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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 535 by ph10, Thu Jun 3 19:18:24 2010 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 42  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 83  string with one of the following five se Line 93  string with one of the following five se
93    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
94    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
95  .sp  .sp
96  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
97  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
98  newline sequence, the pattern  newline sequence, the pattern
99  .sp  .sp
# Line 95  Perl-compatible, are recognized only at Line 105  Perl-compatible, are recognized only at
105  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
106  is used.  is used.
107  .P  .P
108  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
109  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
110  this can be changed; see the description of \eR in the section entitled  affect what the \eR escape sequence matches. By default, this is any Unicode
111    newline sequence, for Perl compatibility. However, this can be changed; see the
112    description of \eR in the section entitled
113  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
114  .\" </a>  .\" </a>
115  "Newline sequences"  "Newline sequences"
# Line 217  one of the following escape sequences th Line 229  one of the following escape sequences th
229    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
230    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
231    \et        tab (hex 09)    \et        tab (hex 09)
232    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
233    \exhh      character with hex code hh    \exhh      character with hex code hh
234    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
235  .sp  .sp
# Line 295  zero, because no more than three octal d Line 307  zero, because no more than three octal d
307  .P  .P
308  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
309  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
310  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
311  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
312  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
313  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
314  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
315  .  .
316  .  .
317  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 342  syntax for referencing a subpattern as a
342  later.  later.
343  .\"  .\"
344  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
345  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
346  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
347  .\" </a>  .\" </a>
348  subroutine  subroutine
# Line 341  subroutine Line 350  subroutine
350  call.  call.
351  .  .
352  .  .
353    .\" HTML <a name="genericchartypes"></a>
354  .SS "Generic character types"  .SS "Generic character types"
355  .rs  .rs
356  .sp  .sp
357  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
358  .sp  .sp
359    \ed     any decimal digit    \ed     any decimal digit
360    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 358  following are always recognized: Line 367  following are always recognized:
367    \ew     any "word" character    \ew     any "word" character
368    \eW     any "non-word" character    \eW     any "non-word" character
369  .sp  .sp
370  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
371  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
372    .\" HTML <a href="#fullstopdot">
373    .\" </a>
374    the "." metacharacter
375    .\"
376    when PCRE_DOTALL is not set.
377  .P  .P
378  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
379    of characters into two disjoint sets. Any given character matches one, and only
380    one, of each pair. The sequences can appear both inside and outside character
381  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
382  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
383  there is no character to match.  there is no character to match.
384  .P  .P
385  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 372  are HT (9), LF (10), FF (12), CR (13), a Line 388  are HT (9), LF (10), FF (12), CR (13), a
388  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
389  does.  does.
390  .P  .P
391  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
392  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
393  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
394  meanings from before UTF-8 support was available, mainly for efficiency  place (see
395  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
396  and \eW.  .\" </a>
397    "Locale support"
398    .\"
399    in the
400    .\" HREF
401    \fBpcreapi\fP
402    .\"
403    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
404    or "french" in Windows, some character codes greater than 128 are used for
405    accented letters, and these are then matched by \ew. The use of locales with
406    Unicode is discouraged.
407    .P
408    By default, in UTF-8 mode, characters with values greater than 128 never match
409    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
410    their original meanings from before UTF-8 support was available, mainly for
411    efficiency reasons. However, if PCRE is compiled with Unicode property support,
412    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
413    properties are used to determine character types, as follows:
414    .sp
415      \ed  any character that \ep{Nd} matches (decimal digit)
416      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
417      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
418    .sp
419    The upper case escapes match the inverse sets of characters. Note that \ed
420    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
421    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
422    \eB because they are defined in terms of \ew and \eW. Matching these sequences
423    is noticeably slower when PCRE_UCP is set.
424  .P  .P
425  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
426  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, which match only ASCII characters by default, these always
427  The horizontal space characters are:  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
428    set. The horizontal space characters are:
429  .sp  .sp
430    U+0009     Horizontal tab    U+0009     Horizontal tab
431    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 456  The vertical space characters are:
456    U+0085     Next line    U+0085     Next line
457    U+2028     Line separator    U+2028     Line separator
458    U+2029     Paragraph separator    U+2029     Paragraph separator
 .P  
 A "word" character is an underscore or any character less than 256 that is a  
 letter or digit. The definition of letters and digits is controlled by PCRE's  
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
459  .  .
