject
2692         string  from  left  to right. Most characters stand for themselves in a         string  from  left  to right. Most characters stand for themselves in a
2693         pattern, and match the corresponding characters in the  subject.  As  a         pattern, and match the corresponding characters in the  subject.  As  a
# Line 2569  PCRE REGULAR EXPRESSION DETAILS Line 2712  PCRE REGULAR EXPRESSION DETAILS
2712    
2713         There are two different sets of metacharacters: those that  are  recog-         There are two different sets of metacharacters: those that  are  recog-
2714         nized  anywhere in the pattern except within square brackets, and those         nized  anywhere in the pattern except within square brackets, and those
2715         that are recognized in square brackets. Outside  square  brackets,  the         that are recognized within square brackets.  Outside  square  brackets,
2716         metacharacters are as follows:         the metacharacters are as follows:
2717    
2718           \      general escape character with several uses           \      general escape character with several uses
2719           ^      assert start of string (or line, in multiline mode)           ^      assert start of string (or line, in multiline mode)
# Line 2692  BACKSLASH Line 2835  BACKSLASH
2835    
2836         Inside a character class, or if the decimal number is  greater  than  9         Inside a character class, or if the decimal number is  greater  than  9
2837         and  there have not been that many capturing subpatterns, PCRE re-reads         and  there have not been that many capturing subpatterns, PCRE re-reads
2838         up to three octal digits following the backslash, ane uses them to gen-         up to three octal digits following the backslash, and uses them to gen-
2839         erate  a data character. Any subsequent digits stand for themselves. In         erate  a data character. Any subsequent digits stand for themselves. In
2840         non-UTF-8 mode, the value of a character specified  in  octal  must  be         non-UTF-8 mode, the value of a character specified  in  octal  must  be
2841         less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For         less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
# Line 2719  BACKSLASH Line 2862  BACKSLASH
2862         All the sequences that define a single character value can be used both         All the sequences that define a single character value can be used both
2863         inside and outside character classes. In addition, inside  a  character         inside and outside character classes. In addition, inside  a  character
2864         class,  the  sequence \b is interpreted as the backspace character (hex         class,  the  sequence \b is interpreted as the backspace character (hex
2865         08), and the sequence \X is interpreted as the character "X". Outside a         08), and the sequences \R and \X are interpreted as the characters  "R"
2866         character class, these sequences have different meanings (see below).         and  "X", respectively. Outside a character class, these sequences have
2867           different meanings (see below).
2868    
2869       Absolute and relative back references
2870    
2871           The sequence \g followed by a positive or negative  number,  optionally
2872           enclosed  in  braces,  is  an absolute or relative back reference. Back
2873           references are discussed later, following the discussion  of  parenthe-
2874           sized subpatterns.
2875    
2876     Generic character types     Generic character types
2877    
2878         The  third  use of backslash is for specifying generic character types.         Another use of backslash is for specifying generic character types. The
2879         The following are always recognized:         following are always recognized:
2880    
2881           \d     any decimal digit           \d     any decimal digit
2882           \D     any character that is not a decimal digit           \D     any character that is not a decimal digit
# Line 2762  BACKSLASH Line 2913  BACKSLASH
2913         code  character  property support is available. The use of locales with         code  character  property support is available. The use of locales with
2914         Unicode is discouraged.         Unicode is discouraged.
2915    
2916       Newline sequences
2917    
2918           Outside a character class, the escape sequence \R matches  any  Unicode
2919           newline sequence. This is an extension to Perl. In non-UTF-8 mode \R is
2920           equivalent to the following:
2921    
2922             (?>\r\n|\n|\x0b|\f|\r|\x85)
2923    
2924           This is an example of an "atomic group", details  of  which  are  given
2925           below.  This particular group matches either the two-character sequence
2926           CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
2927           U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
2928           return, U+000D), or NEL (next line, U+0085). The two-character sequence
2929           is treated as a single unit that cannot be split.
2930    
2931           In  UTF-8  mode, two additional characters whose codepoints are greater
2932           than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
2933           rator,  U+2029).   Unicode character property support is not needed for
2934           these characters to be recognized.
2935    
2936           Inside a character class, \R matches the letter "R".
2937    
2938     Unicode character properties     Unicode character properties
2939    
2940         When PCRE is built with Unicode character property support, three addi-         When PCRE is built with Unicode character property support, three addi-
# Line 2788  BACKSLASH Line 2961  BACKSLASH
2961         Those that are not part of an identified script are lumped together  as         Those that are not part of an identified script are lumped together  as
2962         "Common". The current list of scripts is:         "Common". The current list of scripts is:
2963    
2964         Arabic,  Armenian,  Bengali,  Bopomofo, Braille, Buginese, Buhid, Cana-         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
2965         dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic,  Deseret,         Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
2966         Devanagari,  Ethiopic,  Georgian,  Glagolitic, Gothic, Greek, Gujarati,         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
2967         Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,  Inherited,  Kannada,         Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
2968         Katakana,  Kharoshthi,  Khmer,  Lao, Latin, Limbu, Linear_B, Malayalam,         gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,
2969         Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya,         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
2970         Osmanya,  Runic,  Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag-         Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
2971         banwa,  Tai_Le,  Tamil,  Telugu,  Thaana,  Thai,   Tibetan,   Tifinagh,         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
2972         Ugaritic, Yi.         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
2973    
2974         Each  character has exactly one general category property, specified by         Each  character has exactly one general category property, specified by
2975         a two-letter abbreviation. For compatibility with Perl, negation can be         a two-letter abbreviation. For compatibility with Perl, negation can be
# Line 2889  BACKSLASH Line 3062  BACKSLASH
3062    
3063     Simple assertions     Simple assertions
3064    
3065         The fourth use of backslash is for certain simple assertions. An asser-         The  final use of backslash is for certain simple assertions. An asser-
3066         tion specifies a condition that has to be met at a particular point  in         tion specifies a condition that has to be met at a particular point  in
3067         a  match, without consuming any characters from the subject string. The         a  match, without consuming any characters from the subject string. The
3068         use of subpatterns for more complicated assertions is described  below.         use of subpatterns for more complicated assertions is described  below.
# Line 2897  BACKSLASH Line 3070  BACKSLASH
3070    
3071           \b     matches at a word boundary           \b     matches at a word boundary
3072           \B     matches when not at a word boundary           \B     matches when not at a word boundary
3073           \A     matches at start of subject           \A     matches at the start of the subject
3074           \Z     matches at end of subject or before newline at end           \Z     matches at the end of the subject
3075           \z     matches at end of subject                   also matches before a newline at the end of the subject
3076           \G     matches at first matching position in subject           \z     matches only at the end of the subject
3077             \G     matches at the first matching position in the subject
3078    
3079         These  assertions may not appear in character classes (but note that \b         These  assertions may not appear in character classes (but note that \b
3080         has a different meaning, namely the backspace character, inside a char-         has a different meaning, namely the backspace character, inside a char-
# Line 2997  FULL STOP (PERIOD, DOT) Line 3171  FULL STOP (PERIOD, DOT)
3171         Outside a character class, a dot in the pattern matches any one charac-         Outside a character class, a dot in the pattern matches any one charac-
3172         ter in the subject string except (by default) a character  that  signi-         ter in the subject string except (by default) a character  that  signi-
3173         fies  the  end  of  a line. In UTF-8 mode, the matched character may be         fies  the  end  of  a line. In UTF-8 mode, the matched character may be
3174         more than one byte long. When a line ending  is  defined  as  a  single         more than one byte long.
        character  (CR  or LF), dot never matches that character; when the two-  
        character sequence CRLF is used, dot does not match CR if it is immedi-  
        ately  followed by LF, but otherwise it matches all characters (includ-  
        ing isolated CRs and LFs).  
   
        The behaviour of dot with regard to newlines can  be  changed.  If  the  
        PCRE_DOTALL  option  is  set,  a dot matches any one character, without  
        exception. If newline is defined as the two-character sequence CRLF, it  
        takes two dots to match it.  
3175    
3176         The  handling of dot is entirely independent of the handling of circum-         When a line ending is defined as a single character, dot never  matches
3177         flex and dollar, the only relationship being  that  they  both  involve         that  character; when the two-character sequence CRLF is used, dot does
3178           not match CR if it is immediately followed  by  LF,  but  otherwise  it
3179           matches  all characters (including isolated CRs and LFs). When any Uni-
3180           code line endings are being recognized, dot does not match CR or LF  or
3181           any of the other line ending characters.
3182    
3183           The  behaviour  of  dot  with regard to newlines can be changed. If the
3184           PCRE_DOTALL option is set, a dot matches  any  one  character,  without
3185           exception. If the two-character sequence CRLF is present in the subject
3186           string, it takes two dots to match it.
3187    
3188           The handling of dot is entirely independent of the handling of  circum-
3189           flex  and  dollar,  the  only relationship being that they both involve
3190         newlines. Dot has no special meaning in a character class.         newlines. Dot has no special meaning in a character class.
3191    
3192    
3193  MATCHING A SINGLE BYTE  MATCHING A SINGLE BYTE
3194    
3195         Outside a character class, the escape sequence \C matches any one byte,         Outside a character class, the escape sequence \C matches any one byte,
3196         both in and out of UTF-8 mode. Unlike a dot, it always matches  CR  and         both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any
3197         LF.  The feature is provided in Perl in order to match individual bytes         line-ending characters. The feature is provided in  Perl  in  order  to
3198         in UTF-8 mode.  Because it breaks up UTF-8 characters  into  individual         match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3199         bytes,  what remains in the string may be a malformed UTF-8 string. For         acters into individual bytes, what remains in the string may be a  mal-
3200         this reason, the \C escape sequence is best avoided.         formed  UTF-8  string.  For this reason, the \C escape sequence is best
3201           avoided.
3202    
3203         PCRE does not allow \C to appear in  lookbehind  assertions  (described         PCRE does not allow \C to appear in  lookbehind  assertions  (described
3204         below),  because  in UTF-8 mode this would make it impossible to calcu-         below),  because  in UTF-8 mode this would make it impossible to calcu-
# Line 3067  SQUARE BRACKETS AND CHARACTER CLASSES Line 3245  SQUARE BRACKETS AND CHARACTER CLASSES
3245         PCRE  is  compiled  with Unicode property support as well as with UTF-8         PCRE  is  compiled  with Unicode property support as well as with UTF-8
3246         support.         support.
3247    
3248         Characters that might indicate  line  breaks  (CR  and  LF)  are  never         Characters that might indicate line breaks are  never  treated  in  any
3249         treated  in  any  special way when matching character classes, whatever         special  way  when  matching  character  classes,  whatever line-ending
3250         line-ending sequence is in use, and whatever setting of the PCRE_DOTALL         sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
3251         and PCRE_MULTILINE options is used. A class such as [^a] always matches         PCRE_MULTILINE options is used. A class such as [^a] always matches one
3252         one of these characters.         of these characters.
