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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 572 by ph10, Wed Nov 17 17:55:57 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 56  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
71  .rs  .rs
72  .sp  .sp
# Line 83  string with one of the following five se Line 94  string with one of the following five se
94    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
95    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
96  .sp  .sp
97  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
98  \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
99  newline sequence, the pattern  newline sequence, the pattern
100  .sp  .sp
# Line 95  Perl-compatible, are recognized only at Line 106  Perl-compatible, are recognized only at
106  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
107  is used.  is used.
108  .P  .P
109  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
110  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
111  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
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
115  .\" </a>  .\" </a>
116  "Newline sequences"  "Newline sequences"
# Line 198  Perl, $ and @ cause variable interpolati Line 211  Perl, $ and @ cause variable interpolati
211    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
212  .sp  .sp
213  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
214    An isolated \eE that is not preceded by \eQ is ignored.
215  .  .
216  .  .
217  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 217  one of the following escape sequences th Line 231  one of the following escape sequences th
231    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
232    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
233    \et        tab (hex 09)    \et        tab (hex 09)
234    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
235    \exhh      character with hex code hh    \exhh      character with hex code hh
236    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
237  .sp  .sp
# Line 295  zero, because no more than three octal d Line 309  zero, because no more than three octal d
309  .P  .P
310  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
311  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
312  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
313  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
314  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
315  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
316  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
317  .  .
318  .  .
319  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 344  syntax for referencing a subpattern as a
344  later.  later.
345  .\"  .\"
346  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
347  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
348  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
349  .\" </a>  .\" </a>
350  subroutine  subroutine
# Line 341  subroutine Line 352  subroutine
352  call.  call.
353  .  .
354  .  .
355    .\" HTML <a name="genericchartypes"></a>
356  .SS "Generic character types"  .SS "Generic character types"
357  .rs  .rs
358  .sp  .sp
359  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:  
360  .sp  .sp
361    \ed     any decimal digit    \ed     any decimal digit
362    \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 369  following are always recognized:
369    \ew     any "word" character    \ew     any "word" character
370    \eW     any "non-word" character    \eW     any "non-word" character
371  .sp  .sp
372  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.
373  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
374    .\" HTML <a href="#fullstopdot">
375    .\" </a>
376    the "." metacharacter
377    .\"
378    when PCRE_DOTALL is not set.
379  .P  .P
380  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
381    of characters into two disjoint sets. Any given character matches one, and only
382    one, of each pair. The sequences can appear both inside and outside character
383  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
384  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
385  there is no character to match.  there is no character to match.
386  .P  .P
387  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 390  are HT (9), LF (10), FF (12), CR (13), a
390  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
391  does.  does.
392  .P  .P
393  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.
394  \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
395  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
396  meanings from before UTF-8 support was available, mainly for efficiency  place (see
397  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
398  and \eW.  .\" </a>
399  .P  "Locale support"
400  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
401  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
402  The horizontal space characters are:  .\" HREF
403    \fBpcreapi\fP
404    .\"
405    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
406    or "french" in Windows, some character codes greater than 128 are used for
407    accented letters, and these are then matched by \ew. The use of locales with
408    Unicode is discouraged.
409    .P
410    By default, in UTF-8 mode, characters with values greater than 128 never match
411    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
412    their original meanings from before UTF-8 support was available, mainly for
413    efficiency reasons. However, if PCRE is compiled with Unicode property support,
414    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
415    properties are used to determine character types, as follows:
416    .sp
417      \ed  any character that \ep{Nd} matches (decimal digit)
418      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
419      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
420    .sp
421    The upper case escapes match the inverse sets of characters. Note that \ed
422    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
423    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
424    \eB because they are defined in terms of \ew and \eW. Matching these sequences
425    is noticeably slower when PCRE_UCP is set.
426    .P
427    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
428    release 5.10. In contrast to the other sequences, which match only ASCII
429    characters by default, these always match certain high-valued codepoints in
430    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
431    are:
432  .sp  .sp
433    U+0009     Horizontal tab    U+0009     Horizontal tab
434    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 459  The vertical space characters are:
459    U+0085     Next line    U+0085     Next line
460    U+2028     Line separator    U+2028     Line separator
461    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.  
462  .  .
463  .  .
464  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 436  is discouraged. Line 466  is discouraged.
