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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 491 by ph10, Mon Mar 1 17:45:08 2010 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    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, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 30  The remainder of this document discusses Line 46  The remainder of this document discusses
46  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
48  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
50  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53  .\" HREF  .\" HREF
54  \fBpcrematching\fP  \fBpcrematching\fP
55  .\"  .\"
56  page.  page.
57    .
58    .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76  .P  .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP or
87    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89    .sp
90      (*CR)a.b
91    .sp
92    changes the convention to CR. That pattern matches "a\enb" because LF is no
93    longer a newline. Note that these special settings, which are not
94    Perl-compatible, are recognized only at the very start of a pattern, and that
95    they must be in upper case. If more than one of them is present, the last one
96    is used.
97    .P
98    The newline convention does not affect what the \eR escape sequence matches. By
99    default, this is any Unicode newline sequence, for Perl compatibility. However,
100    this can be changed; see the description of \eR in the section entitled
101    .\" HTML <a href="#newlineseq">
102    .\" </a>
103    "Newline sequences"
104    .\"
105    below. A change of \eR setting can be combined with a change of newline
106    convention.
107    .
108    .
109    .SH "CHARACTERS AND METACHARACTERS"
110    .rs
111    .sp
112  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
113  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
114  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 132  interpreted in some special way.
132  .P  .P
133  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
134  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
135  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
136  as follows:  are as follows:
137  .sp  .sp
138    \e      general escape character with several uses    \e      general escape character with several uses
139    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 164  a character class the only metacharacter
164  .sp  .sp
165  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
166  .  .
167    .
168  .SH BACKSLASH  .SH BACKSLASH
169  .rs  .rs
170  .sp  .sp
# Line 134  The \eQ...\eE sequence is recognized bot Line 207  The \eQ...\eE sequence is recognized bot
207  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
208  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
209  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
210  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
211  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
212  .sp  .sp
213    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
215    \ee        escape (hex 1B)    \ee        escape (hex 1B)
216    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
217    \en        newline (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
221    \exhh      character with hex code hh    \exhh      character with hex code hh
222    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
223  .sp  .sp
# Line 157  Thus \ecz becomes hex 1A, but \ec{ becom Line 229  Thus \ecz becomes hex 1A, but \ec{ becom
229  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
230  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
231  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
232  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
235  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
236  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240  .P  .P
241  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
242  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 190  parenthesized subpatterns. Line 265  parenthesized subpatterns.
265  .P  .P
266  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
267  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
268  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
269  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
271  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 296  zero, because no more than three octal d
296  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
297  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
298  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
299  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
300  these sequences have different meanings  respectively. Outside a character class, these sequences have different
301    meanings
302  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
303  .\" </a>  .\" </a>
304  (see below).  (see below).
305  .\"  .\"
306  .  .
307  .  .
308    .SS "Absolute and relative back references"
309    .rs
310    .sp
311    The sequence \eg followed by an unsigned or a negative number, optionally
312    enclosed in braces, is an absolute or relative back reference. A named back
313    reference can be coded as \eg{name}. Back references are discussed
314    .\" HTML <a href="#backreferences">
315    .\" </a>
316    later,
317    .\"
318    following the discussion of
319    .\" HTML <a href="#subpattern">
320    .\" </a>
321    parenthesized subpatterns.
322    .\"
323    .
324    .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342    .
343    .
344  .SS "Generic character types"  .SS "Generic character types"
345  .rs  .rs
346  .sp  .sp
347  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
348  following are always recognized:  following are always recognized:
349  .sp  .sp
350    \ed     any decimal digit    \ed     any decimal digit
351    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
352      \eh     any horizontal whitespace character
353      \eH     any character that is not a horizontal whitespace character
354    \es     any whitespace character    \es     any whitespace character
355    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
356      \ev     any vertical whitespace character
357      \eV     any character that is not a vertical whitespace character
358    \ew     any "word" character    \ew     any "word" character
359    \eW     any "non-word" character    \eW     any "non-word" character
360  .sp  .sp
# Line 252  there is no character to match. Line 368  there is no character to match.
368  .P  .P
369  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).
370  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
371  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372  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
373  does.)  does.
374    .P
375    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377    character property support is available. These sequences retain their original
378    meanings from before UTF-8 support was available, mainly for efficiency
379    reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381    .P
382    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384    The horizontal space characters are:
385    .sp
386      U+0009     Horizontal tab
387      U+0020     Space
388      U+00A0     Non-break space
389      U+1680     Ogham space mark
390      U+180E     Mongolian vowel separator
391      U+2000     En quad
392      U+2001     Em quad
393      U+2002     En space
394      U+2003     Em space
395      U+2004     Three-per-em space
396      U+2005     Four-per-em space
397      U+2006     Six-per-em space
398      U+2007     Figure space
399      U+2008     Punctuation space
400      U+2009     Thin space
401      U+200A     Hair space
402      U+202F     Narrow no-break space
403      U+205F     Medium mathematical space
404      U+3000     Ideographic space
405    .sp
406    The vertical space characters are:
407    .sp
408      U+000A     Linefeed
409      U+000B     Vertical tab
410      U+000C     Formfeed
411      U+000D     Carriage return
412      U+0085     Next line
413      U+2028     Line separator
414      U+2029     Paragraph separator
415  .P  .P
416  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
417  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 268  in the Line 425  in the
425  .\" HREF  .\" HREF
426  \fBpcreapi\fP  \fBpcreapi\fP
427  .\"  .\"
428  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
430  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
431  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
432  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
433  character property support is available. The use of locales with Unicode is  .
434  discouraged.  .\" HTML <a name="newlineseq"></a>
435    .SS "Newline sequences"
436    .rs
437    .sp
438    Outside a character class, by default, the escape sequence \eR matches any
439    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440    equivalent to the following:
441    .sp
442      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443    .sp
444    This is an example of an "atomic group", details of which are given
445    .\" HTML <a href="#atomicgroup">
446    .\" </a>
447    below.