3253    
3254         The minus (hyphen) character can be used to specify a range of  charac-         The minus (hyphen) character can be used to specify a range of  charac-
3255         ters  in  a  character  class.  For  example,  [d-m] matches any letter         ters  in  a  character  class.  For  example,  [d-m] matches any letter
# Line 3203  INTERNAL OPTION SETTING Line 3381  INTERNAL OPTION SETTING
3381         PCRE extracts it into the global options (and it will therefore show up         PCRE extracts it into the global options (and it will therefore show up
3382         in data extracted by the pcre_fullinfo() function).         in data extracted by the pcre_fullinfo() function).
3383    
3384         An option change within a subpattern affects only that part of the cur-         An  option  change  within a subpattern (see below for a description of
3385         rent pattern that follows it, so         subpatterns) affects only that part of the current pattern that follows
3386           it, so
3387    
3388           (a(?i)b)c           (a(?i)b)c
3389    
3390         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3391         used).   By  this means, options can be made to have different settings         used).  By this means, options can be made to have  different  settings
3392         in different parts of the pattern. Any changes made in one  alternative         in  different parts of the pattern. Any changes made in one alternative
3393         do  carry  on  into subsequent branches within the same subpattern. For         do carry on into subsequent branches within the  same  subpattern.  For
3394         example,         example,
3395    
3396           (a(?i)b|c)           (a(?i)b|c)
3397    
3398         matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the         matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
3399         first  branch  is  abandoned before the option setting. This is because         first branch is abandoned before the option setting.  This  is  because
3400         the effects of option settings happen at compile time. There  would  be         the  effects  of option settings happen at compile time. There would be
3401         some very weird behaviour otherwise.         some very weird behaviour otherwise.
3402    
3403         The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA         The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
3404         can be changed in the same way as the Perl-compatible options by  using         can  be changed in the same way as the Perl-compatible options by using
3405         the characters J, U and X respectively.         the characters J, U and X respectively.
3406    
3407    
# Line 3235  SUBPATTERNS Line 3414  SUBPATTERNS
3414    
3415           cat(aract|erpillar|)           cat(aract|erpillar|)
3416    
3417         matches one of the words "cat", "cataract", or  "caterpillar".  Without         matches  one  of the words "cat", "cataract", or "caterpillar". Without
3418         the  parentheses,  it  would  match "cataract", "erpillar" or the empty         the parentheses, it would match  "cataract",  "erpillar"  or  an  empty
3419         string.         string.
3420    
3421         2. It sets up the subpattern as  a  capturing  subpattern.  This  means         2.  It  sets  up  the  subpattern as a capturing subpattern. This means
3422         that,  when  the  whole  pattern  matches,  that portion of the subject         that, when the whole pattern  matches,  that  portion  of  the  subject
3423         string that matched the subpattern is passed back to the caller via the         string that matched the subpattern is passed back to the caller via the
3424         ovector  argument  of pcre_exec(). Opening parentheses are counted from         ovector argument of pcre_exec(). Opening parentheses are  counted  from
3425         left to right (starting from 1) to obtain  numbers  for  the  capturing         left  to  right  (starting  from 1) to obtain numbers for the capturing
3426         subpatterns.         subpatterns.
3427    
3428         For  example,  if the string "the red king" is matched against the pat-         For example, if the string "the red king" is matched against  the  pat-
3429         tern         tern
3430    
3431           the ((red|white) (king|queen))           the ((red|white) (king|queen))
# Line 3254  SUBPATTERNS Line 3433  SUBPATTERNS
3433         the captured substrings are "red king", "red", and "king", and are num-         the captured substrings are "red king", "red", and "king", and are num-
3434         bered 1, 2, and 3, respectively.         bered 1, 2, and 3, respectively.
3435    
3436         The  fact  that  plain  parentheses  fulfil two functions is not always         The fact that plain parentheses fulfil  two  functions  is  not  always
3437         helpful.  There are often times when a grouping subpattern is  required         helpful.   There are often times when a grouping subpattern is required
3438         without  a capturing requirement. If an opening parenthesis is followed         without a capturing requirement. If an opening parenthesis is  followed
3439         by a question mark and a colon, the subpattern does not do any  captur-         by  a question mark and a colon, the subpattern does not do any captur-
3440         ing,  and  is  not  counted when computing the number of any subsequent         ing, and is not counted when computing the  number  of  any  subsequent
3441         capturing subpatterns. For example, if the string "the white queen"  is         capturing  subpatterns. For example, if the string "the white queen" is
3442         matched against the pattern         matched against the pattern
3443    
3444           the ((?:red|white) (king|queen))           the ((?:red|white) (king|queen))
3445    
3446         the captured substrings are "white queen" and "queen", and are numbered         the captured substrings are "white queen" and "queen", and are numbered
3447         1 and 2. The maximum number of capturing subpatterns is 65535, and  the         1 and 2. The maximum number of capturing subpatterns is 65535.
        maximum  depth  of  nesting of all subpatterns, both capturing and non-  
        capturing, is 200.  
3448    
3449         As a convenient shorthand, if any option settings are required  at  the         As  a  convenient shorthand, if any option settings are required at the
3450         start  of  a  non-capturing  subpattern,  the option letters may appear         start of a non-capturing subpattern,  the  option  letters  may  appear
3451         between the "?" and the ":". Thus the two patterns         between the "?" and the ":". Thus the two patterns
3452    
3453           (?i:saturday|sunday)           (?i:saturday|sunday)
3454           (?:(?i)saturday|sunday)           (?:(?i)saturday|sunday)
3455    
3456         match exactly the same set of strings. Because alternative branches are         match exactly the same set of strings. Because alternative branches are
3457         tried  from  left  to right, and options are not reset until the end of         tried from left to right, and options are not reset until  the  end  of
3458         the subpattern is reached, an option setting in one branch does  affect         the  subpattern is reached, an option setting in one branch does affect
3459         subsequent  branches,  so  the above patterns match "SUNDAY" as well as         subsequent branches, so the above patterns match "SUNDAY"  as  well  as
3460         "Saturday".         "Saturday".
3461    
3462    
3463  NAMED SUBPATTERNS  NAMED SUBPATTERNS
3464    
3465         Identifying capturing parentheses by number is simple, but  it  can  be         Identifying  capturing  parentheses  by number is simple, but it can be
3466         very  hard  to keep track of the numbers in complicated regular expres-         very hard to keep track of the numbers in complicated  regular  expres-
3467         sions. Furthermore, if an  expression  is  modified,  the  numbers  may         sions.  Furthermore,  if  an  expression  is  modified, the numbers may
3468         change.  To help with this difficulty, PCRE supports the naming of sub-         change. To help with this difficulty, PCRE supports the naming of  sub-
3469         patterns, something that Perl  does  not  provide.  The  Python  syntax         patterns. This feature was not added to Perl until release 5.10. Python
3470         (?P<name>...)  is  used. References to capturing parentheses from other         had the feature earlier, and PCRE introduced it at release  4.0,  using
3471         parts of the pattern, such as  backreferences,  recursion,  and  condi-         the  Python syntax. PCRE now supports both the Perl and the Python syn-
3472         tions, can be made by name as well as by number.         tax.
3473    
3474         Names  consist  of  up  to  32 alphanumeric characters and underscores.         In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)
3475         Named capturing parentheses are still  allocated  numbers  as  well  as         or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References
3476         names. The PCRE API provides function calls for extracting the name-to-         to capturing parentheses from other parts of the pattern, such as back-
3477         number translation table from a compiled pattern. There is also a  con-         references,  recursion,  and conditions, can be made by name as well as
3478         venience function for extracting a captured substring by name.         by number.
3479    
3480           Names consist of up to  32  alphanumeric  characters  and  underscores.
3481           Named  capturing  parentheses  are  still  allocated numbers as well as
3482           names, exactly as if the names were not present. The PCRE API  provides
3483           function calls for extracting the name-to-number translation table from
3484           a compiled pattern. There is also a convenience function for extracting
3485           a captured substring by name.
3486    
3487         By  default, a name must be unique within a pattern, but it is possible         By  default, a name must be unique within a pattern, but it is possible
3488         to relax this constraint by setting the PCRE_DUPNAMES option at compile         to relax this constraint by setting the PCRE_DUPNAMES option at compile
# Line 3308  NAMED SUBPATTERNS Line 3492  NAMED SUBPATTERNS
3492         both cases you want to extract the abbreviation. This pattern (ignoring         both cases you want to extract the abbreviation. This pattern (ignoring
3493         the line breaks) does the job:         the line breaks) does the job:
3494    
3495           (?P<DN>Mon|Fri|Sun)(?:day)?|           (?<DN>Mon|Fri|Sun)(?:day)?|
3496           (?P<DN>Tue)(?:sday)?|           (?<DN>Tue)(?:sday)?|
3497           (?P<DN>Wed)(?:nesday)?|           (?<DN>Wed)(?:nesday)?|
3498           (?P<DN>Thu)(?:rsday)?|           (?<DN>Thu)(?:rsday)?|
3499           (?P<DN>Sat)(?:urday)?           (?<DN>Sat)(?:urday)?
3500    
3501         There  are  five capturing substrings, but only one is ever set after a         There  are  five capturing substrings, but only one is ever set after a
3502         match.  The convenience  function  for  extracting  the  data  by  name         match.  The convenience  function  for  extracting  the  data  by  name
3503         returns  the  substring  for  the first, and in this example, the only,         returns  the  substring  for  the first (and in this example, the only)
3504         subpattern of that name that matched.  This  saves  searching  to  find         subpattern of that name that matched.  This  saves  searching  to  find
3505         which  numbered  subpattern  it  was. If you make a reference to a non-         which  numbered  subpattern  it  was. If you make a reference to a non-
3506         unique named subpattern from elsewhere in the  pattern,  the  one  that         unique named subpattern from elsewhere in the  pattern,  the  one  that
# Line 3331  REPETITION Line 3515  REPETITION
3515         following items:         following items:
3516    
3517           a literal data character           a literal data character
3518           the . metacharacter           the dot metacharacter
3519           the \C escape sequence           the \C escape sequence
3520           the \X escape sequence (in UTF-8 mode with Unicode properties)           the \X escape sequence (in UTF-8 mode with Unicode properties)
3521             the \R escape sequence
3522           an escape such as \d that matches a single character           an escape such as \d that matches a single character
3523           a character class           a character class
3524           a back reference (see next section)           a back reference (see next section)
# Line 3373  REPETITION Line 3558  REPETITION
3558         The quantifier {0} is permitted, causing the expression to behave as if         The quantifier {0} is permitted, causing the expression to behave as if
3559         the previous item and the quantifier were not present.         the previous item and the quantifier were not present.