460  .  .
461  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 468  one of the following sequences: Line 495  one of the following sequences:
495    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
496    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
497  .sp  .sp
498  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
499  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
500  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
501  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
502  pattern, and that they must be in upper case. If more than one of them is  pattern, and that they must be in upper case. If more than one of them is
503  present, the last one is used. They can be combined with a change of newline  present, the last one is used. They can be combined with a change of newline
504  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
505  .sp  .sp
506    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
507  .sp  .sp
508  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
509    a character class, \eR is treated as an unrecognized escape sequence, and so
510    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
511  .  .
512  .  .
513  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 525  The extra escape sequences are:
525    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
526  .sp  .sp
527  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
528  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
529  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
530  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
531  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
532    .\" </a>
533    next section).
534    .\"
535    Other Perl properties such as "InMusicalSymbols" are not currently supported by
536    PCRE. Note that \eP{Any} does not match any characters, so always causes a
537    match failure.
538  .P  .P
539  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
540  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 513  Those that are not part of an identified Line 548  Those that are not part of an identified
548  .P  .P
549  Arabic,  Arabic,
550  Armenian,  Armenian,
551    Avestan,
552  Balinese,  Balinese,
553    Bamum,
554  Bengali,  Bengali,
555  Bopomofo,  Bopomofo,
556  Braille,  Braille,
557  Buginese,  Buginese,
558  Buhid,  Buhid,
559  Canadian_Aboriginal,  Canadian_Aboriginal,
560    Carian,
561    Cham,
562  Cherokee,  Cherokee,
563  Common,  Common,
564  Coptic,  Coptic,
# Line 528  Cypriot, Line 567  Cypriot,
567  Cyrillic,  Cyrillic,
568  Deseret,  Deseret,
569  Devanagari,  Devanagari,
570    Egyptian_Hieroglyphs,
571  Ethiopic,  Ethiopic,
572  Georgian,  Georgian,
573  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 580  Hangul,
580  Hanunoo,  Hanunoo,
581  Hebrew,  Hebrew,
582  Hiragana,  Hiragana,
583    Imperial_Aramaic,
584  Inherited,  Inherited,
585    Inscriptional_Pahlavi,
586    Inscriptional_Parthian,
587    Javanese,
588    Kaithi,
589  Kannada,  Kannada,
590  Katakana,  Katakana,
591    Kayah_Li,
592  Kharoshthi,  Kharoshthi,
593  Khmer,  Khmer,
594  Lao,  Lao,
595  Latin,  Latin,
596    Lepcha,
597  Limbu,  Limbu,
598  Linear_B,  Linear_B,
599    Lisu,
600    Lycian,
601    Lydian,
602  Malayalam,  Malayalam,
603    Meetei_Mayek,
604  Mongolian,  Mongolian,
605  Myanmar,  Myanmar,
606  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 608  Nko,
608  Ogham,  Ogham,
609  Old_Italic,  Old_Italic,
610  Old_Persian,  Old_Persian,
611    Old_South_Arabian,
612    Old_Turkic,
613    Ol_Chiki,
614  Oriya,  Oriya,
615  Osmanya,  Osmanya,
616  Phags_Pa,  Phags_Pa,
617  Phoenician,  Phoenician,
618    Rejang,
619  Runic,  Runic,
620    Samaritan,
621    Saurashtra,
622  Shavian,  Shavian,
623  Sinhala,  Sinhala,
624    Sundanese,
625  Syloti_Nagri,  Syloti_Nagri,
626  Syriac,  Syriac,
627  Tagalog,  Tagalog,
628  Tagbanwa,  Tagbanwa,
629  Tai_Le,  Tai_Le,
630    Tai_Tham,
631    Tai_Viet,
632  Tamil,  Tamil,
633  Telugu,  Telugu,
634  Thaana,  Thaana,
# Line 576  Thai, Line 636  Thai,
636  Tibetan,  Tibetan,
637  Tifinagh,  Tifinagh,
638  Ugaritic,  Ugaritic,
639    Vai,
640  Yi.  Yi.
641  .P  .P
642  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
643  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
644  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
645  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
646  .P  .P
647  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
648  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 680  non-UTF-8 mode \eX matches any one chara Line 741  non-UTF-8 mode \eX matches any one chara
741  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
742  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
743  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
744  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
745    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
746    (*UCP).