448    .\"
449    This particular group matches either the two-character sequence CR followed by
450    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
451    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
452    line, U+0085). The two-character sequence is treated as a single unit that
453    cannot be split.
454    .P
455    In UTF-8 mode, two additional characters whose codepoints are greater than 255
456    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
457    Unicode character property support is not needed for these characters to be
458    recognized.
459    .P
460    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462    either at compile time or when the pattern is matched. (BSR is an abbrevation
463    for "backslash R".) This can be made the default when PCRE is built; if this is
464    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465    It is also possible to specify these settings by starting a pattern string with
466    one of the following sequences:
467    .sp
468      (*BSR_ANYCRLF)   CR, LF, or CRLF only
469      (*BSR_UNICODE)   any Unicode newline sequence
470    .sp
471    These override the default and the options given to \fBpcre_compile()\fP or
472    \fBpcre_compile2()\fP, but they can be overridden by options given to
473    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474    which are not Perl-compatible, are recognized only at the very start of a
475    pattern, and that they must be in upper case. If more than one of them is
476    present, the last one is used. They can be combined with a change of newline
477    convention, for example, a pattern can start with:
478    .sp
479      (*ANY)(*BSR_ANYCRLF)
480    .sp
481    Inside a character class, \eR matches the letter "R".
482  .  .
483  .  .
484  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 486  discouraged.
486  .rs  .rs
487  .sp  .sp
488  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
489  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
490  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
491    characters whose codepoints are less than 256, but they do work in this mode.
492    The extra escape sequences are:
493  .sp  .sp
494    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
495    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 307  Those that are not part of an identified Line 513  Those that are not part of an identified
513  .P  .P
514  Arabic,  Arabic,
515  Armenian,  Armenian,
516    Avestan,
517    Balinese,
518    Bamum,
519  Bengali,  Bengali,
520  Bopomofo,  Bopomofo,
521  Braille,  Braille,
522  Buginese,  Buginese,
523  Buhid,  Buhid,
524  Canadian_Aboriginal,  Canadian_Aboriginal,
525    Carian,
526    Cham,
527  Cherokee,  Cherokee,
528  Common,  Common,
529  Coptic,  Coptic,
530    Cuneiform,
531  Cypriot,  Cypriot,
532  Cyrillic,  Cyrillic,
533  Deseret,  Deseret,
534  Devanagari,  Devanagari,
535    Egyptian_Hieroglyphs,
536  Ethiopic,  Ethiopic,
537  Georgian,  Georgian,
538  Glagolitic,  Glagolitic,
# Line 332  Hangul, Line 545  Hangul,
545  Hanunoo,  Hanunoo,
546  Hebrew,  Hebrew,
547  Hiragana,  Hiragana,
548    Imperial_Aramaic,
549  Inherited,  Inherited,
550    Inscriptional_Pahlavi,
551    Inscriptional_Parthian,
552    Javanese,
553    Kaithi,
554  Kannada,  Kannada,
555  Katakana,  Katakana,
556    Kayah_Li,
557  Kharoshthi,  Kharoshthi,
558  Khmer,  Khmer,
559  Lao,  Lao,
560  Latin,  Latin,
561    Lepcha,
562  Limbu,  Limbu,
563  Linear_B,  Linear_B,
564    Lisu,
565    Lycian,
566    Lydian,
567  Malayalam,  Malayalam,
568    Meetei_Mayek,
569  Mongolian,  Mongolian,
570  Myanmar,  Myanmar,
571  New_Tai_Lue,  New_Tai_Lue,
572    Nko,
573  Ogham,  Ogham,
574  Old_Italic,  Old_Italic,
575  Old_Persian,  Old_Persian,
576    Old_South_Arabian,
577    Old_Turkic,
578    Ol_Chiki,
579  Oriya,  Oriya,
580  Osmanya,  Osmanya,
581    Phags_Pa,
582    Phoenician,
583    Rejang,
584  Runic,  Runic,
585    Samaritan,
586    Saurashtra,
587  Shavian,  Shavian,
588  Sinhala,  Sinhala,
589    Sundanese,
590  Syloti_Nagri,  Syloti_Nagri,
591  Syriac,  Syriac,
592  Tagalog,  Tagalog,
593  Tagbanwa,  Tagbanwa,
594  Tai_Le,  Tai_Le,
595    Tai_Tham,
596    Tai_Viet,
597  Tamil,  Tamil,
598  Telugu,  Telugu,
599  Thaana,  Thaana,
# Line 365  Thai, Line 601  Thai,
601  Tibetan,  Tibetan,
602  Tifinagh,  Tifinagh,
603  Ugaritic,  Ugaritic,
604    Vai,
605  Yi.  Yi.
606  .P  .P
607  Each character has exactly one general category property, specified by a  Each character has exactly one general category property, specified by a
# Line 430  The special property L& is also supporte Line 667  The special property L& is also supporte
667  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
668  a modifier or "other".  a modifier or "other".
669  .P  .P
670  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
671    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
672    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
673    (see the discussion of PCRE_NO_UTF8_CHECK in the
674    .\" HREF
675    \fBpcreapi\fP
676    .\"
677    page). Perl does not support the Cs property.
678    .P
679    The long synonyms for property names that Perl supports (such as \ep{Letter})
680  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
681  properties with "Is".  properties with "Is".
682  .P  .P
# Line 454  atomic group Line 700  atomic group
700  (see below).  (see below).
701  .\"  .\"
702  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
703  preceding character.  preceding character. None of them have codepoints less than 256, so in
704    non-UTF-8 mode \eX matches any one character.
705  .P  .P
706  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
707  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 462  why the traditional escape sequences suc Line 709  why the traditional escape sequences suc
709  properties in PCRE.  properties in PCRE.
710  .  .
711  .  .