3560    
3561         For  convenience  (and  historical compatibility) the three most common         For  convenience, the three most common quantifiers have single-charac-
3562         quantifiers have single-character abbreviations:         ter abbreviations:
3563    
3564           *    is equivalent to {0,}           *    is equivalent to {0,}
3565           +    is equivalent to {1,}           +    is equivalent to {1,}
# Line 3426  REPETITION Line 3611  REPETITION
3611         which matches one digit by preference, but can match two if that is the         which matches one digit by preference, but can match two if that is the
3612         only way the rest of the pattern matches.         only way the rest of the pattern matches.
3613    
3614         If the PCRE_UNGREEDY option is set (an option which is not available in         If the PCRE_UNGREEDY option is set (an option that is not available  in
3615         Perl),  the  quantifiers are not greedy by default, but individual ones         Perl),  the  quantifiers are not greedy by default, but individual ones
3616         can be made greedy by following them with a  question  mark.  In  other         can be made greedy by following them with a  question  mark.  In  other
3617         words, it inverts the default behaviour.         words, it inverts the default behaviour.
# Line 3437  REPETITION Line 3622  REPETITION
3622         minimum or maximum.         minimum or maximum.
3623    
3624         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
3625         alent  to Perl's /s) is set, thus allowing the . to match newlines, the         alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
3626         pattern is implicitly anchored, because whatever follows will be  tried         the pattern is implicitly anchored, because whatever  follows  will  be
3627         against  every character position in the subject string, so there is no         tried  against every character position in the subject string, so there
3628         point in retrying the overall match at any position  after  the  first.         is no point in retrying the overall match at  any  position  after  the
3629         PCRE normally treats such a pattern as though it were preceded by \A.         first.  PCRE  normally treats such a pattern as though it were preceded
3630           by \A.
3631    
3632         In  cases  where  it  is known that the subject string contains no new-         In cases where it is known that the subject  string  contains  no  new-
3633         lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-         lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
3634         mization, or alternatively using ^ to indicate anchoring explicitly.         mization, or alternatively using ^ to indicate anchoring explicitly.
3635    
3636         However,  there is one situation where the optimization cannot be used.         However, there is one situation where the optimization cannot be  used.
3637         When .*  is inside capturing parentheses that  are  the  subject  of  a         When  .*   is  inside  capturing  parentheses that are the subject of a
3638         backreference  elsewhere in the pattern, a match at the start may fail,         backreference elsewhere in the pattern, a match at the start  may  fail
3639         and a later one succeed. Consider, for example:         where a later one succeeds. Consider, for example:
3640    
3641           (.*)abc\1           (.*)abc\1
3642    
3643         If the subject is "xyz123abc123" the match point is the fourth  charac-         If  the subject is "xyz123abc123" the match point is the fourth charac-
3644         ter. For this reason, such a pattern is not implicitly anchored.         ter. For this reason, such a pattern is not implicitly anchored.
3645    
3646         When a capturing subpattern is repeated, the value captured is the sub-         When a capturing subpattern is repeated, the value captured is the sub-
# Line 3463  REPETITION Line 3649  REPETITION
3649           (tweedle[dume]{3}\s*)+           (tweedle[dume]{3}\s*)+
3650    
3651         has matched "tweedledum tweedledee" the value of the captured substring         has matched "tweedledum tweedledee" the value of the captured substring
3652         is  "tweedledee".  However,  if there are nested capturing subpatterns,         is "tweedledee". However, if there are  nested  capturing  subpatterns,
3653         the corresponding captured values may have been set in previous  itera-         the  corresponding captured values may have been set in previous itera-
3654         tions. For example, after         tions. For example, after
3655    
3656           /(a|(b))+/           /(a|(b))+/
# Line 3474  REPETITION Line 3660  REPETITION
3660    
3661  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
3662    
3663         With both maximizing and minimizing repetition, failure of what follows         With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
3664         normally causes the repeated item to be re-evaluated to see if  a  dif-         repetition,  failure  of what follows normally causes the repeated item
3665         ferent number of repeats allows the rest of the pattern to match. Some-         to be re-evaluated to see if a different number of repeats  allows  the
3666         times it is useful to prevent this, either to change the nature of  the         rest  of  the pattern to match. Sometimes it is useful to prevent this,
3667         match,  or  to  cause it fail earlier than it otherwise might, when the         either to change the nature of the match, or to cause it  fail  earlier
3668         author of the pattern knows there is no point in carrying on.         than  it otherwise might, when the author of the pattern knows there is
3669           no point in carrying on.
3670    
3671         Consider, for example, the pattern \d+foo when applied to  the  subject         Consider, for example, the pattern \d+foo when applied to  the  subject
3672         line         line
# Line 3493  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE Line 3680  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE
3680         the  means for specifying that once a subpattern has matched, it is not         the  means for specifying that once a subpattern has matched, it is not
3681         to be re-evaluated in this way.         to be re-evaluated in this way.
3682    
3683         If we use atomic grouping for the previous example, the  matcher  would         If we use atomic grouping for the previous example, the  matcher  gives
3684         give up immediately on failing to match "foo" the first time. The nota-         up  immediately  on failing to match "foo" the first time. The notation
3685         tion is a kind of special parenthesis, starting with  (?>  as  in  this         is a kind of special parenthesis, starting with (?> as in this example:
        example:  
3686    
3687           (?>\d+)foo           (?>\d+)foo
3688    
# Line 3528  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE Line 3714  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE
3714         Possessive  quantifiers  are  always  greedy;  the   setting   of   the         Possessive  quantifiers  are  always  greedy;  the   setting   of   the
3715         PCRE_UNGREEDY option is ignored. They are a convenient notation for the         PCRE_UNGREEDY option is ignored. They are a convenient notation for the
3716         simpler forms of atomic group. However, there is no difference  in  the         simpler forms of atomic group. However, there is no difference  in  the
3717         meaning  or  processing  of  a possessive quantifier and the equivalent         meaning  of  a  possessive  quantifier and the equivalent atomic group,
3718         atomic group.         though there may be a performance  difference;  possessive  quantifiers
3719           should be slightly faster.
3720         The possessive quantifier syntax is an extension to  the  Perl  syntax.  
3721         Jeffrey  Friedl originated the idea (and the name) in the first edition         The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
3722         of his book.  Mike McCloskey liked it, so implemented it when he  built         tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
3723         Sun's Java package, and PCRE copied it from there.         edition of his book. Mike McCloskey liked it, so implemented it when he
3724           built Sun's Java package, and PCRE copied it from there. It  ultimately
3725         When  a  pattern  contains an unlimited repeat inside a subpattern that         found its way into Perl at release 5.10.
3726         can itself be repeated an unlimited number of  times,  the  use  of  an  
3727         atomic  group  is  the  only way to avoid some failing matches taking a         PCRE has an optimization that automatically "possessifies" certain sim-
3728           ple pattern constructs. For example, the sequence  A+B  is  treated  as
3729           A++B  because  there is no point in backtracking into a sequence of A's
3730           when B must follow.
3731    
3732           When a pattern contains an unlimited repeat inside  a  subpattern  that
3733           can  itself  be  repeated  an  unlimited number of times, the use of an
3734           atomic group is the only way to avoid some  failing  matches  taking  a
3735         very long time indeed. The pattern         very long time indeed. The pattern
3736    
3737           (\D+|<\d+>)*[!?]           (\D+|<\d+>)*[!?]
3738    
3739         matches an unlimited number of substrings that either consist  of  non-         matches  an  unlimited number of substrings that either consist of non-
3740         digits,  or  digits  enclosed in <>, followed by either ! or ?. When it         digits, or digits enclosed in <>, followed by either ! or  ?.  When  it
3741         matches, it runs quickly. However, if it is applied to         matches, it runs quickly. However, if it is applied to
3742    
3743           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
3744    
3745         it takes a long time before reporting  failure.  This  is  because  the         it  takes  a  long  time  before reporting failure. This is because the
3746         string  can be divided between the internal \D+ repeat and the external         string can be divided between the internal \D+ repeat and the  external
3747         * repeat in a large number of ways, and all  have  to  be  tried.  (The         *  repeat  in  a  large  number of ways, and all have to be tried. (The
3748         example  uses  [!?]  rather than a single character at the end, because         example uses [!?] rather than a single character at  the  end,  because
3749         both PCRE and Perl have an optimization that allows  for  fast  failure         both  PCRE  and  Perl have an optimization that allows for fast failure
3750         when  a single character is used. They remember the last single charac-         when a single character is used. They remember the last single  charac-
3751         ter that is required for a match, and fail early if it is  not  present         ter  that  is required for a match, and fail early if it is not present
3752         in  the  string.)  If  the pattern is changed so that it uses an atomic         in the string.) If the pattern is changed so that  it  uses  an  atomic
3753         group, like this:         group, like this:
3754    
3755           ((?>\D+)|<\d+>)*[!?]           ((?>\D+)|<\d+>)*[!?]
3756    
3757         sequences of non-digits cannot be broken, and failure happens  quickly.         sequences  of non-digits cannot be broken, and failure happens quickly.
3758    
3759    
3760  BACK REFERENCES  BACK REFERENCES
3761    
3762         Outside a character class, a backslash followed by a digit greater than         Outside a character class, a backslash followed by a digit greater than
3763         0 (and possibly further digits) is a back reference to a capturing sub-         0 (and possibly further digits) is a back reference to a capturing sub-
3764         pattern  earlier  (that is, to its left) in the pattern, provided there         pattern earlier (that is, to its left) in the pattern,  provided  there
3765         have been that many previous capturing left parentheses.         have been that many previous capturing left parentheses.
3766    
3767         However, if the decimal number following the backslash is less than 10,         However, if the decimal number following the backslash is less than 10,
3768         it  is  always  taken  as a back reference, and causes an error only if         it is always taken as a back reference, and causes  an  error  only  if
3769         there are not that many capturing left parentheses in the  entire  pat-         there  are  not that many capturing left parentheses in the entire pat-
3770         tern.  In  other words, the parentheses that are referenced need not be         tern. In other words, the parentheses that are referenced need  not  be
3771         to the left of the reference for numbers less than 10. A "forward  back         to  the left of the reference for numbers less than 10. A "forward back
3772         reference"  of  this  type can make sense when a repetition is involved         reference" of this type can make sense when a  repetition  is  involved
3773         and the subpattern to the right has participated in an  earlier  itera-         and  the  subpattern to the right has participated in an earlier itera-
3774         tion.         tion.