747    .
748    .
749    .\" HTML <a name="extraprops"></a>
750    .SS PCRE's additional properties
751    .rs
752    .sp
753    As well as the standard Unicode properties described in the previous
754    section, PCRE supports four more that make it possible to convert traditional
755    escape sequences such as \ew and \es and POSIX character classes to use Unicode
756    properties. PCRE uses these non-standard, non-Perl properties internally when
757    PCRE_UCP is set. They are:
758    .sp
759      Xan   Any alphanumeric character
760      Xps   Any POSIX space character
761      Xsp   Any Perl space character
762      Xwd   Any Perl "word" character
763    .sp
764    Xan matches characters that have either the L (letter) or the N (number)
765    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
766    carriage return, and any other character that has the Z (separator) property.
767    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
768    same characters as Xan, plus underscore.
769  .  .
770  .  .
771  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 711  For example, when the pattern Line 796  For example, when the pattern
796    (foo)\eKbar    (foo)\eKbar
797  .sp  .sp
798  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
799    .P
800    Perl documents that the use of \eK within assertions is "not well defined". In
801    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
802    ignored in negative assertions.
803  .  .
804  .  .
805  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 824  The backslashed assertions are:
824    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
825    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
826  .sp  .sp
827  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
828  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
829    default it matches the corresponding literal character (for example, \eB
830    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
831    escape sequence" error is generated instead.
832  .P  .P
833  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
834  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
835  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
836  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
837  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
838  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
839  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
840    determines which it is. For example, the fragment \eba matches "a" at the start
841    of a word.
842  .P  .P
843  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
844  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 828  end of the subject in both modes, and if Line 922  end of the subject in both modes, and if
922  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
923  .  .
924  .  .
925  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
926    .SH "FULL STOP (PERIOD, DOT) AND \eN"
927  .rs  .rs
928  .sp  .sp
929  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 850  to match it. Line 945  to match it.
945  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
946  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
947  special meaning in a character class.  special meaning in a character class.
948    .P
949    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
950    set. In other words, it matches any one character except one that signifies the
951    end of a line.
952  .  .
953  .  .
954  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 876  the lookbehind. Line 975  the lookbehind.
975  .rs  .rs
976  .sp  .sp
977  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
978  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
979  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
980  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
981  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
982  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
# Line 944  characters in both cases. In UTF-8 mode, Line 1043  characters in both cases. In UTF-8 mode,
1043  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1044  property support.  property support.
1045  .P  .P
1046  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1047  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1048  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1049  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1050  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1051  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1052  .P  .P
1053  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1054  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 969  this notation. For example, Line 1068  this notation. For example,
1068    [01[:alpha:]%]    [01[:alpha:]%]
1069  .sp  .sp
1070  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1071  are  are:
1072  .sp  .sp
1073    alnum    letters and digits    alnum    letters and digits
1074    alpha    letters    alpha    letters
# Line 980  are Line 1079  are
1079    graph    printing characters, excluding space    graph    printing characters, excluding space
1080    lower    lower case letters    lower    lower case letters
1081    print    printing characters, including space    print    printing characters, including space
1082    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1083    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1084    upper    upper case letters    upper    upper case letters
1085    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1100  matches "1", "2", or any non-digit. PCRE
1100  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1101  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1102  .P  .P
1103  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1104  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1105    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1106    character properties are used. This is achieved by replacing the POSIX classes
1107    by other sequences, as follows:
1108    .sp
1109      [:alnum:]  becomes  \ep{Xan}
1110      [:alpha:]  becomes  \ep{L}
1111      [:blank:]  becomes  \eh
1112      [:digit:]  becomes  \ep{Nd}
1113      [:lower:]  becomes  \ep{Ll}
1114      [:space:]  becomes  \ep{Xps}
1115      [:upper:]  becomes  \ep{Lu}
1116      [:word:]   becomes  \ep{Xwd}
1117    .sp
1118    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1119    classes are unchanged, and match only characters with code points less than
1120    128.
1121  .  .
1122  .  .