712    .\" HTML <a name="resetmatchstart"></a>
713    .SS "Resetting the match start"
714    .rs
715    .sp
716    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
717    matched characters not to be included in the final matched sequence. For
718    example, the pattern:
719    .sp
720      foo\eKbar
721    .sp
722    matches "foobar", but reports that it has matched "bar". This feature is
723    similar to a lookbehind assertion
724    .\" HTML <a href="#lookbehind">
725    .\" </a>
726    (described below).
727    .\"
728    However, in this case, the part of the subject before the real match does not
729    have to be of fixed length, as lookbehind assertions do. The use of \eK does
730    not interfere with the setting of
731    .\" HTML <a href="#subpattern">
732    .\" </a>
733    captured substrings.
734    .\"
735    For example, when the pattern
736    .sp
737      (foo)\eKbar
738    .sp
739    matches "foobar", the first substring is still set to "foo".
740    .
741    .
742  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
743  .SS "Simple assertions"  .SS "Simple assertions"
744  .rs  .rs
745  .sp  .sp
746  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
747  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
748  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
749  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 755  The backslashed assertions are:
755  .sp  .sp
756    \eb     matches at a word boundary    \eb     matches at a word boundary
757    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
758    \eA     matches at start of subject    \eA     matches at the start of the subject
759    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
760    \ez     matches at end of subject            also matches before a newline at the end of the subject
761    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
762      \eG     matches at the first matching position in the subject
763  .sp  .sp
764  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
765  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 489  different meaning, namely the backspace Line 767  different meaning, namely the backspace
767  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
768  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
769  \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
770  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
771    separte "start of word" or "end of word" metasequence. However, whatever
772    follows \eb normally determines which it is. For example, the fragment
773    \eba matches "a" at the start of a word.
774  .P  .P
775  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
776  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 578  end of the subject in both modes, and if Line 859  end of the subject in both modes, and if
859  .sp  .sp
860  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
861  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
862  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
863  a line ending is defined as a single character (CR or LF), dot never matches  .P
864  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
865  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
866  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
867    (including isolated CRs and LFs). When any Unicode line endings are being
868    recognized, dot does not match CR or LF or any of the other line ending
869    characters.
870  .P  .P
871  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
872  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
873  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
874    to match it.
875  .P  .P
876  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
877  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
# Line 597  special meaning in a character class. Line 882  special meaning in a character class.
882  .rs  .rs
883  .sp  .sp
884  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
885  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
886  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
887  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
888  the string may be a malformed UTF-8 string. For this reason, the \eC escape  what remains in the string may be a malformed UTF-8 string. For this reason,
889  sequence is best avoided.  the \eC escape sequence is best avoided.
890  .P  .P
891  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
892  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 617  the lookbehind. Line 902  the lookbehind.
902  .rs  .rs
903  .sp  .sp
904  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
905  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
906  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
907  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
908  escaped with a backslash.  a member of the class, it should be the first data character in the class
909    (after an initial circumflex, if present) or escaped with a backslash.
910  .P  .P
911  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
912  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
913  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
914  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
915  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
916  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 634  For example, the character class [aeiou] Line 920  For example, the character class [aeiou]
920  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
921  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
922  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
923  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
924  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
925  string.  string.
926  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 934  caseful version would. In UTF-8 mode, PC
934  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
935  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
936  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
937  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
938  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
939  UTF-8 support.  with UTF-8 support.
940  .P  .P
941  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
942  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
943  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
944  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
945  .P  .P
946  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
947  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 679  example [\ex{100}-\ex{2ff}]. Line 965  example [\ex{100}-\ex{2ff}].
965  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
966  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
967  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
968  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
969  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
970  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
971  property support.  property support.
# Line 769  alternative in the subpattern. Line 1055  alternative in the subpattern.
1055  .rs  .rs
1056  .sp  .sp
1057  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1058  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1059  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1060    The option letters are
1061  .sp  .sp
1062    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1063    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1071  PCRE_MULTILINE while unsetting PCRE_DOTA
1071  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1072  unset.  unset.
1073  .P  .P
1074  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1075  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1076  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1077  the global options (and it will therefore show up in data extracted by the  .P
1078  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1079    subpattern parentheses), the change applies to the remainder of the pattern
1080    that follows. If the change is placed right at the start of a pattern, PCRE
1081    extracts it into the global options (and it will therefore show up in data
1082    extracted by the \fBpcre_fullinfo()\fP function).
1083  .P  .P
1084  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1085  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1086  .sp  .sp
1087    (a(?i)b)c    (a(?i)b)c
1088  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1098  branch is abandoned before the option se
1098  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1099  behaviour otherwise.  behaviour otherwise.
1100  .P  .P
1101  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1102  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1103  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1104    the application has set or what has been defaulted. Details are given in the
1105    section entitled
1106    .\" HTML <a href="#newlineseq">
1107    .\" </a>
1108    "Newline sequences"
1109    .\"
1110    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1111    mode; this is equivalent to setting the PCRE_UTF8 option.
1112  .  .
1113  .  .
1114  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1123  Turning part of a pattern into a subpatt
1123    cat(aract|erpillar|)    cat(aract|erpillar|)
1124  .sp  .sp
1125  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1126  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1127  .sp  .sp
1128  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
1129  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
# Line 849  the string "the white queen" is matched Line 1148  the string "the white queen" is matched
1148    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1149  .sp  .sp
1150  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1151  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1152  .P  .P
1153  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1154  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 865  is reached, an option setting in one bra Line 1163  is reached, an option setting in one bra
1163  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1164  .  .
1165  .  .