466  .rs  .rs
467  .sp  .sp
468  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
469  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
470  .sp  .sp
471    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
472  .sp  .sp
# Line 468  one of the following sequences: Line 497  one of the following sequences:
497    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
498    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
499  .sp  .sp
500  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
501  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
502  \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,
503  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
504  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
505  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
506  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
507  .sp  .sp
508    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
509  .sp  .sp
510  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
511    a character class, \eR is treated as an unrecognized escape sequence, and so
512    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
513  .  .
514  .  .
515  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 527  The extra escape sequences are:
527    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
528  .sp  .sp
529  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
530  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
531  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
532  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
533  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
534    .\" </a>
535    next section).
536    .\"
537    Other Perl properties such as "InMusicalSymbols" are not currently supported by
538    PCRE. Note that \eP{Any} does not match any characters, so always causes a
539    match failure.
540  .P  .P
541  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
542  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 550  Those that are not part of an identified
550  .P  .P
551  Arabic,  Arabic,
552  Armenian,  Armenian,
553    Avestan,
554  Balinese,  Balinese,
555    Bamum,
556  Bengali,  Bengali,
557  Bopomofo,  Bopomofo,
558  Braille,  Braille,
559  Buginese,  Buginese,
560  Buhid,  Buhid,
561  Canadian_Aboriginal,  Canadian_Aboriginal,
562    Carian,
563    Cham,
564  Cherokee,  Cherokee,
565  Common,  Common,
566  Coptic,  Coptic,
# Line 528  Cypriot, Line 569  Cypriot,
569  Cyrillic,  Cyrillic,
570  Deseret,  Deseret,
571  Devanagari,  Devanagari,
572    Egyptian_Hieroglyphs,
573  Ethiopic,  Ethiopic,
574  Georgian,  Georgian,
575  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 582  Hangul,
582  Hanunoo,  Hanunoo,
583  Hebrew,  Hebrew,
584  Hiragana,  Hiragana,
585    Imperial_Aramaic,
586  Inherited,  Inherited,
587    Inscriptional_Pahlavi,
588    Inscriptional_Parthian,
589    Javanese,
590    Kaithi,
591  Kannada,  Kannada,
592  Katakana,  Katakana,
593    Kayah_Li,
594  Kharoshthi,  Kharoshthi,
595  Khmer,  Khmer,
596  Lao,  Lao,
597  Latin,  Latin,
598    Lepcha,
599  Limbu,  Limbu,
600  Linear_B,  Linear_B,
601    Lisu,
602    Lycian,
603    Lydian,
604  Malayalam,  Malayalam,
605    Meetei_Mayek,
606  Mongolian,  Mongolian,
607  Myanmar,  Myanmar,
608  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 610  Nko,
610  Ogham,  Ogham,
611  Old_Italic,  Old_Italic,
612  Old_Persian,  Old_Persian,
613    Old_South_Arabian,
614    Old_Turkic,
615    Ol_Chiki,
616  Oriya,  Oriya,
617  Osmanya,  Osmanya,
618  Phags_Pa,  Phags_Pa,
619  Phoenician,  Phoenician,
620    Rejang,
621  Runic,  Runic,
622    Samaritan,
623    Saurashtra,
624  Shavian,  Shavian,
625  Sinhala,  Sinhala,
626    Sundanese,
627  Syloti_Nagri,  Syloti_Nagri,
628  Syriac,  Syriac,
629  Tagalog,  Tagalog,
630  Tagbanwa,  Tagbanwa,
631  Tai_Le,  Tai_Le,
632    Tai_Tham,
633    Tai_Viet,
634  Tamil,  Tamil,
635  Telugu,  Telugu,
636  Thaana,  Thaana,
# Line 576  Thai, Line 638  Thai,
638  Tibetan,  Tibetan,
639  Tifinagh,  Tifinagh,
640  Ugaritic,  Ugaritic,
641    Vai,
642  Yi.  Yi.
643  .P  .P
644  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
645  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
646  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
647  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
648  .P  .P
649  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
650  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 743  non-UTF-8 mode \eX matches any one chara
743  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
744  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
745  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
746  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
747    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
748    (*UCP).
749    .
750    .