1123  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1081  section entitled Line 1196  section entitled
1196  .\" </a>  .\" </a>
1197  "Newline sequences"  "Newline sequences"
1198  .\"  .\"
1199  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1200  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1201    PCRE_UTF8 and the PCRE_UCP options, respectively.
1202  .  .
1203  .  .
1204  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1163  stored. Line 1279  stored.
1279    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1280    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1281  .sp  .sp
1282  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1283  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1284  "defdef":  or "defdef":
1285  .sp  .sp
1286    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1287  .sp  .sp
1288  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1289  refers to the first one in the pattern with the given number. The following  refers to the first one in the pattern with the given number. The following
1290  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1291  .sp  .sp
1292    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1293  .sp  .sp
1294    If a
1295    .\" HTML <a href="#conditions">
1296    .\" </a>
1297    condition test
1298    .\"
1299    for a subpattern's having matched refers to a non-unique number, the test is
1300    true if any of the subpatterns of that number have matched.
1301  .P  .P
1302  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1303  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1304  .  .
1305  .  .
# Line 1189  if an expression is modified, the number Line 1312  if an expression is modified, the number
1312  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1313  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1314  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1315  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1316    have different names, but PCRE does not.
1317  .P  .P
1318  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1319  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1320  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1321  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1322  .\" </a>  .\" </a>
1323  backreferences,  back references,
1324  .\"  .\"
1325  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1326  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1340  extracting the name-to-number translatio
1340  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1341  .P  .P
1342  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1343  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1344  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1345  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1346  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1347  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1348    name, and in both cases you want to extract the abbreviation. This pattern
1349    (ignoring the line breaks) does the job:
1350  .sp  .sp
1351    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1352    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1360  subpattern, as described in the previous
1360  .P  .P
1361  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1362  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1363  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1364  make a reference to a non-unique named subpattern from elsewhere in the  .P
1365  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1366  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1367    used. In the absence of duplicate numbers (see the previous section) this is
1368    the one with the lowest number. If you use a named reference in a condition
1369    test (see the
1370    .\"
1371    .\" HTML <a href="#conditions">
1372    .\" </a>
1373    section about conditions
1374    .\"
1375    below), either to check whether a subpattern has matched, or to check for
1376    recursion, all subpatterns with the same name are tested. If the condition is
1377    true for any one of them, the overall condition is true. This is the same
1378    behaviour as testing by number. For further details of the interfaces for
1379    handling named subpatterns, see the
1380  .\" HREF  .\" HREF
1381  \fBpcreapi\fP  \fBpcreapi\fP
1382  .\"  .\"
1383  documentation.  documentation.
1384  .P  .P
1385  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1386  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1387  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1388    are given to subpatterns with the same number. However, you can give the same
1389    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1390  .  .
1391  .  .
1392  .SH REPETITION  .SH REPETITION
# Line 1263  items: Line 1404  items:
1404    a character class    a character class
1405    a back reference (see next section)    a back reference (see next section)
1406    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1407    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1408  .sp  .sp
1409  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1410  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1515  worth setting PCRE_DOTALL in order to ob
1515  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1516  .P  .P
1517  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1518  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1519  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1520  succeeds. Consider, for example:  succeeds. Consider, for example:
1521  .sp  .sp
# Line 1589  references to it always fail by default. Line 1730  references to it always fail by default.
1730  .sp  .sp
1731    (a|(bc))\e2    (a|(bc))\e2
1732  .sp  .sp
1733  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1734  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1735  unset value matches an empty string.  unset value matches an empty string.
1736  .P  .P
1737  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
# Line 1603  whitespace. Otherwise, the \eg{ syntax o Line 1744  whitespace. Otherwise, the \eg{ syntax o
1744  "Comments"  "Comments"
1745  .\"  .\"
1746  below) can be used.  below) can be used.
1747  .P  .
1748    .SS "Recursive back references"
1749    .rs
1750    .sp
1751  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1752  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1753  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1617  to the previous iteration. In order for Line 1761  to the previous iteration. In order for
1761  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1762  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1763  minimum of zero.  minimum of zero.
1764    .P
1765    Back references of this type cause the group that they reference to be treated
1766    as an
1767    .\" HTML <a href="#atomicgroup">
1768    .\" </a>
1769    atomic group.
1770    .\"
1771    Once the whole group has been matched, a subsequent matching failure cannot
1772    cause backtracking into the middle of the group.