1166    .\" HTML <a name="dupsubpatternnumber"></a>
1167    .SH "DUPLICATE SUBPATTERN NUMBERS"
1168    .rs
1169    .sp
1170    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1171    the same numbers for its capturing parentheses. Such a subpattern starts with
1172    (?| and is itself a non-capturing subpattern. For example, consider this
1173    pattern:
1174    .sp
1175      (?|(Sat)ur|(Sun))day
1176    .sp
1177    Because the two alternatives are inside a (?| group, both sets of capturing
1178    parentheses are numbered one. Thus, when the pattern matches, you can look
1179    at captured substring number one, whichever alternative matched. This construct
1180    is useful when you want to capture part, but not all, of one of a number of
1181    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1182    number is reset at the start of each branch. The numbers of any capturing
1183    buffers that follow the subpattern start after the highest number used in any
1184    branch. The following example is taken from the Perl documentation.
1185    The numbers underneath show in which buffer the captured content will be
1186    stored.
1187    .sp
1188      # before  ---------------branch-reset----------- after
1189      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1190      # 1            2         2  3        2     3     4
1191    .sp
1192    A back reference to a numbered subpattern uses the most recent value that is
1193    set for that number by any subpattern. The following pattern matches "abcabc"
1194    or "defdef":
1195    .sp
1196      /(?|(abc)|(def))\e1/
1197    .sp
1198    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1199    refers to the first one in the pattern with the given number. The following
1200    pattern matches "abcabc" or "defabc":
1201    .sp
1202      /(?|(abc)|(def))(?1)/
1203    .sp
1204    If a
1205    .\" HTML <a href="#conditions">
1206    .\" </a>
1207    condition test
1208    .\"
1209    for a subpattern's having matched refers to a non-unique number, the test is
1210    true if any of the subpatterns of that number have matched.
1211    .P
1212    An alternative approach to using this "branch reset" feature is to use
1213    duplicate named subpatterns, as described in the next section.
1214    .
1215    .
1216  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1217  .rs  .rs
1218  .sp  .sp
1219  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1220  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1221  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1222  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1223  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1224    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1225    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1226    have different names, but PCRE does not.
1227    .P
1228    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1229    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1230  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1231  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1232  .\" </a>  .\" </a>
1233  backreferences,  back references,
1234  .\"  .\"
1235  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1236  .\" </a>  .\" </a>
# Line 890  conditions, Line 1244  conditions,
1244  can be made by name as well as by number.  can be made by name as well as by number.
1245  .P  .P
1246  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1247  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1248  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1249  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1250  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1251  .P  .P
1252  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
1253  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
1254  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
1255  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
1256  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
1257  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
1258  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1259    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1260    (?P<DN>Tue)(?:sday)?|  .sp
1261    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1262    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1263    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1264      (?<DN>Thu)(?:rsday)?|
1265      (?<DN>Sat)(?:urday)?
1266  .sp  .sp
1267  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1268    (An alternative way of solving this problem is to use a "branch reset"
1269    subpattern, as described in the previous section.)
1270    .P
1271  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1272  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
1273  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1274  make a reference to a non-unique named subpattern from elsewhere in the  .P
1275  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
1276  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1277    used. In the absence of duplicate numbers (see the previous section) this is
1278    the one with the lowest number. If you use a named reference in a condition
1279    test (see the
1280    .\"
1281    .\" HTML <a href="#conditions">
1282    .\" </a>
1283    section about conditions
1284    .\"
1285    below), either to check whether a subpattern has matched, or to check for
1286    recursion, all subpatterns with the same name are tested. If the condition is
1287    true for any one of them, the overall condition is true. This is the same
1288    behaviour as testing by number. For further details of the interfaces for
1289    handling named subpatterns, see the
1290  .\" HREF  .\" HREF
1291  \fBpcreapi\fP  \fBpcreapi\fP
1292  .\"  .\"
1293  documentation.  documentation.
1294    .P
1295    \fBWarning:\fP You cannot use different names to distinguish between two
1296    subpatterns with the same number because PCRE uses only the numbers when
1297    matching. For this reason, an error is given at compile time if different names
1298    are given to subpatterns with the same number. However, you can give the same
1299    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1300  .  .
1301  .  .
1302  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1306  Repetition is specified by quantifiers,
1306  items:  items:
1307  .sp  .sp
1308    a literal data character    a literal data character
1309    the . metacharacter    the dot metacharacter
1310    the \eC escape sequence    the \eC escape sequence
1311    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1312      the \eR escape sequence
1313    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1314    a character class    a character class
1315    a back reference (see next section)    a back reference (see next section)
1316    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1317      a recursive or "subroutine" call to a subpattern
1318  .sp  .sp
1319  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1320  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 966  support is available, \eX{3} matches thr Line 1346  support is available, \eX{3} matches thr
1346  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1347  .P  .P
1348  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1349  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1350    subpatterns that are referenced as
1351    .\" HTML <a href="#subpatternsassubroutines">
1352    .\" </a>
1353    subroutines
1354    .\"
1355    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1356    quantifier are omitted from the compiled pattern.
1357  .P  .P
1358  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1359  quantifiers have single-character abbreviations:  abbreviations:
1360  .sp  .sp
1361    *    is equivalent to {0,}    *    is equivalent to {0,}
1362    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1404  own right. Because it has two uses, it c
1404  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1405  way the rest of the pattern matches.  way the rest of the pattern matches.
1406  .P  .P
1407  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1408  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1409  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1410  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1414  is greater than 1 or with a limited maxi
1414  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1415  .P  .P
1416  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1417  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1418  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1419  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1420  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1038  worth setting PCRE_DOTALL in order to ob Line 1425  worth setting PCRE_DOTALL in order to ob
1425  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1426  .P  .P
1427  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1428  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1429  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1430  succeed. Consider, for example:  succeeds. Consider, for example:
1431  .sp  .sp
1432    (.*)abc\e1    (.*)abc\e1
1433  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1453  matches "aba" the value of the second ca
1453  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1454  .rs  .rs
1455  .sp  .sp
1456  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1457  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1458  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1459  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1460  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1461  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1462  .P  .P
1463  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1464  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1470  item, and then with 4, and so on, before
1470  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1471  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1472  .P  .P
1473  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1474  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1475  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1476  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1500  previous example can be rewritten as
1500  .sp  .sp
1501    \ed++foo    \ed++foo
1502  .sp  .sp
1503    Note that a possessive quantifier can be used with an entire group, for
1504    example:
1505    .sp
1506      (abc|xyz){2,3}+
1507    .sp
1508  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1509  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1510  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1511  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1512  .P  difference; possessive quantifiers should be slightly faster.