751    .\" HTML <a name="extraprops"></a>
752    .SS PCRE's additional properties
753    .rs
754    .sp
755    As well as the standard Unicode properties described in the previous
756    section, PCRE supports four more that make it possible to convert traditional
757    escape sequences such as \ew and \es and POSIX character classes to use Unicode
758    properties. PCRE uses these non-standard, non-Perl properties internally when
759    PCRE_UCP is set. They are:
760    .sp
761      Xan   Any alphanumeric character
762      Xps   Any POSIX space character
763      Xsp   Any Perl space character
764      Xwd   Any Perl "word" character
765    .sp
766    Xan matches characters that have either the L (letter) or the N (number)
767    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
768    carriage return, and any other character that has the Z (separator) property.
769    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
770    same characters as Xan, plus underscore.
771  .  .
772  .  .
773  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
774  .SS "Resetting the match start"  .SS "Resetting the match start"
775  .rs  .rs
776  .sp  .sp
777  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
778  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
779  .sp  .sp
780    foo\eKbar    foo\eKbar
781  .sp  .sp
# Line 711  For example, when the pattern Line 797  For example, when the pattern
797    (foo)\eKbar    (foo)\eKbar
798  .sp  .sp
799  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
800    .P
801    Perl documents that the use of \eK within assertions is "not well defined". In
802    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
803    ignored in negative assertions.
804  .  .
805  .  .
806  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 825  The backslashed assertions are:
825    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
826    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
827  .sp  .sp
828  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
829  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
830    default it matches the corresponding literal character (for example, \eB
831    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
832    escape sequence" error is generated instead.
833  .P  .P
834  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
835  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
836  \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
837  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
838  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
839  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
840  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
841    determines which it is. For example, the fragment \eba matches "a" at the start
842    of a word.
843  .P  .P
844  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
845  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 923  end of the subject in both modes, and if
923  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
924  .  .
925  .  .
926  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
927    .SH "FULL STOP (PERIOD, DOT) AND \eN"
928  .rs  .rs
929  .sp  .sp
930  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 946  to match it.
946  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
947  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
948  special meaning in a character class.  special meaning in a character class.
949    .P
950    The escape sequence \eN behaves like a dot, except that it is not affected by
951    the PCRE_DOTALL option. In other words, it matches any character except one
952    that signifies the end of a line.
953  .  .
954  .  .
955  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 858  special meaning in a character class. Line 958  special meaning in a character class.
958  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
959  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
960  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
961  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
962  what remains in the string may be a malformed UTF-8 string. For this reason,  rest of the string may start with a malformed UTF-8 character. For this reason,
963  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
964  .P  .P
965  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 876  the lookbehind. Line 976  the lookbehind.
976  .rs  .rs
977  .sp  .sp
978  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
979  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.
980  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
981  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
982  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
983  (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 1044  characters in both cases. In UTF-8 mode,
1044  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
1045  property support.  property support.
1046  .P  .P
1047  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
1048  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
1049  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1050  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
1051  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.
1052  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1053  .P  .P
1054  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1055  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 1069  this notation. For example,
1069    [01[:alpha:]%]    [01[:alpha:]%]
1070  .sp  .sp
1071  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1072  are  are:
1073  .sp  .sp
1074    alnum    letters and digits    alnum    letters and digits
1075    alpha    letters    alpha    letters
# Line 980  are Line 1080  are
1080    graph    printing characters, excluding space    graph    printing characters, excluding space
1081    lower    lower case letters    lower    lower case letters
1082    print    printing characters, including space    print    printing characters, including space
1083    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1084    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1085    upper    upper case letters    upper    upper case letters
1086    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1101  matches "1", "2", or any non-digit. PCRE
1101  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
1102  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1103  .P  .P
1104  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
1105  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1106    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1107    character properties are used. This is achieved by replacing the POSIX classes
1108    by other sequences, as follows:
1109    .sp
1110      [:alnum:]  becomes  \ep{Xan}
1111      [:alpha:]  becomes  \ep{L}
1112      [:blank:]  becomes  \eh
1113      [:digit:]  becomes  \ep{Nd}
1114      [:lower:]  becomes  \ep{Ll}
1115      [:space:]  becomes  \ep{Xps}
1116      [:upper:]  becomes  \ep{Lu}
1117      [:word:]   becomes  \ep{Xwd}
1118    .sp
1119    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1120    classes are unchanged, and match only characters with code points less than
1121    128.
1122  .  .
1123  .  .