1773  .  .
1774  .  .
1775  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1712  In some cases, the Perl 5.10 escape sequ Line 1865  In some cases, the Perl 5.10 escape sequ
1865  .\" </a>  .\" </a>
1866  (see above)  (see above)
1867  .\"  .\"
1868  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
1869  restriction.  restriction.
1870  .P  .P
1871  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1730  different numbers of bytes, are also not Line 1883  different numbers of bytes, are also not
1883  "Subroutine"  "Subroutine"
1884  .\"  .\"
1885  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1886  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1887  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1888  .\" </a>  .\" </a>
1889  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 1956  characters that are not "999".
1956  .sp  .sp
1957  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1958  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1959  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1960  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1961  .sp  .sp
1962    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1821  recursion, a pseudo-condition called DEF Line 1974  recursion, a pseudo-condition called DEF
1974  .sp  .sp
1975  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1976  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1977  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
1978  (see the earlier  (see the earlier
1979  .\"  .\"
1980  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1981  .\" </a>  .\" </a>
# Line 1874  Rewriting the above example to use a nam Line 2027  Rewriting the above example to use a nam
2027  .sp  .sp
2028    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2029  .sp  .sp
2030    If the name used in a condition of this kind is a duplicate, the test is
2031    applied to all subpatterns of the same name, and is true if any one of them has
2032    matched.
2033  .  .
2034  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2035  .rs  .rs
# Line 1887  letter R, for example: Line 2043  letter R, for example:
2043  .sp  .sp
2044  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2045  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2046  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2047    applied to all subpatterns of the same name, and is true if any one of them is
2048    the most recent recursion.
2049  .P  .P
2050  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2051  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2052  .\" </a>  .\" </a>
2053  Recursive patterns  The syntax for recursive patterns
2054  .\"  .\"
2055  are described below.  is described below.
2056  .  .
2057  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2058  .rs  .rs
# Line 1903  If the condition is the string (DEFINE), Line 2061  If the condition is the string (DEFINE),
2061  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2062  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2063  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2064  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
2065  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2066  .\" </a>  .\" </a>
2067  "subroutines"  "subroutines"
# Line 1980  this kind of recursion was subsequently Line 2138  this kind of recursion was subsequently
2138  .P  .P
2139  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2140  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2141  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2142  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2143  .\" </a>  .\" </a>
2144  "subroutine"  "subroutine"
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2154  PCRE_EXTENDED option is set so that whit
2154  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2155  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2156  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2157  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2158  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2159  .P  .P
2160  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 2044  the match runs for a very long time inde Line 2202  the match runs for a very long time inde
2202  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2203  before failure can be reported.  before failure can be reported.
2204  .P  .P
2205  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2206  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2207  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2208  .\" HREF  .\" HREF
2209  \fBpcrecallout\fP  \fBpcrecallout\fP
2210  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2212  documentation). If the pattern above is
2212  .sp  .sp
2213    (ab(cd)ef)    (ab(cd)ef)
2214  .sp  .sp
2215  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2216  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2217  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2218    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2219       ^                        ^  .P
2220       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2221  .sp  obtain extra memory to store data during a recursion, which it does by using
2222  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2223  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2224  .P  .P
2225  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2226  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2087  is the actual recursive call. Line 2241  is the actual recursive call.
2241  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2242  treated as an atomic group. That is, once it has matched some of the subject  treated as an atomic group. That is, once it has matched some of the subject
2243  string, it is never re-entered, even if it contains untried alternatives and  string, it is never re-entered, even if it contains untried alternatives and
2244  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2245  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2246  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2247  .sp  .sp
2248    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2249  .sp  .sp
2250  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2251  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2252  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2253  subject string "abcba":  subject string "abcba":
2254  .P  .P
2255  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2256  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2257  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2258  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2259  tests are not part of the recursion).  tests are not part of the recursion).
2260  .P  .P
2261  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2262  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2263  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2264  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2265  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2113  alternatives in the other order, things Line 2267  alternatives in the other order, things
2267  .sp  .sp
2268    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2269  .sp  .sp
2270  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2271  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2272  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2273  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2274  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2275  .P  .P
2276  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that matches all palindromic strings, not just those
2277  with an odd number of characters, it is tempting to change the pattern to this:  with an odd number of characters, it is tempting to change the pattern to this:
2278  .sp  .sp
2279    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2280  .sp  .sp
2281  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2282  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2283  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2284  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2285  .sp  .sp
2286    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2287  .sp  .sp
2288  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2289  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2290  .sp  .sp
2291    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2292  .sp  .sp
2293  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2294  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2295  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2296  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2297  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2298  gone into a loop.  gone into a loop.