1513  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1514  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1515  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1516  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1517    package, and PCRE copied it from there. It ultimately found its way into Perl
1518    at release 5.10.
1519    .P
1520    PCRE has an optimization that automatically "possessifies" certain simple
1521    pattern constructs. For example, the sequence A+B is treated as A++B because
1522    there is no point in backtracking into a sequence of A's when B must follow.
1523  .P  .P
1524  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1525  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1167  numbers less than 10. A "forward back re Line 1565  numbers less than 10. A "forward back re
1565  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1566  in an earlier iteration.  in an earlier iteration.
1567  .P  .P
1568  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1569  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1570  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1571  "Non-printing characters"  "Non-printing characters"
1572  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1573  .\" </a>  .\" </a>
1574  above  above
1575  .\"  .\"
1576  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1577    no such problem when named parentheses are used. A back reference to any
1578    subpattern is possible using named parentheses (see below).
1579    .P
1580    Another way of avoiding the ambiguity inherent in the use of digits following a
1581    backslash is to use the \eg escape sequence, which is a feature introduced in
1582    Perl 5.10. This escape must be followed by an unsigned number or a negative
1583    number, optionally enclosed in braces. These examples are all identical:
1584    .sp
1585      (ring), \e1
1586      (ring), \eg1
1587      (ring), \eg{1}
1588    .sp
1589    An unsigned number specifies an absolute reference without the ambiguity that
1590    is present in the older syntax. It is also useful when literal digits follow
1591    the reference. A negative number is a relative reference. Consider this
1592    example:
1593    .sp
1594      (abc(def)ghi)\eg{-1}
1595    .sp
1596    The sequence \eg{-1} is a reference to the most recently started capturing
1597    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1598    would be equivalent to \e1. The use of relative references can be helpful in
1599    long patterns, and also in patterns that are created by joining together
1600    fragments that contain references within themselves.
1601  .P  .P
1602  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1603  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1619  back reference, the case of letters is r
1619  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1620  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1621  .P  .P
1622  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1623  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1624    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1625    back reference syntax, in which \eg can be used for both numeric and named
1626    references, is also supported. We could rewrite the above example in any of
1627    the following ways:
1628  .sp  .sp
1629      (?<p1>(?i)rah)\es+\ek<p1>
1630      (?'p1'(?i)rah)\es+\ek{p1}
1631    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1632      (?<p1>(?i)rah)\es+\eg{p1}
1633  .sp  .sp
1634  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1635  after the reference.  after the reference.
1636  .P  .P
1637  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1638  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1639  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1640  .sp  .sp
1641    (a|(bc))\e2    (a|(bc))\e2
1642  .sp  .sp
1643  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1644  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1645  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1646  with a digit character, some delimiter must be used to terminate the back  .P
1647  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1648  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1649    If the pattern continues with a digit character, some delimiter must be used to
1650    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1651    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1652  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1653  .\" </a>  .\" </a>
1654  "Comments"  "Comments"
1655  .\"  .\"
1656  below) can be used.  below) can be used.
1657  .P  .
1658    .SS "Recursive back references"
1659    .rs
1660    .sp
1661  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
1662  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.
1663  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1236  to the previous iteration. In order for Line 1671  to the previous iteration. In order for
1671  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
1672  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
1673  minimum of zero.  minimum of zero.
1674    .P
1675    Back references of this type cause the group that they reference to be treated
1676    as an
1677    .\" HTML <a href="#atomicgroup">
1678    .\" </a>
1679    atomic group.
1680    .\"
1681    Once the whole group has been matched, a subsequent matching failure cannot
1682    cause backtracking into the middle of the group.
1683  .  .
1684  .  .
1685  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1289  lookbehind assertion is needed to achiev Line 1733  lookbehind assertion is needed to achiev
1733  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
1734  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
1735  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.
1736    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1737    synonym for (?!).
1738  .  .
1739  .  .
1740  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1759  is permitted, but
1759  .sp  .sp
1760  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1761  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
1762  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1763  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1764  .sp  .sp
1765    (?<=ab(c|de))    (?<=ab(c|de))
1766  .sp  .sp
1767  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1768  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1769    branches:
1770  .sp  .sp
1771    (?<=abc|abde)    (?<=abc|abde)
1772  .sp  .sp
1773    In some cases, the Perl 5.10 escape sequence \eK
1774    .\" HTML <a href="#resetmatchstart">
1775    .\" </a>
1776    (see above)
1777    .\"
1778    can be used instead of a lookbehind assertion to get round the fixed-length
1779    restriction.
1780    .P
1781  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1782  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1783  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1784  match is deemed to fail.  assertion fails.
1785  .P  .P
1786  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1787  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1788  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1789  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1790    .P
1791    .\" HTML <a href="#subpatternsassubroutines">
1792    .\" </a>
1793    "Subroutine"
1794    .\"
1795    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1796    as the subpattern matches a fixed-length string.
1797    .\" HTML <a href="#recursion">
1798    .\" </a>
1799    Recursion,
1800    .\"
1801    however, is not supported.
1802  .P  .P
1803  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1804  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching of fixed-length strings at the end of subject
1805  such as  strings. Consider a simple pattern such as
1806  .sp  .sp
1807    abcd$    abcd$
1808  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1818  then all but the last two characters, an
1818  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1819  if the pattern is written as  if the pattern is written as
1820  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1821    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1822  .sp  .sp
1823  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1824  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1825  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1826  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1403  characters that are not "999". Line 1866  characters that are not "999".