1124  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1056  extracts it into the global options (and Line 1172  extracts it into the global options (and
1172  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1173  .P  .P
1174  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1175  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1176  .sp  .sp
1177    (a(?i)b)c    (a(?i)b)c
1178  .sp  .sp
# Line 1081  section entitled Line 1197  section entitled
1197  .\" </a>  .\" </a>
1198  "Newline sequences"  "Newline sequences"
1199  .\"  .\"
1200  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
1201  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1202    PCRE_UTF8 and the PCRE_UCP options, respectively.
1203  .  .
1204  .  .
1205  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1096  Turning part of a pattern into a subpatt Line 1213  Turning part of a pattern into a subpatt
1213  .sp  .sp
1214    cat(aract|erpillar|)    cat(aract|erpillar|)
1215  .sp  .sp
1216  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1217  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1218  .sp  .sp
1219  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1220  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1221  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1222  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1223  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1224  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1225  .sp  .sp
1226    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1227  .sp  .sp
# Line 1154  at captured substring number one, whiche Line 1270  at captured substring number one, whiche
1270  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1271  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1272  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1273  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1274  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1275  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1276  .sp  .sp
1277    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1278    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1279    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1280  .sp  .sp
1281  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
1282  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1283  "defdef":  or "defdef":
1284  .sp  .sp
1285    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1286  .sp  .sp
1287  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1288  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
1289  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1290  .sp  .sp
1291    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1292  .sp  .sp
1293    If a
1294    .\" HTML <a href="#conditions">
1295    .\" </a>
1296    condition test
1297    .\"
1298    for a subpattern's having matched refers to a non-unique number, the test is
1299    true if any of the subpatterns of that number have matched.
1300  .P  .P
1301  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1302  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1303  .  .
1304  .  .
# Line 1189  if an expression is modified, the number Line 1311  if an expression is modified, the number
1311  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1312  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
1313  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
1314  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1315    have different names, but PCRE does not.
1316  .P  .P
1317  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
1318  (?'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
1319  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1320  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1321  .\" </a>  .\" </a>
1322  backreferences,  back references,
1323  .\"  .\"
1324  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1325  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1339  extracting the name-to-number translatio
1339  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1340  .P  .P
1341  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
1342  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
1343  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
1344  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
1345  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
1346  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
1347    name, and in both cases you want to extract the abbreviation. This pattern
1348    (ignoring the line breaks) does the job:
1349  .sp  .sp
1350    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1351    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1359  subpattern, as described in the previous
1359  .P  .P
1360  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1361  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
1362  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1363  make a reference to a non-unique named subpattern from elsewhere in the  .P
1364  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
1365  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1366    used. In the absence of duplicate numbers (see the previous section) this is
1367    the one with the lowest number. If you use a named reference in a condition
1368    test (see the
1369    .\"
1370    .\" HTML <a href="#conditions">
1371    .\" </a>
1372    section about conditions
1373    .\"
1374    below), either to check whether a subpattern has matched, or to check for
1375    recursion, all subpatterns with the same name are tested. If the condition is
1376    true for any one of them, the overall condition is true. This is the same
1377    behaviour as testing by number. For further details of the interfaces for
1378    handling named subpatterns, see the
1379  .\" HREF  .\" HREF
1380  \fBpcreapi\fP  \fBpcreapi\fP
1381  .\"  .\"
1382  documentation.  documentation.
1383  .P  .P
1384  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1385  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1386  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1387    are given to subpatterns with the same number. However, you can give the same
1388    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1389  .  .
1390  .  .
1391  .SH REPETITION  .SH REPETITION
# Line 1259  items: Line 1399  items:
1399    the \eC escape sequence    the \eC escape sequence
1400    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1401    the \eR escape sequence    the \eR escape sequence
1402    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1403    a character class    a character class
1404    a back reference (see next section)    a back reference (see next section)
1405    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1406    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1407  .sp  .sp
1408  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1409  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1301  subpatterns that are referenced as Line 1441  subpatterns that are referenced as
1441  .\" </a>  .\" </a>
1442  subroutines  subroutines
1443  .\"  .\"
1444  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1445  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1446    .\" </a>
1447    "Defining subpatterns for use by reference only"
1448    .\"
1449    below). Items other than subpatterns that have a {0} quantifier are omitted
1450    from the compiled pattern.