3775    
3776         It is not possible to have a numerical "forward back reference" to sub-         It is not possible to have a numerical "forward back  reference"  to  a
3777         pattern whose number is 10 or more. However, a back  reference  to  any         subpattern  whose  number  is  10  or  more using this syntax because a
3778         subpattern  is  possible  using named parentheses (see below). See also         sequence such as \50 is interpreted as a character  defined  in  octal.
3779         the subsection entitled "Non-printing  characters"  above  for  further         See the subsection entitled "Non-printing characters" above for further
3780         details of the handling of digits following a backslash.         details of the handling of digits following a backslash.  There  is  no
3781           such  problem  when named parentheses are used. A back reference to any
3782           subpattern is possible using named parentheses (see below).
3783    
3784           Another way of avoiding the ambiguity inherent in  the  use  of  digits
3785           following a backslash is to use the \g escape sequence, which is a fea-
3786           ture introduced in Perl 5.10. This escape must be followed by  a  posi-
3787           tive  or  a negative number, optionally enclosed in braces. These exam-
3788           ples are all identical:
3789    
3790             (ring), \1
3791             (ring), \g1
3792             (ring), \g{1}
3793    
3794           A positive number specifies an absolute reference without the ambiguity
3795           that  is  present  in  the older syntax. It is also useful when literal
3796           digits follow the reference. A negative number is a relative reference.
3797           Consider this example:
3798    
3799             (abc(def)ghi)\g{-1}
3800    
3801           The sequence \g{-1} is a reference to the most recently started captur-
3802           ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,
3803           \g{-2} would be equivalent to \1. The use of relative references can be
3804           helpful in long patterns, and also in  patterns  that  are  created  by
3805           joining together fragments that contain references within themselves.
3806    
3807         A  back  reference matches whatever actually matched the capturing sub-         A  back  reference matches whatever actually matched the capturing sub-
3808         pattern in the current subject string, rather  than  anything  matching         pattern in the current subject string, rather  than  anything  matching
# Line 3603  BACK REFERENCES Line 3821  BACK REFERENCES
3821         matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the         matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
3822         original capturing subpattern is matched caselessly.         original capturing subpattern is matched caselessly.
3823    
3824         Back references to named subpatterns use the Python  syntax  (?P=name).         Back references to named subpatterns use the Perl  syntax  \k<name>  or
3825         We could rewrite the above example as follows:         \k'name'  or  the  Python  syntax (?P=name). We could rewrite the above
3826           example in either of the following ways:
3827    
3828             (?<p1>(?i)rah)\s+\k<p1>
3829           (?P<p1>(?i)rah)\s+(?P=p1)           (?P<p1>(?i)rah)\s+(?P=p1)
3830    
3831         A  subpattern  that  is  referenced  by  name may appear in the pattern         A subpattern that is referenced by  name  may  appear  in  the  pattern
3832         before or after the reference.         before or after the reference.
3833    
3834         There may be more than one back reference to the same subpattern. If  a         There  may be more than one back reference to the same subpattern. If a
3835         subpattern  has  not actually been used in a particular match, any back         subpattern has not actually been used in a particular match,  any  back
3836         references to it always fail. For example, the pattern         references to it always fail. For example, the pattern
3837    
3838           (a|(bc))\2           (a|(bc))\2
3839    
3840         always fails if it starts to match "a" rather than "bc". Because  there         always  fails if it starts to match "a" rather than "bc". Because there
3841         may  be  many  capturing parentheses in a pattern, all digits following         may be many capturing parentheses in a pattern,  all  digits  following
3842         the backslash are taken as part of a potential back  reference  number.         the  backslash  are taken as part of a potential back reference number.
3843         If the pattern continues with a digit character, some delimiter must be         If the pattern continues with a digit character, some delimiter must be
3844         used to terminate the back reference. If the  PCRE_EXTENDED  option  is         used  to  terminate  the back reference. If the PCRE_EXTENDED option is
3845         set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-         set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-
3846         ments" below) can be used.         ments" below) can be used.
3847    
3848         A back reference that occurs inside the parentheses to which it  refers         A  back reference that occurs inside the parentheses to which it refers
3849         fails  when  the subpattern is first used, so, for example, (a\1) never         fails when the subpattern is first used, so, for example,  (a\1)  never
3850         matches.  However, such references can be useful inside  repeated  sub-         matches.   However,  such references can be useful inside repeated sub-
3851         patterns. For example, the pattern         patterns. For example, the pattern
3852    
3853           (a|b\1)+           (a|b\1)+
3854    
3855         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
3856         ation of the subpattern,  the  back  reference  matches  the  character         ation  of  the  subpattern,  the  back  reference matches the character
3857         string  corresponding  to  the previous iteration. In order for this to         string corresponding to the previous iteration. In order  for  this  to
3858         work, the pattern must be such that the first iteration does  not  need         work,  the  pattern must be such that the first iteration does not need
3859         to  match the back reference. This can be done using alternation, as in         to match the back reference. This can be done using alternation, as  in
3860         the example above, or by a quantifier with a minimum of zero.         the example above, or by a quantifier with a minimum of zero.
3861    
3862    
3863  ASSERTIONS  ASSERTIONS
3864    
3865         An assertion is a test on the characters  following  or  preceding  the         An  assertion  is  a  test on the characters following or preceding the
3866         current  matching  point that does not actually consume any characters.         current matching point that does not actually consume  any  characters.
3867         The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are         The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are
3868         described above.         described above.
3869    
3870         More  complicated  assertions  are  coded as subpatterns. There are two         More complicated assertions are coded as  subpatterns.  There  are  two
3871         kinds: those that look ahead of the current  position  in  the  subject         kinds:  those  that  look  ahead of the current position in the subject
3872         string,  and  those  that  look  behind  it. An assertion subpattern is         string, and those that look  behind  it.  An  assertion  subpattern  is
3873         matched in the normal way, except that it does not  cause  the  current         matched  in  the  normal way, except that it does not cause the current
3874         matching position to be changed.         matching position to be changed.
3875    
3876         Assertion  subpatterns  are  not  capturing subpatterns, and may not be         Assertion subpatterns are not capturing subpatterns,  and  may  not  be
3877         repeated, because it makes no sense to assert the  same  thing  several         repeated,  because  it  makes no sense to assert the same thing several
3878         times.  If  any kind of assertion contains capturing subpatterns within         times. If any kind of assertion contains capturing  subpatterns  within
3879         it, these are counted for the purposes of numbering the capturing  sub-         it,  these are counted for the purposes of numbering the capturing sub-
3880         patterns in the whole pattern.  However, substring capturing is carried         patterns in the whole pattern.  However, substring capturing is carried
3881         out only for positive assertions, because it does not  make  sense  for         out  only  for  positive assertions, because it does not make sense for
3882         negative assertions.         negative assertions.
3883    
3884     Lookahead assertions     Lookahead assertions
# Line 3668  ASSERTIONS Line 3888  ASSERTIONS
3888    
3889           \w+(?=;)           \w+(?=;)
3890    
3891         matches a word followed by a semicolon, but does not include the  semi-         matches  a word followed by a semicolon, but does not include the semi-
3892         colon in the match, and         colon in the match, and
3893    
3894           foo(?!bar)           foo(?!bar)
3895    
3896         matches  any  occurrence  of  "foo" that is not followed by "bar". Note         matches any occurrence of "foo" that is not  followed  by  "bar".  Note
3897         that the apparently similar pattern         that the apparently similar pattern
3898    
3899           (?!foo)bar           (?!foo)bar
3900    
3901         does not find an occurrence of "bar"  that  is  preceded  by  something         does  not  find  an  occurrence  of "bar" that is preceded by something
3902         other  than "foo"; it finds any occurrence of "bar" whatsoever, because         other than "foo"; it finds any occurrence of "bar" whatsoever,  because
3903         the assertion (?!foo) is always true when the next three characters are         the assertion (?!foo) is always true when the next three characters are
3904         "bar". A lookbehind assertion is needed to achieve the other effect.         "bar". A lookbehind assertion is needed to achieve the other effect.
3905    
3906         If you want to force a matching failure at some point in a pattern, the         If you want to force a matching failure at some point in a pattern, the
3907         most convenient way to do it is  with  (?!)  because  an  empty  string         most  convenient  way  to  do  it  is with (?!) because an empty string
3908         always  matches, so an assertion that requires there not to be an empty         always matches, so an assertion that requires there not to be an  empty
3909         string must always fail.         string must always fail.
3910    
3911     Lookbehind assertions     Lookbehind assertions
3912    
3913         Lookbehind assertions start with (?<= for positive assertions and  (?<!         Lookbehind  assertions start with (?<= for positive assertions and (?<!
3914         for negative assertions. For example,         for negative assertions. For example,
3915    
3916           (?<!foo)bar           (?<!foo)bar
3917    
3918         does  find  an  occurrence  of "bar" that is not preceded by "foo". The         does find an occurrence of "bar" that is not  preceded  by  "foo".  The
3919         contents of a lookbehind assertion are restricted  such  that  all  the         contents  of  a  lookbehind  assertion are restricted such that all the
3920         strings it matches must have a fixed length. However, if there are sev-         strings it matches must have a fixed length. However, if there are sev-
3921         eral top-level alternatives, they do not all  have  to  have  the  same         eral  top-level  alternatives,  they  do  not all have to have the same
3922         fixed length. Thus         fixed length. Thus
3923    
3924           (?<=bullock|donkey)           (?<=bullock|donkey)
# Line 3707  ASSERTIONS Line 3927  ASSERTIONS
3927    
3928           (?<!dogs?|cats?)           (?<!dogs?|cats?)
3929    
3930         causes  an  error at compile time. Branches that match different length         causes an error at compile time. Branches that match  different  length
3931         strings are permitted only at the top level of a lookbehind  assertion.         strings  are permitted only at the top level of a lookbehind assertion.
3932         This  is  an  extension  compared  with  Perl (at least for 5.8), which         This is an extension compared with  Perl  (at  least  for  5.8),  which
3933         requires all branches to match the same length of string. An  assertion         requires  all branches to match the same length of string. An assertion
3934         such as         such as
3935    
3936           (?<=ab(c|de))           (?<=ab(c|de))
3937    
3938         is  not  permitted,  because  its single top-level branch can match two         is not permitted, because its single top-level  branch  can  match  two
3939         different lengths, but it is acceptable if rewritten to  use  two  top-         different  lengths,  but  it is acceptable if rewritten to use two top-
3940         level branches:         level branches:
3941    
3942           (?<=abc|abde)           (?<=abc|abde)
3943    
3944         The  implementation  of lookbehind assertions is, for each alternative,         The implementation of lookbehind assertions is, for  each  alternative,
3945         to temporarily move the current position back by the  fixed  width  and         to  temporarily  move the current position back by the fixed length and
3946         then try to match. If there are insufficient characters before the cur-         then try to match. If there are insufficient characters before the cur-
3947         rent position, the match is deemed to fail.         rent position, the assertion fails.