1866  .sp  .sp
1867  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1868  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1869  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1870  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1871  .sp  .sp
1872    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1873    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1413  If the condition is satisfied, the yes-p Line 1876  If the condition is satisfied, the yes-p
1876  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
1877  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1878  .P  .P
1879  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1880  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1881  underscores, the condition is satisfied if the capturing subpattern of that  .
1882  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1883  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1884  named subpattern; if it cannot find one and the text consists entirely of  .sp
1885  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
1886  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
1887  recommended.  matched. If there is more than one capturing subpattern with the same number
1888    (see the earlier
1889    .\"
1890    .\" HTML <a href="#recursion">
1891    .\" </a>
1892    section about duplicate subpattern numbers),
1893    .\"
1894    the condition is true if any of them have been set. An alternative notation is
1895    to precede the digits with a plus or minus sign. In this case, the subpattern
1896    number is relative rather than absolute. The most recently opened parentheses
1897    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1898    looping constructs it can also make sense to refer to subsequent groups with
1899    constructs such as (?(+2).
1900  .P  .P
1901  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1902  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 1437  or not. If they did, that is, if subject Line 1912  or not. If they did, that is, if subject
1912  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
1913  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1914  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1915  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1916  named subpattern gives this:  .P
1917    If you were embedding this pattern in a larger one, you could use a relative
1918    reference:
1919    .sp
1920      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1921    .sp
1922    This makes the fragment independent of the parentheses in the larger pattern.
1923    .
1924    .SS "Checking for a used subpattern by name"
1925    .rs
1926    .sp
1927    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1928    subpattern by name. For compatibility with earlier versions of PCRE, which had
1929    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1930    there is a possible ambiguity with this syntax, because subpattern names may
1931    consist entirely of digits. PCRE looks first for a named subpattern; if it
1932    cannot find one and the name consists entirely of digits, PCRE looks for a
1933    subpattern of that number, which must be greater than zero. Using subpattern
1934    names that consist entirely of digits is not recommended.
1935    .P
1936    Rewriting the above example to use a named subpattern gives this:
1937  .sp  .sp
1938    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1939    .sp
1940    If the name used in a condition of this kind is a duplicate, the test is
1941    applied to all subpatterns of the same name, and is true if any one of them has
1942    matched.
1943    .
1944    .SS "Checking for pattern recursion"
1945    .rs
1946  .sp  .sp
1947  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
1948  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
1949  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
1950  Recursive patterns are described in the next section.  letter R, for example:
1951    .sp
1952      (?(R3)...) or (?(R&name)...)
1953    .sp
1954    the condition is true if the most recent recursion is into a subpattern whose
1955    number or name is given. This condition does not check the entire recursion
1956    stack. If the name used in a condition of this kind is a duplicate, the test is
1957    applied to all subpatterns of the same name, and is true if any one of them is
1958    the most recent recursion.
1959  .P  .P
1960  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
1961    .\" HTML <a href="#recursion">
1962    .\" </a>
1963    The syntax for recursive patterns
1964    .\"
1965    is described below.
1966    .
1967    .SS "Defining subpatterns for use by reference only"
1968    .rs
1969    .sp
1970    If the condition is the string (DEFINE), and there is no subpattern with the
1971    name DEFINE, the condition is always false. In this case, there may be only one
1972    alternative in the subpattern. It is always skipped if control reaches this
1973    point in the pattern; the idea of DEFINE is that it can be used to define
1974    "subroutines" that can be referenced from elsewhere. (The use of
1975    .\" HTML <a href="#subpatternsassubroutines">
1976    .\" </a>
1977    "subroutines"
1978    .\"
1979    is described below.) For example, a pattern to match an IPv4 address could be
1980    written like this (ignore whitespace and line breaks):
1981    .sp
1982      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1983      \eb (?&byte) (\e.(?&byte)){3} \eb
1984    .sp
1985    The first part of the pattern is a DEFINE group inside which a another group
1986    named "byte" is defined. This matches an individual component of an IPv4
1987    address (a number less than 256). When matching takes place, this part of the
1988    pattern is skipped because DEFINE acts like a false condition. The rest of the
1989    pattern uses references to the named group to match the four dot-separated
1990    components of an IPv4 address, insisting on a word boundary at each end.
1991    .
1992    .SS "Assertion conditions"
1993    .rs
1994    .sp
1995    If the condition is not in any of the above formats, it must be an assertion.
1996  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1997  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1998  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 2028  next newline in the pattern.
2028  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2029  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2030  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2031  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2032  that allows regular expressions to recurse (amongst other things). It does this  .P
2033  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2034  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2035  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2036    pattern using code interpolation to solve the parentheses problem can be
2037    created like this:
2038  .sp  .sp
2039    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2040  .sp  .sp
2041  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2042  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2043  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2044  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2045  .P  supports special syntax for recursion of the entire pattern, and also for
2046  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2047  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2048  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2049  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2050  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2051  .P  provided that it occurs inside that subpattern. (If not, it is a
2052  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2053  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2054  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
2055    .\"
2056    call, which is described in the next section.) The special item (?R) or (?0) is
2057    a recursive call of the entire regular expression.
2058  .P  .P
2059  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2060  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2061  .sp  .sp
2062    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2063  .sp  .sp
2064  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2065  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
2066  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2067  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2068    to avoid backtracking into sequences of non-parentheses.
2069  .P  .P
2070  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
2071  pattern, so instead you could use this:  pattern, so instead you could use this:
2072  .sp  .sp
2073    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2074  .sp  .sp
2075  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2076  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2077  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2078  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2079  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references (a Perl 5.10 feature).
2080  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2081  .sp  most recently opened parentheses preceding the recursion. In other words, a
2082    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2083  .sp  it is encountered.