1451  .P  .P
1452  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1453  abbreviations:  abbreviations:
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1519  worth setting PCRE_DOTALL in order to ob
1519  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1520  .P  .P
1521  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1522  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1523  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
1524  succeeds. Consider, for example:  succeeds. Consider, for example:
1525  .sp  .sp
# Line 1527  no such problem when named parentheses a Line 1672  no such problem when named parentheses a
1672  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1673  .P  .P
1674  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1675  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1676  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1677  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1678  .sp  .sp
1679    (ring), \e1    (ring), \e1
1680    (ring), \eg1    (ring), \eg1
# Line 1543  example: Line 1688  example:
1688    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1689  .sp  .sp
1690  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1691  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1692  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1693  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1694  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1695  .P  .P
1696  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1697  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1589  references to it always fail by default. Line 1734  references to it always fail by default.
1734  .sp  .sp
1735    (a|(bc))\e2    (a|(bc))\e2
1736  .sp  .sp
1737  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
1738  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
1739  unset value matches an empty string.  unset value matches an empty string.
1740  .P  .P
1741  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 1748  whitespace. Otherwise, the \eg{ syntax o
1748  "Comments"  "Comments"
1749  .\"  .\"
1750  below) can be used.  below) can be used.
1751  .P  .
1752    .SS "Recursive back references"
1753    .rs
1754    .sp
1755  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
1756  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.
1757  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 1765  to the previous iteration. In order for
1765  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
1766  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
1767  minimum of zero.  minimum of zero.
1768    .P
1769    Back references of this type cause the group that they reference to be treated
1770    as an
1771    .\" HTML <a href="#atomicgroup">
1772    .\" </a>
1773    atomic group.
1774    .\"
1775    Once the whole group has been matched, a subsequent matching failure cannot
1776    cause backtracking into the middle of the group.
1777  .  .
1778  .  .
1779  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1670  lookbehind assertion is needed to achiev Line 1827  lookbehind assertion is needed to achiev
1827  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1828  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1829  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1830  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1831  .  .
1832  .  .
1833  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1696  is permitted, but Line 1852  is permitted, but
1852  .sp  .sp
1853  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1854  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1855  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1856  match the same length of string. An assertion such as  length of string. An assertion such as
1857  .sp  .sp
1858    (?<=ab(c|de))    (?<=ab(c|de))
1859  .sp  .sp
# Line 1707  branches: Line 1863  branches:
1863  .sp  .sp
1864    (?<=abc|abde)    (?<=abc|abde)
1865  .sp  .sp
1866  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1867  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1868  .\" </a>  .\" </a>
1869  (see above)  (see above)
1870  .\"  .\"
1871  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
1872  restriction.  restriction.
1873  .P  .P
1874  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 1886  different numbers of bytes, are also not
1886  "Subroutine"  "Subroutine"
1887  .\"  .\"
1888  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
1889  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1890  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1891  .\" </a>  .\" </a>
1892  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 1959  characters that are not "999".
1959  .sp  .sp
1960  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1961  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1962  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1963  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1964  .sp  .sp
1965    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1811  already been matched. The two possible f Line 1967  already been matched. The two possible f
1967  .sp  .sp
1968  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1969  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1970  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1971    itself contain nested subpatterns of any form, including conditional
1972    subpatterns; the restriction to two alternatives applies only at the level of
1973    the condition. This pattern fragment is an example where the alternatives are
1974    complex:
1975    .sp
1976      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1977    .sp
1978  .P  .P
1979  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
1980  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1821  recursion, a pseudo-condition called DEF Line 1984  recursion, a pseudo-condition called DEF
1984  .sp  .sp
1985  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
1986  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1987  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
1988  (see the earlier  (see the earlier
1989  .\"  .\"
1990  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1991  .\" </a>  .\" </a>
1992  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
1993  .\"  .\"
1994  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
1995  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
1996  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
1997  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
1998  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
1999  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2000    zero in any of these forms is not used; it provokes a compile-time error.)
2001  .P  .P
2002  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2003  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1844  three parts for ease of discussion: Line 2008  three parts for ease of discussion:
2008  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2009  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2010  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2011  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2012  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2013  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2014  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2015  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1874  Rewriting the above example to use a nam Line 2038  Rewriting the above example to use a nam
2038  .sp  .sp
2039    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2040  .sp  .sp
2041    If the name used in a condition of this kind is a duplicate, the test is
2042    applied to all subpatterns of the same name, and is true if any one of them has
2043    matched.
2044  .  .