3948    
3949         PCRE does not allow the \C escape (which matches a single byte in UTF-8         PCRE does not allow the \C escape (which matches a single byte in UTF-8
3950         mode)  to appear in lookbehind assertions, because it makes it impossi-         mode) to appear in lookbehind assertions, because it makes it  impossi-
3951         ble to calculate the length of the lookbehind. The \X escape, which can         ble  to  calculate the length of the lookbehind. The \X and \R escapes,
3952         match different numbers of bytes, is also not permitted.         which can match different numbers of bytes, are also not permitted.
3953    
3954         Atomic  groups can be used in conjunction with lookbehind assertions to         Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
3955         specify efficient matching at the end of the subject string. Consider a         assertions  to  specify  efficient  matching  at the end of the subject
3956         simple pattern such as         string. Consider a simple pattern such as
3957    
3958           abcd$           abcd$
3959    
3960         when  applied  to  a  long string that does not match. Because matching         when applied to a long string that does  not  match.  Because  matching
3961         proceeds from left to right, PCRE will look for each "a" in the subject         proceeds from left to right, PCRE will look for each "a" in the subject
3962         and  then  see  if what follows matches the rest of the pattern. If the         and then see if what follows matches the rest of the  pattern.  If  the
3963         pattern is specified as         pattern is specified as
3964    
3965           ^.*abcd$           ^.*abcd$
3966    
3967         the initial .* matches the entire string at first, but when this  fails         the  initial .* matches the entire string at first, but when this fails
3968         (because there is no following "a"), it backtracks to match all but the         (because there is no following "a"), it backtracks to match all but the
3969         last character, then all but the last two characters, and so  on.  Once         last  character,  then all but the last two characters, and so on. Once
3970         again  the search for "a" covers the entire string, from right to left,         again the search for "a" covers the entire string, from right to  left,
3971         so we are no better off. However, if the pattern is written as         so we are no better off. However, if the pattern is written as
3972    
          ^(?>.*)(?<=abcd)  
   
        or, equivalently, using the possessive quantifier syntax,  
   
3973           ^.*+(?<=abcd)           ^.*+(?<=abcd)
3974    
3975         there can be no backtracking for the .* item; it  can  match  only  the         there  can  be  no backtracking for the .*+ item; it can match only the
3976         entire  string.  The subsequent lookbehind assertion does a single test         entire string. The subsequent lookbehind assertion does a  single  test
3977         on the last four characters. If it fails, the match fails  immediately.         on  the last four characters. If it fails, the match fails immediately.
3978         For  long  strings, this approach makes a significant difference to the         For long strings, this approach makes a significant difference  to  the
3979         processing time.         processing time.
3980    
3981     Using multiple assertions     Using multiple assertions
# Line 3768  ASSERTIONS Line 3984  ASSERTIONS
3984    
3985           (?<=\d{3})(?<!999)foo           (?<=\d{3})(?<!999)foo
3986    
3987         matches "foo" preceded by three digits that are not "999". Notice  that         matches  "foo" preceded by three digits that are not "999". Notice that
3988         each  of  the  assertions is applied independently at the same point in         each of the assertions is applied independently at the  same  point  in
3989         the subject string. First there is a  check  that  the  previous  three         the  subject  string.  First  there  is a check that the previous three
3990         characters  are  all  digits,  and  then there is a check that the same         characters are all digits, and then there is  a  check  that  the  same
3991         three characters are not "999".  This pattern does not match "foo" pre-         three characters are not "999".  This pattern does not match "foo" pre-
3992         ceded  by  six  characters,  the first of which are digits and the last         ceded by six characters, the first of which are  digits  and  the  last
3993         three of which are not "999". For example, it  doesn't  match  "123abc-         three  of  which  are not "999". For example, it doesn't match "123abc-
3994         foo". A pattern to do that is         foo". A pattern to do that is
3995    
3996           (?<=\d{3}...)(?<!999)foo           (?<=\d{3}...)(?<!999)foo
3997    
3998         This  time  the  first assertion looks at the preceding six characters,         This time the first assertion looks at the  preceding  six  characters,
3999         checking that the first three are digits, and then the second assertion         checking that the first three are digits, and then the second assertion
4000         checks that the preceding three characters are not "999".         checks that the preceding three characters are not "999".
4001    
# Line 3787  ASSERTIONS Line 4003  ASSERTIONS
4003    
4004           (?<=(?<!foo)bar)baz           (?<=(?<!foo)bar)baz
4005    
4006         matches  an occurrence of "baz" that is preceded by "bar" which in turn         matches an occurrence of "baz" that is preceded by "bar" which in  turn
4007         is not preceded by "foo", while         is not preceded by "foo", while
4008    
4009           (?<=\d{3}(?!999)...)foo           (?<=\d{3}(?!999)...)foo
4010    
4011         is another pattern that matches "foo" preceded by three digits and  any         is  another pattern that matches "foo" preceded by three digits and any
4012         three characters that are not "999".         three characters that are not "999".
4013    
4014    
4015  CONDITIONAL SUBPATTERNS  CONDITIONAL SUBPATTERNS
4016    
4017         It  is possible to cause the matching process to obey a subpattern con-         It is possible to cause the matching process to obey a subpattern  con-
4018         ditionally or to choose between two alternative subpatterns,  depending         ditionally  or to choose between two alternative subpatterns, depending
4019         on  the result of an assertion, or whether a previous capturing subpat-         on the result of an assertion, or whether a previous capturing  subpat-
4020         tern matched or not. The two possible forms of  conditional  subpattern         tern  matched  or not. The two possible forms of conditional subpattern
4021         are         are
4022    
4023           (?(condition)yes-pattern)           (?(condition)yes-pattern)
4024           (?(condition)yes-pattern|no-pattern)           (?(condition)yes-pattern|no-pattern)
4025    
4026         If  the  condition is satisfied, the yes-pattern is used; otherwise the         If the condition is satisfied, the yes-pattern is used;  otherwise  the
4027         no-pattern (if present) is used. If there are more  than  two  alterna-         no-pattern  (if  present)  is used. If there are more than two alterna-
4028         tives in the subpattern, a compile-time error occurs.         tives in the subpattern, a compile-time error occurs.
4029    
4030         There are three kinds of condition. If the text between the parentheses         There are four kinds of condition: references  to  subpatterns,  refer-
4031         consists of a sequence of digits, or a sequence of alphanumeric charac-         ences to recursion, a pseudo-condition called DEFINE, and assertions.
4032         ters  and underscores, the condition is satisfied if the capturing sub-  
4033         pattern of that number or name has previously matched. There is a  pos-     Checking for a used subpattern by number
4034         sible  ambiguity here, because subpattern names may consist entirely of  
4035         digits. PCRE looks first for a named subpattern; if it cannot find  one         If  the  text between the parentheses consists of a sequence of digits,
4036         and  the text consists entirely of digits, it looks for a subpattern of         the condition is true if the capturing subpattern of  that  number  has
4037         that number, which must be greater than zero.  Using  subpattern  names         previously matched.
        that consist entirely of digits is not recommended.  
4038    
4039         Consider  the  following  pattern, which contains non-significant white         Consider  the  following  pattern, which contains non-significant white
4040         space to make it more readable (assume the PCRE_EXTENDED option) and to         space to make it more readable (assume the PCRE_EXTENDED option) and to
# Line 3836  CONDITIONAL SUBPATTERNS Line 4051  CONDITIONAL SUBPATTERNS
4051         tern  is  executed  and  a  closing parenthesis is required. Otherwise,         tern  is  executed  and  a  closing parenthesis is required. Otherwise,
4052         since no-pattern is not present, the  subpattern  matches  nothing.  In         since no-pattern is not present, the  subpattern  matches  nothing.  In
4053         other  words,  this  pattern  matches  a  sequence  of non-parentheses,         other  words,  this  pattern  matches  a  sequence  of non-parentheses,
4054         optionally enclosed in parentheses. Rewriting it to use a named subpat-         optionally enclosed in parentheses.
4055         tern gives this:  
4056       Checking for a used subpattern by name
4057    
4058           Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
4059           used  subpattern  by  name.  For compatibility with earlier versions of
4060           PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
4061           also  recognized. However, there is a possible ambiguity with this syn-
4062           tax, because subpattern names may  consist  entirely  of  digits.  PCRE
4063           looks  first for a named subpattern; if it cannot find one and the name
4064           consists entirely of digits, PCRE looks for a subpattern of  that  num-
4065           ber,  which must be greater than zero. Using subpattern names that con-
4066           sist entirely of digits is not recommended.
4067    
4068           (?P<OPEN> \( )?    [^()]+    (?(OPEN) \) )         Rewriting the above example to use a named subpattern gives this:
4069    
4070             (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )
4071    
4072    
4073       Checking for pattern recursion
4074    
4075         If the condition is the string (R), and there is no subpattern with the         If the condition is the string (R), and there is no subpattern with the
4076         name R, the condition is satisfied if a recursive call to  the  pattern         name  R, the condition is true if a recursive call to the whole pattern
4077         or  subpattern  has  been made. At "top level", the condition is false.         or any subpattern has been made. If digits or a name preceded by amper-
4078         This is a PCRE extension.  Recursive patterns are described in the next         sand follow the letter R, for example:
4079         section.  
4080             (?(R3)...) or (?(R&name)...)
4081    
4082           the  condition is true if the most recent recursion is into the subpat-
4083           tern whose number or name is given. This condition does not  check  the
4084           entire recursion stack.
4085    
4086           At  "top  level", all these recursion test conditions are false. Recur-
4087           sive patterns are described below.
4088    
4089       Defining subpatterns for use by reference only
4090    
4091           If the condition is the string (DEFINE), and  there  is  no  subpattern
4092           with  the  name  DEFINE,  the  condition is always false. In this case,
4093           there may be only one alternative  in  the  subpattern.  It  is  always
4094           skipped  if  control  reaches  this  point  in the pattern; the idea of
4095           DEFINE is that it can be used to define "subroutines" that can be  ref-
4096           erenced  from elsewhere. (The use of "subroutines" is described below.)