2084  This particular example pattern contains nested unlimited repeats, and so the  .P
2085  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2086  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
2087  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2088    .\" HTML <a href="#subpatternsassubroutines">
2089    .\" </a>
2090    "subroutine"
2091    .\"
2092    calls, as described in the next section.
2093    .P
2094    An alternative approach is to use named parentheses instead. The Perl syntax
2095    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2096    could rewrite the above example as follows:
2097    .sp
2098      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2099    .sp
2100    If there is more than one subpattern with the same name, the earliest one is
2101    used.
2102    .P
2103    This particular example pattern that we have been looking at contains nested
2104    unlimited repeats, and so the use of a possessive quantifier for matching
2105    strings of non-parentheses is important when applying the pattern to strings
2106    that do not match. For example, when this pattern is applied to
2107  .sp  .sp
2108    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2109  .sp  .sp
2110  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2111  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2112  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
2113  before failure can be reported.  before failure can be reported.
2114  .P  .P
2115  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
2116  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
2117  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2118  .\" HREF  .\" HREF
2119  \fBpcrecallout\fP  \fBpcrecallout\fP
2120  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2122  documentation). If the pattern above is
2122  .sp  .sp
2123    (ab(cd)ef)    (ab(cd)ef)
2124  .sp  .sp
2125  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
2126  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
2127  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2128    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2129       ^                        ^  .P
2130       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2131  .sp  obtain extra memory to store data during a recursion, which it does by using
2132  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
2133  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.  
2134  .P  .P
2135  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.
2136  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1578  different alternatives for the recursive Line 2144  different alternatives for the recursive
2144  is the actual recursive call.  is the actual recursive call.
2145  .  .
2146  .  .
2147    .\" HTML <a name="recursiondifference"></a>
2148    .SS "Recursion difference from Perl"
2149    .rs
2150    .sp
2151    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2152    treated as an atomic group. That is, once it has matched some of the subject
2153    string, it is never re-entered, even if it contains untried alternatives and
2154    there is a subsequent matching failure. This can be illustrated by the
2155    following pattern, which purports to match a palindromic string that contains
2156    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2157    .sp
2158      ^(.|(.)(?1)\e2)$
2159    .sp
2160    The idea is that it either matches a single character, or two identical
2161    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2162    it does not if the pattern is longer than three characters. Consider the
2163    subject string "abcba":
2164    .P
2165    At the top level, the first character is matched, but as it is not at the end
2166    of the string, the first alternative fails; the second alternative is taken
2167    and the recursion kicks in. The recursive call to subpattern 1 successfully
2168    matches the next character ("b"). (Note that the beginning and end of line
2169    tests are not part of the recursion).
2170    .P
2171    Back at the top level, the next character ("c") is compared with what
2172    subpattern 2 matched, which was "a". This fails. Because the recursion is
2173    treated as an atomic group, there are now no backtracking points, and so the
2174    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2175    try the second alternative.) However, if the pattern is written with the
2176    alternatives in the other order, things are different:
2177    .sp
2178      ^((.)(?1)\e2|.)$
2179    .sp
2180    This time, the recursing alternative is tried first, and continues to recurse
2181    until it runs out of characters, at which point the recursion fails. But this
2182    time we do have another alternative to try at the higher level. That is the big
2183    difference: in the previous case the remaining alternative is at a deeper
2184    recursion level, which PCRE cannot use.
2185    .P
2186    To change the pattern so that matches all palindromic strings, not just those
2187    with an odd number of characters, it is tempting to change the pattern to this:
2188    .sp
2189      ^((.)(?1)\e2|.?)$
2190    .sp
2191    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2192    deeper recursion has matched a single character, it cannot be entered again in
2193    order to match an empty string. The solution is to separate the two cases, and
2194    write out the odd and even cases as alternatives at the higher level:
2195    .sp
2196      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2197    .sp
2198    If you want to match typical palindromic phrases, the pattern has to ignore all
2199    non-word characters, which can be done like this:
2200    .sp
2201      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2202    .sp
2203    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2204    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2205    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2206    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2207    more) to match typical phrases, and Perl takes so long that you think it has
2208    gone into a loop.
2209    .P
2210    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2211    string does not start with a palindrome that is shorter than the entire string.
2212    For example, although "abcba" is correctly matched, if the subject is "ababa",
2213    PCRE finds the palindrome "aba" at the start, then fails at top level because
2214    the end of the string does not follow. Once again, it cannot jump back into the
2215    recursion to try other alternatives, so the entire match fails.
2216    .
2217    .
2218  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2219  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2220  .rs  .rs
2221  .sp  .sp
2222  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
2223  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2224  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2225  pattern  before or after the reference. A numbered reference can be absolute or
2226    relative, as in these examples:
2227    .sp
2228      (...(absolute)...)...(?2)...
2229      (...(relative)...)...(?-1)...
2230      (...(?+1)...(relative)...
2231    .sp
2232    An earlier example pointed out that the pattern
2233  .sp  .sp
2234    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2235  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2239  matches "sense and sensibility" and "res
2239    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2240  .sp  .sp
2241  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
2242  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
 which they refer. However, named references can refer to later subpatterns.  
2243  .P  .P
2244  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2245  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
2246  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
2247  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2248    call revert to their previous values afterwards.
2249    .P
2250    When a subpattern is used as a subroutine, processing options such as
2251    case-independence are fixed when the subpattern is defined. They cannot be
2252    changed for different calls. For example, consider this pattern:
2253    .sp
2254      (abc)(?i:(?-1))
2255    .sp
2256    It matches "abcabc". It does not match "abcABC" because the change of
2257    processing option does not affect the called subpattern.
2258    .
2259    .