2045  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2046  .rs  .rs
# Line 1887  letter R, for example: Line 2054  letter R, for example:
2054  .sp  .sp
2055  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
2056  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
2057  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2058    applied to all subpatterns of the same name, and is true if any one of them is
2059    the most recent recursion.
2060  .P  .P
2061  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2062  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2063  .\" </a>  .\" </a>
2064  Recursive patterns  The syntax for recursive patterns
2065  .\"  .\"
2066  are described below.  is described below.
2067  .  .
2068    .\" HTML <a name="subdefine"></a>
2069  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2070  .rs  .rs
2071  .sp  .sp
# Line 1903  If the condition is the string (DEFINE), Line 2073  If the condition is the string (DEFINE),
2073  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
2074  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2075  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
2076  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
2077  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2078  .\" </a>  .\" </a>
2079  "subroutines"  "subroutines"
2080  .\"  .\"
2081  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2082  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2083    breaks):
2084  .sp  .sp
2085    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2086    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1944  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2115  dd-aaa-dd or dd-dd-dd, where aaa are let
2115  .SH COMMENTS  .SH COMMENTS
2116  .rs  .rs
2117  .sp  .sp
2118  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2119  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2120  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2121    subpattern name or number. The characters that make up a comment play no part
2122    in the pattern matching.
2123  .P  .P
2124  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2125  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2126  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2127    this case continues to immediately after the next newline character or
2128    character sequence in the pattern. Which characters are interpreted as newlines
2129    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2130    sequence at the start of the pattern, as described in the section entitled
2131    .\" HTML <a href="#newlines">
2132    .\" </a>
2133    "Newline conventions"
2134    .\"
2135    above. Note that the end of this type of comment is a literal newline sequence
2136    in the pattern; escape sequences that happen to represent a newline do not
2137    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2138    default newline convention is in force:
2139    .sp
2140      abc #comment \en still comment
2141    .sp
2142    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2143    a newline in the pattern. The sequence \en is still literal at this stage, so
2144    it does not terminate the comment. Only an actual character with the code value
2145    0x0a (the default newline) does so.
2146  .  .
2147  .  .
2148  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1980  this kind of recursion was subsequently Line 2172  this kind of recursion was subsequently
2172  .P  .P
2173  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
2174  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,
2175  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2176  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2177  .\" </a>  .\" </a>
2178  "subroutine"  "subroutine"
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2188  PCRE_EXTENDED option is set so that whit
2188  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2189  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
2190  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2191  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
2192  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2193  .P  .P
2194  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 2008  We have put the pattern into parentheses Line 2200  We have put the pattern into parentheses
2200  them instead of the whole pattern.  them instead of the whole pattern.
2201  .P  .P
2202  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2203  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2204  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2205  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2206  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2207  .P  .P
2208  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2209  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
# Line 2044  the match runs for a very long time inde Line 2235  the match runs for a very long time inde
2235  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
2236  before failure can be reported.  before failure can be reported.
2237  .P  .P
2238  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
2239  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
2240  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  
2241  .\" HREF  .\" HREF
2242  \fBpcrecallout\fP  \fBpcrecallout\fP
2243  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2245  documentation). If the pattern above is
2245  .sp  .sp
2246    (ab(cd)ef)    (ab(cd)ef)
2247  .sp  .sp
2248  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
2249  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
2250  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2251    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2252       ^                        ^  .P
2253       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2254  .sp  obtain extra memory to store data during a recursion, which it does by using
2255  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
2256  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.  
2257  .P  .P
2258  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.
2259  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 2274  is the actual recursive call.
2274  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
2275  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
2276  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
2277  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2278  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2279  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2280  .sp  .sp
2281    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2282  .sp  .sp
2283  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
2284  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2285  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
2286  subject string "abcba":  subject string "abcba":
2287  .P  .P
2288  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
2289  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2290  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2291  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
2292  tests are not part of the recursion).  tests are not part of the recursion).
2293  .P  .P
2294  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
2295  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2296  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
2297  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
2298  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 2300  alternatives in the other order, things
2300  .sp  .sp
2301    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2302  .sp  .sp
2303  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2304  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
2305  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
2306  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2307  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2308  .P  .P
2309  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2310  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2311    this:
2312  .sp  .sp
2313    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2314  .sp  .sp
2315  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
2316  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
2317  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
2318  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:
2319  .sp  .sp
2320    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2321  .sp  .sp
2322  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
2323  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2324  .sp  .sp
2325    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2326  .sp  .sp
2327  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
2328  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
2329  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2330  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
2331  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
2332  gone into a loop.  gone into a loop.