4097           For example, a pattern to match an IPv4 address could be  written  like
4098           this (ignore whitespace and line breaks):
4099    
4100             (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4101             \b (?&byte) (\.(?&byte)){3} \b
4102    
4103           The  first part of the pattern is a DEFINE group inside which a another
4104           group named "byte" is defined. This matches an individual component  of
4105           an  IPv4  address  (a number less than 256). When matching takes place,
4106           this part of the pattern is skipped because DEFINE acts  like  a  false
4107           condition.
4108    
4109           The rest of the pattern uses references to the named group to match the
4110           four dot-separated components of an IPv4 address, insisting on  a  word
4111           boundary at each end.
4112    
4113         If  the  condition  is  not  a sequence of digits or (R), it must be an     Assertion conditions
4114    
4115           If  the  condition  is  not  in any of the above formats, it must be an
4116         assertion.  This may be a positive or negative lookahead or  lookbehind         assertion.  This may be a positive or negative lookahead or  lookbehind
4117         assertion.  Consider  this  pattern,  again  containing non-significant         assertion.  Consider  this  pattern,  again  containing non-significant
4118         white space, and with the two alternatives on the second line:         white space, and with the two alternatives on the second line:
# Line 3882  RECURSIVE PATTERNS Line 4147  RECURSIVE PATTERNS
4147         unlimited nested parentheses. Without the use of  recursion,  the  best         unlimited nested parentheses. Without the use of  recursion,  the  best
4148         that  can  be  done  is  to use a pattern that matches up to some fixed         that  can  be  done  is  to use a pattern that matches up to some fixed
4149         depth of nesting. It is not possible to  handle  an  arbitrary  nesting         depth of nesting. It is not possible to  handle  an  arbitrary  nesting
4150         depth.  Perl  provides  a  facility  that allows regular expressions to         depth.
4151         recurse (amongst other things). It does this by interpolating Perl code  
4152         in the expression at run time, and the code can refer to the expression         For some time, Perl has provided a facility that allows regular expres-
4153         itself. A Perl pattern to solve the parentheses problem can be  created         sions to recurse (amongst other things). It does this by  interpolating
4154         like this:         Perl  code in the expression at run time, and the code can refer to the
4155           expression itself. A Perl pattern using code interpolation to solve the
4156           parentheses problem can be created like this:
4157    
4158           $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;           $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4159    
4160         The (?p{...}) item interpolates Perl code at run time, and in this case         The (?p{...}) item interpolates Perl code at run time, and in this case
4161         refers recursively to the pattern in which it appears. Obviously,  PCRE         refers recursively to the pattern in which it appears.
        cannot  support  the  interpolation  of Perl code. Instead, it supports  
        some special syntax for recursion of the entire pattern, and  also  for  
        individual subpattern recursion.  
4162    
4163         The  special item that consists of (? followed by a number greater than         Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4164           it  supports  special  syntax  for recursion of the entire pattern, and
4165           also for individual subpattern recursion.  After  its  introduction  in
4166           PCRE  and  Python,  this  kind of recursion was introduced into Perl at
4167           release 5.10.
4168    
4169           A special item that consists of (? followed by a  number  greater  than
4170         zero and a closing parenthesis is a recursive call of the subpattern of         zero and a closing parenthesis is a recursive call of the subpattern of
4171         the  given  number, provided that it occurs inside that subpattern. (If         the given number, provided that it occurs inside that  subpattern.  (If
4172         not, it is a "subroutine" call, which is described  in  the  next  sec-         not,  it  is  a  "subroutine" call, which is described in the next sec-
4173         tion.)  The special item (?R) is a recursive call of the entire regular         tion.) The special item (?R) or (?0) is a recursive call of the  entire
4174         expression.         regular expression.
4175    
4176         A recursive subpattern call is always treated as an atomic group.  That         In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
4177         is,  once  it  has  matched some of the subject string, it is never re-         always treated as an atomic group. That is, once it has matched some of
4178         entered, even if it contains untried alternatives and there is a subse-         the subject string, it is never re-entered, even if it contains untried
4179         quent matching failure.         alternatives and there is a subsequent matching failure.
4180    
4181         This  PCRE  pattern  solves  the nested parentheses problem (assume the         This PCRE pattern solves the nested  parentheses  problem  (assume  the
4182         PCRE_EXTENDED option is set so that white space is ignored):         PCRE_EXTENDED option is set so that white space is ignored):
4183    
4184           \( ( (?>[^()]+) | (?R) )* \)           \( ( (?>[^()]+) | (?R) )* \)
4185    
4186         First it matches an opening parenthesis. Then it matches any number  of         First  it matches an opening parenthesis. Then it matches any number of
4187         substrings  which  can  either  be  a sequence of non-parentheses, or a         substrings which can either be a  sequence  of  non-parentheses,  or  a
4188         recursive match of the pattern itself (that is, a  correctly  parenthe-         recursive  match  of the pattern itself (that is, a correctly parenthe-
4189         sized substring).  Finally there is a closing parenthesis.         sized substring).  Finally there is a closing parenthesis.
4190    
4191         If  this  were  part of a larger pattern, you would not want to recurse         If this were part of a larger pattern, you would not  want  to  recurse
4192         the entire pattern, so instead you could use this:         the entire pattern, so instead you could use this:
4193    
4194           ( \( ( (?>[^()]+) | (?1) )* \) )           ( \( ( (?>[^()]+) | (?1) )* \) )
4195    
4196         We have put the pattern into parentheses, and caused the  recursion  to         We  have  put the pattern into parentheses, and caused the recursion to
4197         refer  to them instead of the whole pattern. In a larger pattern, keep-         refer to them instead of the whole pattern. In a larger pattern,  keep-
4198         ing track of parenthesis numbers can be tricky. It may be  more  conve-         ing  track  of parenthesis numbers can be tricky. It may be more conve-
4199         nient  to use named parentheses instead. For this, PCRE uses (?P>name),         nient to use named parentheses instead. The Perl  syntax  for  this  is
4200         which is an extension to the Python syntax that  PCRE  uses  for  named         (?&name);  PCRE's  earlier syntax (?P>name) is also supported. We could
4201         parentheses (Perl does not provide named parentheses). We could rewrite         rewrite the above example as follows:
4202         the above example as follows:  
4203             (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4204           (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )  
4205           If there is more than one subpattern with the same name,  the  earliest
4206         This particular example pattern contains nested unlimited repeats,  and         one  is used. This particular example pattern contains nested unlimited
4207         so  the  use of atomic grouping for matching strings of non-parentheses         repeats, and so the use of atomic grouping for matching strings of non-
4208         is important when applying the pattern to strings that  do  not  match.         parentheses  is  important when applying the pattern to strings that do
4209         For example, when this pattern is applied to         not match. For example, when this pattern is applied to
4210    
4211           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4212    
4213         it  yields "no match" quickly. However, if atomic grouping is not used,         it yields "no match" quickly. However, if atomic grouping is not  used,
4214         the match runs for a very long time indeed because there  are  so  many         the  match  runs  for a very long time indeed because there are so many
4215         different  ways  the  + and * repeats can carve up the subject, and all         different ways the + and * repeats can carve up the  subject,  and  all
4216         have to be tested before failure can be reported.         have to be tested before failure can be reported.
4217    
4218         At the end of a match, the values set for any capturing subpatterns are         At the end of a match, the values set for any capturing subpatterns are
4219         those from the outermost level of the recursion at which the subpattern         those from the outermost level of the recursion at which the subpattern
4220         value is set.  If you want to obtain  intermediate  values,  a  callout         value  is  set.   If  you want to obtain intermediate values, a callout
4221         function can be used (see the next section and the pcrecallout documen-         function can be used (see below and the pcrecallout documentation).  If
4222         tation). If the pattern above is matched against         the pattern above is matched against
4223    
4224           (ab(cd)ef)           (ab(cd)ef)
4225    
4226         the value for the capturing parentheses is  "ef",  which  is  the  last         the  value  for  the  capturing  parentheses is "ef", which is the last
4227         value  taken  on at the top level. If additional parentheses are added,         value taken on at the top level. If additional parentheses  are  added,
4228         giving         giving
4229    
4230           \( ( ( (?>[^()]+) | (?R) )* ) \)           \( ( ( (?>[^()]+) | (?R) )* ) \)
4231              ^                        ^              ^                        ^
4232              ^                        ^              ^                        ^
4233    
4234         the string they capture is "ab(cd)ef", the contents of  the  top  level         the  string  they  capture is "ab(cd)ef", the contents of the top level
4235         parentheses.  If there are more than 15 capturing parentheses in a pat-         parentheses. If there are more than 15 capturing parentheses in a  pat-
4236         tern, PCRE has to obtain extra memory to store data during a recursion,         tern, PCRE has to obtain extra memory to store data during a recursion,
4237         which  it  does  by  using pcre_malloc, freeing it via pcre_free after-         which it does by using pcre_malloc, freeing  it  via  pcre_free  after-
4238         wards. If  no  memory  can  be  obtained,  the  match  fails  with  the         wards.  If  no  memory  can  be  obtained,  the  match  fails  with the
4239         PCRE_ERROR_NOMEMORY error.         PCRE_ERROR_NOMEMORY error.
4240    
4241         Do  not  confuse  the (?R) item with the condition (R), which tests for         Do not confuse the (?R) item with the condition (R),  which  tests  for
4242         recursion.  Consider this pattern, which matches text in  angle  brack-         recursion.   Consider  this pattern, which matches text in angle brack-
4243         ets,  allowing for arbitrary nesting. Only digits are allowed in nested         ets, allowing for arbitrary nesting. Only digits are allowed in  nested
4244         brackets (that is, when recursing), whereas any characters are  permit-         brackets  (that is, when recursing), whereas any characters are permit-
4245         ted at the outer level.         ted at the outer level.
4246    
4247           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
4248    
4249         In  this  pattern, (?(R) is the start of a conditional subpattern, with         In this pattern, (?(R) is the start of a conditional  subpattern,  with
4250         two different alternatives for the recursive and  non-recursive  cases.         two  different  alternatives for the recursive and non-recursive cases.
4251         The (?R) item is the actual recursive call.         The (?R) item is the actual recursive call.
4252    
4253    
4254  SUBPATTERNS AS SUBROUTINES  SUBPATTERNS AS SUBROUTINES
4255    
4256         If the syntax for a recursive subpattern reference (either by number or         If the syntax for a recursive subpattern reference (either by number or
4257         by name) is used outside the parentheses to which it refers,  it  oper-         by  name)  is used outside the parentheses to which it refers, it oper-
4258         ates  like  a  subroutine in a programming language. An earlier example         ates like a subroutine in a programming language. The "called"  subpat-
4259           tern  may  be defined before or after the reference. An earlier example
4260         pointed out that the pattern         pointed out that the pattern
4261    
4262           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
# Line 3996  SUBPATTERNS AS SUBROUTINES Line 4267  SUBPATTERNS AS SUBROUTINES
4267           (sens|respons)e and (?1)ibility           (sens|respons)e and (?1)ibility
4268    
4269         is  used, it does match "sense and responsibility" as well as the other         is  used, it does match "sense and responsibility" as well as the other
4270         two strings. Such references, if given  numerically,  must  follow  the         two strings. Another example is  given  in  the  discussion  of  DEFINE
4271         subpattern  to which they refer. However, named references can refer to         above.
        later subpatterns.  