# Line 2177  matches "sense and sensibility" and "res Line 2331  matches "sense and sensibility" and "res
2331  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2332  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2333  .P  .P
2334  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2335  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2336  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2337  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2338    call revert to their previous values afterwards.
2339  .P  .P
2340  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2341  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 2249  description of the interface to the call Line 2404  description of the interface to the call
2404  documentation.  documentation.
2405  .  .
2406  .  .
2407    .\" HTML <a name="backtrackcontrol"></a>
2408  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2409  .rs  .rs
2410  .sp  .sp
# Line 2264  a backtracking algorithm. With the excep Line 2420  a backtracking algorithm. With the excep
2420  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2421  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2422  .P  .P
2423  If any of these verbs are used in an assertion subpattern, their effect is  If any of these verbs are used in an assertion or subroutine subpattern
2424  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2425  Note that assertion subpatterns are processed as anchored at the point where  it does not extend to the surrounding pattern. Note that such subpatterns are
2426  they are tested.  processed as anchored at the point where they are tested.
2427  .P  .P
2428  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2429  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2430  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2431  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. An name is a sequence of
2432  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2433    parenthesis immediately follows the colon, the effect is as if the colon were
2434    not there. Any number of these verbs may occur in a pattern.
2435    .P
2436    PCRE contains some optimizations that are used to speed up matching by running
2437    some checks at the start of each match attempt. For example, it may know the
2438    minimum length of matching subject, or that a particular character must be
2439    present. When one of these optimizations suppresses the running of a match, any
2440    included backtracking verbs will not, of course, be processed. You can suppress
2441    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2442    when calling \fBpcre_exec()\fP.
2443    .
2444  .  .
2445  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2446  .rs  .rs
2447  .sp  .sp
2448  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2449    followed by a name.
2450  .sp  .sp
2451     (*ACCEPT)     (*ACCEPT)
2452  .sp  .sp
# Line 2289  captured. (This feature was added to PCR Line 2457  captured. (This feature was added to PCR
2457  .sp  .sp
2458    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2459  .sp  .sp
2460  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2461  the outer parentheses.  the outer parentheses.
2462  .sp  .sp
2463    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2305  callout feature, as for example in this Line 2473  callout feature, as for example in this
2473  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2474  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2475  .  .
2476    .
2477    .SS "Recording which path was taken"
2478    .rs
2479    .sp
2480    There is one verb whose main purpose is to track how a match was arrived at,
2481    though it also has a secondary use in conjunction with advancing the match
2482    starting point (see (*SKIP) below).
2483    .sp
2484      (*MARK:NAME) or (*:NAME)
2485    .sp
2486    A name is always required with this verb. There may be as many instances of
2487    (*MARK) as you like in a pattern, and their names do not have to be unique.
2488    .P
2489    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2490    to the caller via the \fIpcre_extra\fP data structure, as described in the
2491    .\" HTML <a href="pcreapi.html#extradata">
2492    .\" </a>
2493    section on \fIpcre_extra\fP
2494    .\"
2495    in the
2496    .\" HREF
2497    \fBpcreapi\fP
2498    .\"
2499    documentation. No data is returned for a partial match. Here is an example of
2500    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2501    outputting of (*MARK) data:
2502    .sp
2503      /X(*MARK:A)Y|X(*MARK:B)Z/K
2504      XY
2505       0: XY
2506      MK: A
2507      XZ
2508       0: XZ
2509      MK: B
2510    .sp
2511    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2512    indicates which of the two alternatives matched. This is a more efficient way
2513    of obtaining this information than putting each alternative in its own
2514    capturing parentheses.