# Line 2177  matches "sense and sensibility" and "res Line 2365  matches "sense and sensibility" and "res
2365  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
2366  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2367  .P  .P
2368  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2369  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
2370  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
2371  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2372    call revert to their previous values afterwards.
2373  .P  .P
2374  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2375  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 2438  description of the interface to the call
2438  documentation.  documentation.
2439  .  .
2440  .  .
2441    .\" HTML <a name="backtrackcontrol"></a>
2442  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2443  .rs  .rs
2444  .sp  .sp
# Line 2264  a backtracking algorithm. With the excep Line 2454  a backtracking algorithm. With the excep
2454  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2455  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2456  .P  .P
2457  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
2458  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2459  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
2460  they are tested.  processed as anchored at the point where they are tested.
2461  .P  .P
2462  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
2463  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2464  (*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,
2465  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
2466  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2467    parenthesis immediately follows the colon, the effect is as if the colon were
2468    not there. Any number of these verbs may occur in a pattern.
2469    .P
2470    PCRE contains some optimizations that are used to speed up matching by running
2471    some checks at the start of each match attempt. For example, it may know the
2472    minimum length of matching subject, or that a particular character must be
2473    present. When one of these optimizations suppresses the running of a match, any
2474    included backtracking verbs will not, of course, be processed. You can suppress
2475    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2476    when calling \fBpcre_exec()\fP.
2477    .
2478  .  .
2479  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2480  .rs  .rs
2481  .sp  .sp
2482  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2483    followed by a name.
2484  .sp  .sp
2485     (*ACCEPT)     (*ACCEPT)
2486  .sp  .sp
# Line 2289  captured. (This feature was added to PCR Line 2491  captured. (This feature was added to PCR
2491  .sp  .sp
2492    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2493  .sp  .sp
2494  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
2495  the outer parentheses.  the outer parentheses.
2496  .sp  .sp
2497    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2305  callout feature, as for example in this Line 2507  callout feature, as for example in this
2507  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
2508  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2509  .  .
2510    .
2511    .SS "Recording which path was taken"
2512    .rs
2513    .sp
2514    There is one verb whose main purpose is to track how a match was arrived at,
2515    though it also has a secondary use in conjunction with advancing the match
2516    starting point (see (*SKIP) below).
2517    .sp
2518      (*MARK:NAME) or (*:NAME)
2519    .sp
2520    A name is always required with this verb. There may be as many instances of
2521    (*MARK) as you like in a pattern, and their names do not have to be unique.
2522    .P
2523    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2524    to the caller via the \fIpcre_extra\fP data structure, as described in the
2525    .\" HTML <a href="pcreapi.html#extradata">
2526    .\" </a>
2527    section on \fIpcre_extra\fP
2528    .\"
2529    in the
2530    .\" HREF
2531    \fBpcreapi\fP
2532    .\"
2533    documentation. No data is returned for a partial match. Here is an example of
2534    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2535    outputting of (*MARK) data:
2536    .sp
2537      /X(*MARK:A)Y|X(*MARK:B)Z/K
2538      XY
2539       0: XY
2540      MK: A
2541      XZ
2542       0: XZ
2543      MK: B
2544    .sp
2545    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2546    indicates which of the two alternatives matched. This is a more efficient way
2547    of obtaining this information than putting each alternative in its own
2548    capturing parentheses.
2549    .P
2550    A name may also be returned after a failed match if the final path through the
2551    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2552    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2553    starting point for matching is advanced, the final check is often with an empty
2554    string, causing a failure before (*MARK) is reached. For example:
2555    .sp
2556      /X(*MARK:A)Y|X(*MARK:B)Z/K
2557      XP
2558      No match
2559    .sp
2560    There are three potential starting points for this match (starting with X,
2561    starting with P, and with an empty string). If the pattern is anchored, the
2562    result is different:
2563    .sp
2564      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2565      XP
2566      No match, mark = B
2567    .sp
2568    PCRE's start-of-match optimizations can also interfere with this. For example,
2569    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2570    subject length for a match, a shorter subject will not be scanned at all.
2571    .P
2572    Note that similar anomalies (though different in detail) exist in Perl, no
2573    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2574    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2575    .
2576    .