4272    
4273         Like recursive subpatterns, a "subroutine" call is always treated as an         Like recursive subpatterns, a "subroutine" call is always treated as an
4274         atomic  group. That is, once it has matched some of the subject string,         atomic group. That is, once it has matched some of the subject  string,
4275         it is never re-entered, even if it contains  untried  alternatives  and         it  is  never  re-entered, even if it contains untried alternatives and
4276         there is a subsequent matching failure.         there is a subsequent matching failure.
4277    
4278           When a subpattern is used as a subroutine, processing options  such  as
4279           case-independence are fixed when the subpattern is defined. They cannot
4280           be changed for different calls. For example, consider this pattern:
4281    
4282             (abc)(?i:(?1))
4283    
4284           It matches "abcabc". It does not match "abcABC" because the  change  of
4285           processing option does not affect the called subpattern.
4286    
4287    
4288  CALLOUTS  CALLOUTS
4289    
# Line 4040  CALLOUTS Line 4319  CALLOUTS
4319         gether. A complete description of the interface to the callout function         gether. A complete description of the interface to the callout function
4320         is given in the pcrecallout documentation.         is given in the pcrecallout documentation.
4321    
4322  Last updated: 06 June 2006  
4323  Copyright (c) 1997-2006 University of Cambridge.  SEE ALSO
4324    
4325           pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4326    
4327    
4328    AUTHOR
4329    
4330           Philip Hazel
4331           University Computing Service
4332           Cambridge CB2 3QH, England.
4333    
4334    
4335    REVISION
4336    
4337           Last updated: 06 March 2007
4338           Copyright (c) 1997-2007 University of Cambridge.
4339  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
4340    
4341    
# Line 4146  EXAMPLE OF PARTIAL MATCHING USING PCRETE Line 4440  EXAMPLE OF PARTIAL MATCHING USING PCRETE
4440         The first data string is matched  completely,  so  pcretest  shows  the         The first data string is matched  completely,  so  pcretest  shows  the
4441         matched  substrings.  The  remaining four strings do not match the com-         matched  substrings.  The  remaining four strings do not match the com-
4442         plete pattern, but the first two are partial matches.  The  same  test,         plete pattern, but the first two are partial matches.  The  same  test,
4443         using  DFA  matching (by means of the \D escape sequence), produces the         using  pcre_dfa_exec()  matching  (by means of the \D escape sequence),
4444         following output:         produces the following output:
4445    
4446             re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/             re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/
4447           data> 25jun04\P\D           data> 25jun04\P\D
# Line 4169  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe Line 4463  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe
4463    
4464         When a partial match has been found using pcre_dfa_exec(), it is possi-         When a partial match has been found using pcre_dfa_exec(), it is possi-
4465         ble to continue the match by  providing  additional  subject  data  and         ble to continue the match by  providing  additional  subject  data  and
4466         calling  pcre_dfa_exec() again with the PCRE_DFA_RESTART option and the         calling  pcre_dfa_exec()  again  with the same compiled regular expres-
4467         same working space (where details of the  previous  partial  match  are         sion, this time setting the PCRE_DFA_RESTART option. You must also pass
4468         stored).  Here  is  an  example  using  pcretest,  where  the \R escape         the  same working space as before, because this is where details of the
4469         sequence sets the PCRE_DFA_RESTART option and the  \D  escape  sequence         previous partial match are stored. Here is an example  using  pcretest,
4470         requests the use of pcre_dfa_exec():         using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
4471           \D are as above):
4472    
4473             re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/             re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/
4474           data> 23ja\P\D           data> 23ja\P\D
# Line 4181  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe Line 4476  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe
4476           data> n05\R\D           data> n05\R\D
4477            0: n05            0: n05
4478    
4479         The  first  call has "23ja" as the subject, and requests partial match-         The first call has "23ja" as the subject, and requests  partial  match-
4480         ing; the second call  has  "n05"  as  the  subject  for  the  continued         ing;  the  second  call  has  "n05"  as  the  subject for the continued
4481         (restarted)  match.   Notice  that when the match is complete, only the         (restarted) match.  Notice that when the match is  complete,  only  the
4482         last part is shown; PCRE does  not  retain  the  previously  partially-         last  part  is  shown;  PCRE  does not retain the previously partially-
4483         matched  string. It is up to the calling program to do that if it needs         matched string. It is up to the calling program to do that if it  needs
4484         to.         to.
4485    
4486         This facility can  be  used  to  pass  very  long  subject  strings  to         You  can  set  PCRE_PARTIAL  with  PCRE_DFA_RESTART to continue partial
4487         pcre_dfa_exec(). However, some care is needed for certain types of pat-         matching over multiple segments. This facility can be used to pass very
4488         tern.         long  subject  strings to pcre_dfa_exec(). However, some care is needed
4489           for certain types of pattern.
4490    
4491         1. If the pattern contains tests for the beginning or end  of  a  line,         1. If the pattern contains tests for the beginning or end  of  a  line,
4492         you  need  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-         you  need  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
# Line 4199  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe Line 4495  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe
4495    
4496         2.  If  the  pattern contains backward assertions (including \b or \B),         2.  If  the  pattern contains backward assertions (including \b or \B),
4497         you need to arrange for some overlap in the subject  strings  to  allow         you need to arrange for some overlap in the subject  strings  to  allow
4498         for  this.  For example, you could pass the subject in chunks that were         for  this.  For  example, you could pass the subject in chunks that are
4499         500 bytes long, but in a buffer of 700 bytes, with the starting  offset         500 bytes long, but in a buffer of 700 bytes, with the starting  offset
4500         set to 200 and the previous 200 bytes at the start of the buffer.         set to 200 and the previous 200 bytes at the start of the buffer.
4501    
# Line 4249  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe Line 4545  MULTI-SEGMENT MATCHING WITH pcre_dfa_exe
4545    
4546         where no string can be a partial match for both alternatives.         where no string can be a partial match for both alternatives.
4547    
4548  Last updated: 16 January 2006  
4549  Copyright (c) 1997-2006 University of Cambridge.  AUTHOR
4550    
4551           Philip Hazel
4552           University Computing Service
4553           Cambridge CB2 3QH, England.
4554    
4555    
4556    REVISION
4557    
4558           Last updated: 06 March 2007
4559           Copyright (c) 1997-2007 University of Cambridge.
4560  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
4561    
4562    
# Line 4361  COMPATIBILITY WITH DIFFERENT PCRE RELEAS Line 4667  COMPATIBILITY WITH DIFFERENT PCRE RELEAS
4667         makes  up  a  compiled pattern was changed for release 5.0. If you have         makes  up  a  compiled pattern was changed for release 5.0. If you have
4668         any saved patterns that were compiled with  previous  releases  (not  a         any saved patterns that were compiled with  previous  releases  (not  a
4669         facility  that  was  previously advertised), you will have to recompile         facility  that  was  previously advertised), you will have to recompile
4670         them for release 5.0. However, from now on, it should  be  possible  to         them for release 5.0 and above.
4671         make changes in a compatible manner.  
4672           If you have any saved patterns in UTF-8 mode that use  \p  or  \P  that
4673           were  compiled  with any release up to and including 6.4, you will have
4674           to recompile them for release 6.5 and above.
4675    
4676           All saved patterns from earlier releases must be recompiled for release
4677           7.0  or  higher,  because  there was an internal reorganization at that
4678           release.
4679    
4680    
4681    AUTHOR
4682    
4683           Philip Hazel
4684           University Computing Service
4685           Cambridge CB2 3QH, England.
4686    
4687    
4688         Notwithstanding the above, if you have any saved patterns in UTF-8 mode  REVISION
        that use \p or \P that were compiled with any release up to and includ-  
        ing 6.4, you will have to recompile them for release 6.5 and above.  
4689    
4690  Last updated: 01 February 2006         Last updated: 06 March 2007
4691  Copyright (c) 1997-2006 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
4692  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
4693    
4694    
# Line 4382  NAME Line 4701  NAME
4701    
4702  PCRE PERFORMANCE  PCRE PERFORMANCE
4703    
4704         Certain  items  that may appear in regular expression patterns are more         Two  aspects  of performance are discussed below: memory usage and pro-
4705         efficient than others. It is more efficient to use  a  character  class         cessing time. The way you express your pattern as a regular  expression
4706         like  [aeiou]  than  a set of alternatives such as (a|e|i|o|u). In gen-         can affect both of them.
4707         eral, the simplest construction that provides the required behaviour is  
4708         usually  the  most  efficient.  Jeffrey Friedl's book contains a lot of  
4709         useful general discussion  about  optimizing  regular  expressions  for  MEMORY USAGE
4710         efficient  performance. This document contains a few observations about  
4711         PCRE.         Patterns are compiled by PCRE into a reasonably efficient byte code, so
4712           that most simple patterns do not use much memory. However, there is one
4713           case where memory usage can be unexpectedly large. When a parenthesized
4714           subpattern has a quantifier with a minimum greater than 1 and/or a lim-
4715           ited  maximum,  the  whole subpattern is repeated in the compiled code.
4716           For example, the pattern
4717    
4718             (abc|def){2,4}
4719    
4720           is compiled as if it were
4721    
4722             (abc|def)(abc|def)((abc|def)(abc|def)?)?
4723    
4724           (Technical aside: It is done this way so that backtrack  points  within
4725           each of the repetitions can be independently maintained.)
4726    
4727           For  regular expressions whose quantifiers use only small numbers, this
4728           is not usually a problem. However, if the numbers are large,  and  par-
4729           ticularly  if  such repetitions are nested, the memory usage can become
4730           an embarrassment. For example, the very simple pattern
4731    
4732             ((ab){1,1000}c){1,3}
4733    
4734           uses 51K bytes when compiled. When PCRE is compiled  with  its  default
4735           internal  pointer  size of two bytes, the size limit on a compiled pat-
4736           tern is 64K, and this is reached with the above pattern  if  the  outer
4737           repetition is increased from 3 to 4. PCRE can be compiled to use larger
4738           internal pointers and thus handle larger compiled patterns, but  it  is
4739           better to try to rewrite your pattern to use less memory if you can.