2515    .P
2516    A name may also be returned after a failed match if the final path through the
2517    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2518    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2519    starting point for matching is advanced, the final check is often with an empty
2520    string, causing a failure before (*MARK) is reached. For example:
2521    .sp
2522      /X(*MARK:A)Y|X(*MARK:B)Z/K
2523      XP
2524      No match
2525    .sp
2526    There are three potential starting points for this match (starting with X,
2527    starting with P, and with an empty string). If the pattern is anchored, the
2528    result is different:
2529    .sp
2530      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2531      XP
2532      No match, mark = B
2533    .sp
2534    PCRE's start-of-match optimizations can also interfere with this. For example,
2535    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2536    subject length for a match, a shorter subject will not be scanned at all.
2537    .P
2538    Note that similar anomalies (though different in detail) exist in Perl, no
2539    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2540    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2541    .
2542    .
2543  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2544  .rs  .rs
2545  .sp  .sp
2546  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2547  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2548  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2549    the verb. However, when one of these verbs appears inside an atomic group, its
2550    effect is confined to that group, because once the group has been matched,
2551    there is never any backtracking into it. In this situation, backtracking can
2552    "jump back" to the left of the entire atomic group. (Remember also, as stated
2553    above, that this localization also applies in subroutine calls and assertions.)
2554    .P
2555    These verbs differ in exactly what kind of failure occurs when backtracking
2556    reaches them.
2557  .sp  .sp
2558    (*COMMIT)    (*COMMIT)
2559  .sp  .sp
2560  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2561  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2562  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2563  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2564  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2565  .sp  .sp
2566    a+(*COMMIT)b    a+(*COMMIT)b
2567  .sp  .sp
2568  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2569  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2570  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2571    (*PRUNE)  match failure.
2572  .sp  .P
2573  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2574  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2575  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2576  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2577  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2578  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2579  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2580  be expressed in any other way.    xyzabc\eY
2581      No match
2582    .sp
2583    PCRE knows that any match must start with "a", so the optimization skips along
2584    the subject to "a" before running the first match attempt, which succeeds. When
2585    the optimization is disabled by the \eY escape in the second subject, the match
2586    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2587    starting points.
2588    .sp
2589      (*PRUNE) or (*PRUNE:NAME)
2590    .sp
2591    This verb causes the match to fail at the current starting position in the
2592    subject if the rest of the pattern does not match. If the pattern is
2593    unanchored, the normal "bumpalong" advance to the next starting character then
2594    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2595    reached, or when matching to the right of (*PRUNE), but if there is no match to
2596    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2597    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2598    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2599    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2600    match fails completely; the name is passed back if this is the final attempt.
2601    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2602    pattern (*PRUNE) has the same effect as (*COMMIT).
2603  .sp  .sp
2604    (*SKIP)    (*SKIP)
2605  .sp  .sp
2606  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2607  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2608  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2609  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2610    successful match. Consider:
2611  .sp  .sp
2612    a+(*SKIP)b    a+(*SKIP)b
2613  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2618  effect as this example; although it woul
2618  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2619  instead of skipping on to "c".  instead of skipping on to "c".
2620  .sp  .sp
2621    (*THEN)    (*SKIP:NAME)
2622    .sp
2623    When (*SKIP) has an associated name, its behaviour is modified. If the
2624    following pattern fails to match, the previous path through the pattern is
2625    searched for the most recent (*MARK) that has the same name. If one is found,
2626    the "bumpalong" advance is to the subject position that corresponds to that
2627    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2628    matching name is found, normal "bumpalong" of one character happens (the
2629    (*SKIP) is ignored).
2630    .sp
2631      (*THEN) or (*THEN:NAME)
2632  .sp  .sp
2633  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next alternation if the rest of the pattern does
2634  not match. That is, it cancels pending backtracking, but only within the  not match. That is, it cancels pending backtracking, but only within the
# Line 2363  for a pattern-based if-then-else block: Line 2639  for a pattern-based if-then-else block:
2639  .sp  .sp
2640  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2641  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure the matcher skips to the
2642  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2643  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2644    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2645    like (*PRUNE).
2646  .  .
2647  .  .
2648  .SH "SEE ALSO"  .SH "SEE ALSO"
2649  .rs  .rs
2650  .sp  .sp
2651  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2652  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2653  .  .
2654  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2666  Cambridge CB2 3QH, England.
2666  .rs  .rs
2667  .sp  .sp
2668  .nf  .nf
2669  Last updated: 30 September 2009  Last updated: 18 May 2010
2670  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2671  .fi  .fi

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