2260    .\" HTML <a name="onigurumasubroutines"></a>
2261    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2262    .rs
2263    .sp
2264    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2265    a number enclosed either in angle brackets or single quotes, is an alternative
2266    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2267    are two of the examples used above, rewritten using this syntax:
2268    .sp
2269      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2270      (sens|respons)e and \eg'1'ibility
2271    .sp
2272    PCRE supports an extension to Oniguruma: if a number is preceded by a
2273    plus or a minus sign it is taken as a relative reference. For example:
2274    .sp
2275      (abc)(?i:\eg<-1>)
2276    .sp
2277    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2278    synonymous. The former is a back reference; the latter is a subroutine call.
2279  .  .
2280  .  .
2281  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2296  function is to be called. If you want to
2296  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2297  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2298  .sp  .sp
2299    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2300  .sp  .sp
2301  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2302  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1638  description of the interface to the call Line 2312  description of the interface to the call
2312  \fBpcrecallout\fP  \fBpcrecallout\fP
2313  .\"  .\"
2314  documentation.  documentation.
2315    .
2316    .
2317    .SH "BACKTRACKING CONTROL"
2318    .rs
2319    .sp
2320    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2321    are described in the Perl documentation as "experimental and subject to change
2322    or removal in a future version of Perl". It goes on to say: "Their usage in
2323    production code should be noted to avoid problems during upgrades." The same
2324    remarks apply to the PCRE features described in this section.
2325    .P
2326    Since these verbs are specifically related to backtracking, most of them can be
2327    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2328    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2329    failing negative assertion, they cause an error if encountered by
2330    \fBpcre_dfa_exec()\fP.
2331  .P  .P
2332  .in 0  If any of these verbs are used in an assertion or subroutine subpattern
2333  Last updated: 06 June 2006  (including recursive subpatterns), their effect is confined to that subpattern;
2334  .br  it does not extend to the surrounding pattern. Note that such subpatterns are
2335  Copyright (c) 1997-2006 University of Cambridge.  processed as anchored at the point where they are tested.
2336    .P
2337    The new verbs make use of what was previously invalid syntax: an opening
2338    parenthesis followed by an asterisk. In Perl, they are generally of the form
2339    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2340    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2341    are two kinds:
2342    .
2343    .SS "Verbs that act immediately"
2344    .rs
2345    .sp
2346    The following verbs act as soon as they are encountered:
2347    .sp
2348       (*ACCEPT)
2349    .sp
2350    This verb causes the match to end successfully, skipping the remainder of the
2351    pattern. When inside a recursion, only the innermost pattern is ended
2352    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2353    captured. (This feature was added to PCRE at release 8.00.) For example:
2354    .sp
2355      A((?:A|B(*ACCEPT)|C)D)
2356    .sp
2357    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2358    the outer parentheses.
2359    .sp
2360      (*FAIL) or (*F)
2361    .sp
2362    This verb causes the match to fail, forcing backtracking to occur. It is
2363    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2364    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2365    Perl features that are not present in PCRE. The nearest equivalent is the
2366    callout feature, as for example in this pattern:
2367    .sp
2368      a+(?C)(*FAIL)
2369    .sp
2370    A match with the string "aaaa" always fails, but the callout is taken before
2371    each backtrack happens (in this example, 10 times).
2372    .
2373    .SS "Verbs that act after backtracking"
2374    .rs
2375    .sp
2376    The following verbs do nothing when they are encountered. Matching continues
2377    with what follows, but if there is no subsequent match, a failure is forced.
2378    The verbs differ in exactly what kind of failure occurs.
2379    .sp
2380      (*COMMIT)
2381    .sp
2382    This verb causes the whole match to fail outright if the rest of the pattern
2383    does not match. Even if the pattern is unanchored, no further attempts to find
2384    a match by advancing the starting point take place. Once (*COMMIT) has been
2385    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2386    starting point, or not at all. For example:
2387    .sp
2388      a+(*COMMIT)b
2389    .sp
2390    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2391    dynamic anchor, or "I've started, so I must finish."
2392    .sp
2393      (*PRUNE)
2394    .sp
2395    This verb causes the match to fail at the current position if the rest of the
2396    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2397    advance to the next starting character then happens. Backtracking can occur as
2398    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2399    if there is no match to the right, backtracking cannot cross (*PRUNE).
2400    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2401    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2402    be expressed in any other way.
2403    .sp
2404      (*SKIP)
2405    .sp
2406    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2407    "bumpalong" advance is not to the next character, but to the position in the
2408    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2409    was matched leading up to it cannot be part of a successful match. Consider:
2410    .sp
2411      a+(*SKIP)b
2412    .sp
2413    If the subject is "aaaac...", after the first match attempt fails (starting at
2414    the first character in the string), the starting point skips on to start the
2415    next attempt at "c". Note that a possessive quantifer does not have the same
2416    effect as this example; although it would suppress backtracking during the
2417    first match attempt, the second attempt would start at the second character
2418    instead of skipping on to "c".
2419    .sp
2420      (*THEN)
2421    .sp
2422    This verb causes a skip to the next alternation if the rest of the pattern does
2423    not match. That is, it cancels pending backtracking, but only within the
2424    current alternation. Its name comes from the observation that it can be used
2425    for a pattern-based if-then-else block:
2426    .sp
2427      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2428    .sp
2429    If the COND1 pattern matches, FOO is tried (and possibly further items after
2430    the end of the group if FOO succeeds); on failure the matcher skips to the
2431    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2432    is used outside of any alternation, it acts exactly like (*PRUNE).
2433    .
2434    .
2435    .SH "SEE ALSO"
2436    .rs
2437    .sp
2438    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2439    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2440    .
2441    .
2442    .SH AUTHOR
2443    .rs
2444    .sp
2445    .nf
2446    Philip Hazel
2447    University Computing Service
2448    Cambridge CB2 3QH, England.
2449    .fi
2450    .
2451    .
2452    .SH REVISION
2453    .rs
2454    .sp
2455    .nf
2456    Last updated: 01 March 2010
2457    Copyright (c) 1997-2010 University of Cambridge.
2458    .fi

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