4740    
4741           One  way  of reducing the memory usage for such patterns is to make use
4742           of PCRE's "subroutine" facility. Re-writing the above pattern as
4743    
4744             ((ab)(?2){0,999}c)(?1){0,2}
4745    
4746           reduces the memory requirements to 18K, and indeed it remains under 20K
4747           even  with the outer repetition increased to 100. However, this pattern
4748           is not exactly equivalent, because the "subroutine" calls  are  treated
4749           as  atomic groups into which there can be no backtracking if there is a
4750           subsequent matching failure. Therefore, PCRE cannot  do  this  kind  of
4751           rewriting  automatically.   Furthermore,  there is a noticeable loss of
4752           speed when executing the modified pattern. Nevertheless, if the  atomic
4753           grouping  is  not  a  problem and the loss of speed is acceptable, this
4754           kind of rewriting will allow you to process patterns that  PCRE  cannot
4755           otherwise handle.
4756    
4757    
4758    PROCESSING TIME
4759    
4760           Certain  items  in regular expression patterns are processed more effi-
4761           ciently than others. It is more efficient to use a character class like
4762           [aeiou]   than   a   set   of  single-character  alternatives  such  as
4763           (a|e|i|o|u). In general, the simplest construction  that  provides  the
4764           required behaviour is usually the most efficient. Jeffrey Friedl's book
4765           contains a lot of useful general discussion  about  optimizing  regular
4766           expressions  for  efficient  performance.  This document contains a few
4767           observations about PCRE.
4768    
4769         Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is         Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
4770         slow,  because PCRE has to scan a structure that contains data for over         slow,  because PCRE has to scan a structure that contains data for over
# Line 4423  PCRE PERFORMANCE Line 4798  PCRE PERFORMANCE
4798         take  a  long time to run when applied to a string that does not match.         take  a  long time to run when applied to a string that does not match.
4799         Consider the pattern fragment         Consider the pattern fragment
4800    
4801           (a+)*           ^(a+)*
4802    
4803         This can match "aaaa" in 33 different ways, and this  number  increases         This can match "aaaa" in 16 different ways, and this  number  increases
4804         very  rapidly  as the string gets longer. (The * repeat can match 0, 1,         very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
4805         2, 3, or 4 times, and for each of those  cases  other  than  0,  the  +         2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
4806         repeats  can  match  different numbers of times.) When the remainder of         repeats  can  match  different numbers of times.) When the remainder of
4807         the pattern is such that the entire match is going to fail, PCRE has in         the pattern is such that the entire match is going to fail, PCRE has in
4808         principle  to  try  every  possible  variation,  and  this  can take an         principle  to  try  every  possible  variation,  and  this  can take an
4809         extremely long time.         extremely long time, even for relatively short strings.
4810    
4811         An optimization catches some of the more simple cases such as         An optimization catches some of the more simple cases such as
4812    
# Line 4452  PCRE PERFORMANCE Line 4827  PCRE PERFORMANCE
4827         In many cases, the solution to this kind of performance issue is to use         In many cases, the solution to this kind of performance issue is to use
4828         an atomic group or a possessive quantifier.         an atomic group or a possessive quantifier.
4829    
4830  Last updated: 28 February 2005  
4831  Copyright (c) 1997-2005 University of Cambridge.  AUTHOR
4832    
4833           Philip Hazel
4834           University Computing Service
4835           Cambridge CB2 3QH, England.
4836    
4837    
4838    REVISION
4839    
4840           Last updated: 06 March 2007
4841           Copyright (c) 1997-2007 University of Cambridge.
4842  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
4843    
4844    
# Line 4666  MEMORY USAGE Line 5051  MEMORY USAGE
5051  AUTHOR  AUTHOR
5052    
5053         Philip Hazel         Philip Hazel
5054         University Computing Service,         University Computing Service
5055         Cambridge CB2 3QG, England.         Cambridge CB2 3QH, England.
5056    
5057    
5058    REVISION
5059    
5060  Last updated: 16 January 2006         Last updated: 06 March 2007
5061  Copyright (c) 1997-2006 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
5062  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5063    
5064    
# Line 4762  MATCHING INTERFACE Line 5150  MATCHING INTERFACE
5150              number of sub-patterns, "i"th captured sub-pattern is              number of sub-patterns, "i"th captured sub-pattern is
5151              ignored.              ignored.
5152    
5153           CAVEAT: An optional sub-pattern that does  not  exist  in  the  matched
5154           string  is  assigned  the  empty  string. Therefore, the following will
5155           return false (because the empty string is not a valid number):
5156    
5157              int number;
5158              pcrecpp::RE::FullMatch("abc", "[a-z]+(\d+)?", &number);
5159    
5160         The matching interface supports at most 16 arguments per call.  If  you         The matching interface supports at most 16 arguments per call.  If  you
5161         need    more,    consider    using    the    more   general   interface         need    more,    consider    using    the    more   general   interface
5162         pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.         pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
5163    
5164    
5165    QUOTING METACHARACTERS
5166    
5167           You can use the "QuoteMeta" operation to insert backslashes before  all
5168           potentially  meaningful  characters  in  a string. The returned string,
5169           used as a regular expression, will exactly match the original string.
5170    
5171             Example:
5172                string quoted = RE::QuoteMeta(unquoted);
5173    
5174           Note that it's legal to escape a character even if it  has  no  special
5175           meaning  in  a  regular expression -- so this function does that. (This
5176           also makes it identical to the perl function  of  the  same  name;  see
5177           "perldoc    -f    quotemeta".)    For   example,   "1.5-2.0?"   becomes
5178           "1\.5\-2\.0\?".
5179    
5180    
5181  PARTIAL MATCHES  PARTIAL MATCHES
5182    
5183         You can use the "PartialMatch" operation when you want the  pattern  to         You can use the "PartialMatch" operation when you want the  pattern  to
# Line 4978  REPLACING PARTS OF STRINGS Line 5389  REPLACING PARTS OF STRINGS
5389  AUTHOR  AUTHOR
5390    
5391         The C++ wrapper was contributed by Google Inc.         The C++ wrapper was contributed by Google Inc.
5392         Copyright (c) 2005 Google Inc.         Copyright (c) 2007 Google Inc.
5393    
5394    
5395    REVISION
5396    
5397           Last updated: 06 March 2007
5398  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5399    
5400    
# Line 5045  PCRE SAMPLE PROGRAM Line 5461  PCRE SAMPLE PROGRAM
5461    
5462         (for example) to the compile command to get round this problem.         (for example) to the compile command to get round this problem.
5463    
5464  Last updated: 09 September 2004  
5465  Copyright (c) 1997-2004 University of Cambridge.  AUTHOR
5466    
5467           Philip Hazel
5468           University Computing Service
5469           Cambridge CB2 3QH, England.
5470    
5471    
5472    REVISION
5473    
5474           Last updated: 06 March 2007
5475           Copyright (c) 1997-2007 University of Cambridge.
5476  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5477  PCRESTACK(3)                                                      PCRESTACK(3)  PCRESTACK(3)                                                      PCRESTACK(3)
5478    
# Line 5099  PCRE DISCUSSION OF STACK USAGE Line 5525  PCRE DISCUSSION OF STACK USAGE
5525         ter. For a long string, a lot of stack is required. Consider  now  this         ter. For a long string, a lot of stack is required. Consider  now  this
5526         rewritten pattern, which matches exactly the same strings:         rewritten pattern, which matches exactly the same strings:
5527    
5528           ([^<]++|<(?!inet))           ([^<]++|<(?!inet))+
5529    
5530         This  uses very much less stack, because runs of characters that do not         This  uses very much less stack, because runs of characters that do not
5531         contain "<" are "swallowed" in one item inside the parentheses.  Recur-         contain "<" are "swallowed" in one item inside the parentheses.  Recur-
# Line 5108  PCRE DISCUSSION OF STACK USAGE Line 5534  PCRE DISCUSSION OF STACK USAGE
5534         quantifier  is  used  to stop any backtracking into the runs of non-"<"         quantifier  is  used  to stop any backtracking into the runs of non-"<"
5535         characters, but that is not related to stack usage.         characters, but that is not related to stack usage.
5536    
5537           This example shows that one way of avoiding stack problems when  match-
5538           ing long subject strings is to write repeated parenthesized subpatterns
5539           to match more than one character whenever possible.
5540    
5541         In environments where stack memory is constrained, you  might  want  to         In environments where stack memory is constrained, you  might  want  to
5542         compile  PCRE to use heap memory instead of stack for remembering back-         compile  PCRE to use heap memory instead of stack for remembering back-
5543         up points. This makes it run a lot more slowly, however. Details of how         up points. This makes it run a lot more slowly, however. Details of how
5544         to do this are given in the pcrebuild documentation.         to do this are given in the pcrebuild documentation.
5545    
5546         In Unix-like environments, there is not often a problem with the stack,         In  Unix-like environments, there is not often a problem with the stack
5547         though the default limit on stack size varies from  system  to  system.         unless very long strings are involved,  though  the  default  limit  on
5548         Values  from 8Mb to 64Mb are common. You can find your default limit by         stack  size  varies  from system to system. Values from 8Mb to 64Mb are
5549         running the command:         common. You can find your default limit by running the command:
5550    
5551           ulimit -s           ulimit -s
5552    
5553         The effect of running out of stack is often SIGSEGV,  though  sometimes         Unfortunately, the effect of running out of  stack  is  often  SIGSEGV,
5554         an error message is given. You can normally increase the limit on stack         though  sometimes  a more explicit error message is given. You can nor-
5555         size by code such as this:         mally increase the limit on stack size by code such as this:
5556    
5557           struct rlimit rlim;           struct rlimit rlim;
5558           getrlimit(RLIMIT_STACK, &rlim);           getrlimit(RLIMIT_STACK, &rlim);
# Line 5144  PCRE DISCUSSION OF STACK USAGE Line 5574  PCRE DISCUSSION OF STACK USAGE
5574         recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you         recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you
5575         should  set  the  limit at 16000 recursions. A 64Mb stack, on the other         should  set  the  limit at 16000 recursions. A 64Mb stack, on the other
5576         hand, can support around 128000 recursions. The pcretest  test  program         hand, can support around 128000 recursions. The pcretest  test  program
5577         has  a command line option (-S) that can be used to increase its stack.         has a command line option (-S) that can be used to increase the size of
5578           its stack.
5579    
5580    
5581    AUTHOR
5582    
5583           Philip Hazel
5584           University Computing Service
5585           Cambridge CB2 3QH, England.
5586    
5587    
5588    REVISION
5589    
5590  Last updated: 29 June 2006         Last updated: 12 March 2007
5591  Copyright (c) 1997-2006 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
5592  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5593    
5594    

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