2577  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2578  .rs  .rs
2579  .sp  .sp
2580  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2581  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
2582  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
2583    the verb. However, when one of these verbs appears inside an atomic group, its
2584    effect is confined to that group, because once the group has been matched,
2585    there is never any backtracking into it. In this situation, backtracking can
2586    "jump back" to the left of the entire atomic group. (Remember also, as stated
2587    above, that this localization also applies in subroutine calls and assertions.)
2588    .P
2589    These verbs differ in exactly what kind of failure occurs when backtracking
2590    reaches them.
2591  .sp  .sp
2592    (*COMMIT)    (*COMMIT)
2593  .sp  .sp
2594  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
2595  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
2596  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
2597  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
2598  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2599  .sp  .sp
2600    a+(*COMMIT)b    a+(*COMMIT)b
2601  .sp  .sp
2602  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
2603  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
2604  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2605    (*PRUNE)  match failure.
2606  .sp  .P
2607  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,
2608  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
2609  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2610  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2611  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2612  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2613  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2614  be expressed in any other way.    xyzabc\eY
2615      No match
2616    .sp
2617    PCRE knows that any match must start with "a", so the optimization skips along
2618    the subject to "a" before running the first match attempt, which succeeds. When
2619    the optimization is disabled by the \eY escape in the second subject, the match
2620    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2621    starting points.
2622    .sp
2623      (*PRUNE) or (*PRUNE:NAME)
2624    .sp
2625    This verb causes the match to fail at the current starting position in the
2626    subject if the rest of the pattern does not match. If the pattern is
2627    unanchored, the normal "bumpalong" advance to the next starting character then
2628    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2629    reached, or when matching to the right of (*PRUNE), but if there is no match to
2630    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2631    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2632    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2633    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2634    match fails completely; the name is passed back if this is the final attempt.
2635    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2636    pattern (*PRUNE) has the same effect as (*COMMIT).
2637  .sp  .sp
2638    (*SKIP)    (*SKIP)
2639  .sp  .sp
2640  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
2641  "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,
2642  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2643  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
2644    successful match. Consider:
2645  .sp  .sp
2646    a+(*SKIP)b    a+(*SKIP)b
2647  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2652  effect as this example; although it woul
2652  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2653  instead of skipping on to "c".  instead of skipping on to "c".
2654  .sp  .sp
2655    (*THEN)    (*SKIP:NAME)
2656  .sp  .sp
2657  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2658  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2659  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2660  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2661    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2662    matching name is found, normal "bumpalong" of one character happens (the
2663    (*SKIP) is ignored).
2664    .sp
2665      (*THEN) or (*THEN:NAME)
2666    .sp
2667    This verb causes a skip to the next alternation in the innermost enclosing
2668    group if the rest of the pattern does not match. That is, it cancels pending
2669    backtracking, but only within the current alternation. Its name comes from the
2670    observation that it can be used for a pattern-based if-then-else block:
2671  .sp  .sp
2672    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2673  .sp  .sp
2674  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
2675  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
2676  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2677  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
2678    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2679    like (*PRUNE).
2680    .
2681    .P
2682    The above verbs provide four different "strengths" of control when subsequent
2683    matching fails. (*THEN) is the weakest, carrying on the match at the next
2684    alternation. (*PRUNE) comes next, failing the match at the current starting
2685    position, but allowing an advance to the next character (for an unanchored
2686    pattern). (*SKIP) is similar, except that the advance may be more than one
2687    character. (*COMMIT) is the strongest, causing the entire match to fail.
2688    .P
2689    If more than one is present in a pattern, the "stongest" one wins. For example,
2690    consider this pattern, where A, B, etc. are complex pattern fragments:
2691    .sp
2692      (A(*COMMIT)B(*THEN)C|D)
2693    .sp
2694    Once A has matched, PCRE is committed to this match, at the current starting
2695    position. If subsequently B matches, but C does not, the normal (*THEN) action
2696    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2697    overrides.
2698  .  .
2699  .  .
2700  .SH "SEE ALSO"  .SH "SEE ALSO"
2701  .rs  .rs
2702  .sp  .sp
2703  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2704  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2705  .  .
2706  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2718  Cambridge CB2 3QH, England.
2718  .rs  .rs
2719  .sp  .sp
2720  .nf  .nf
2721  Last updated: 30 September 2009  Last updated: 17 November 2010
2722  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2723  .fi  .fi

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