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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 507 by ph10, Wed Mar 10 16:08:01 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    .P
741    Perl documents that the use of \eK within assertions is "not well defined". In
742    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
743    ignored in negative assertions.
744    .
745    .
746  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
747  .SS "Simple assertions"  .SS "Simple assertions"
748  .rs  .rs
749  .sp  .sp
750  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
751  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,
752  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
753  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 759  The backslashed assertions are:
759  .sp  .sp
760    \eb     matches at a word boundary    \eb     matches at a word boundary
761    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
762    \eA     matches at start of subject    \eA     matches at the start of the subject
763    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
764    \ez     matches at end of subject            also matches before a newline at the end of the subject
765    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
766      \eG     matches at the first matching position in the subject
767  .sp  .sp
768  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
769  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 771  different meaning, namely the backspace
771  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
772  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
773  \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
774  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
775    separte "start of word" or "end of word" metasequence. However, whatever
776    follows \eb normally determines which it is. For example, the fragment
777    \eba matches "a" at the start of a word.
778  .P  .P
779  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
780  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 863  end of the subject in both modes, and if
863  .sp  .sp
864  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
865  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
866  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.
867  a line ending is defined as a single character (CR or LF), dot never matches  .P
868  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
869  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
870  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
871    (including isolated CRs and LFs). When any Unicode line endings are being
872    recognized, dot does not match CR or LF or any of the other line ending
873    characters.
874  .P  .P
875  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
876  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
877  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
878    to match it.
879  .P  .P
880  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
881  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 886  special meaning in a character class.
886  .rs  .rs
887  .sp  .sp
888  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
889  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
890  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
891  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,
892  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,
893  sequence is best avoided.  the \eC escape sequence is best avoided.
894  .P  .P
895  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
896  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 617  the lookbehind. Line 906  the lookbehind.
906  .rs  .rs
907  .sp  .sp
908  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
909  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.
910  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
911  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
912  escaped with a backslash.  a member of the class, it should be the first data character in the class
913    (after an initial circumflex, if present) or escaped with a backslash.
914  .P  .P
915  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
916  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
917  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
918  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
919  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
920  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 924  For example, the character class [aeiou]
924  [^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
925  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
926  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
927  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
928  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
929  string.  string.
930  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 938  caseful version would. In UTF-8 mode, PC
938  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
939  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
940  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
941  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,
942  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
943  UTF-8 support.  with UTF-8 support.
944  .P  .P
945  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
946  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
947  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
948  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
949  .P  .P
950  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
951  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 969  example [\ex{100}-\ex{2ff}].
969  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
970  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
971  [][\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
972  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
973  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
974  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
975  property support.  property support.
# Line 769  alternative in the subpattern. Line 1059  alternative in the subpattern.
1059  .rs  .rs
1060  .sp  .sp
1061  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1062  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
1063  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1064    The option letters are
1065  .sp  .sp
1066    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1067    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1075  PCRE_MULTILINE while unsetting PCRE_DOTA
1075  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
1076  unset.  unset.
1077  .P  .P
1078  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
1079  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
1080  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1081  the global options (and it will therefore show up in data extracted by the  .P
1082  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1083    subpattern parentheses), the change applies to the remainder of the pattern
1084    that follows. If the change is placed right at the start of a pattern, PCRE
1085    extracts it into the global options (and it will therefore show up in data
1086    extracted by the \fBpcre_fullinfo()\fP function).
1087  .P  .P
1088  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
1089  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1090  .sp  .sp
1091    (a(?i)b)c    (a(?i)b)c
1092  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1102  branch is abandoned before the option se
1102  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1103  behaviour otherwise.  behaviour otherwise.
1104  .P  .P
1105  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
1106  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
1107  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1108    the application has set or what has been defaulted. Details are given in the
1109    section entitled
1110    .\" HTML <a href="#newlineseq">
1111    .\" </a>
1112    "Newline sequences"
1113    .\"
1114    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1115    mode; this is equivalent to setting the PCRE_UTF8 option.
1116  .  .
1117  .  .
1118  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1127  Turning part of a pattern into a subpatt
1127    cat(aract|erpillar|)    cat(aract|erpillar|)
1128  .sp  .sp
1129  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1130  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1131  .sp  .sp
1132  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
1133  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 1152  the string "the white queen" is matched
1152    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1153  .sp  .sp
1154  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
1155  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.  
1156  .P  .P
1157  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
1158  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 1167  is reached, an option setting in one bra
1167  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1168  .  .
1169  .  .
1170    .\" HTML <a name="dupsubpatternnumber"></a>
1171    .SH "DUPLICATE SUBPATTERN NUMBERS"
1172    .rs
1173    .sp
1174    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1175    the same numbers for its capturing parentheses. Such a subpattern starts with
1176    (?| and is itself a non-capturing subpattern. For example, consider this
1177    pattern:
1178    .sp
1179      (?|(Sat)ur|(Sun))day
1180    .sp
1181    Because the two alternatives are inside a (?| group, both sets of capturing
1182    parentheses are numbered one. Thus, when the pattern matches, you can look
1183    at captured substring number one, whichever alternative matched. This construct
1184    is useful when you want to capture part, but not all, of one of a number of
1185    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1186    number is reset at the start of each branch. The numbers of any capturing
1187    buffers that follow the subpattern start after the highest number used in any
1188    branch. The following example is taken from the Perl documentation.
1189    The numbers underneath show in which buffer the captured content will be
1190    stored.
1191    .sp
1192      # before  ---------------branch-reset----------- after
1193      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1194      # 1            2         2  3        2     3     4
1195    .sp
1196    A back reference to a numbered subpattern uses the most recent value that is
1197    set for that number by any subpattern. The following pattern matches "abcabc"
1198    or "defdef":
1199    .sp
1200      /(?|(abc)|(def))\e1/
1201    .sp
1202    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1203    refers to the first one in the pattern with the given number. The following
1204    pattern matches "abcabc" or "defabc":
1205    .sp
1206      /(?|(abc)|(def))(?1)/
1207    .sp
1208    If a
1209    .\" HTML <a href="#conditions">
1210    .\" </a>
1211    condition test
1212    .\"
1213    for a subpattern's having matched refers to a non-unique number, the test is
1214    true if any of the subpatterns of that number have matched.
1215    .P
1216    An alternative approach to using this "branch reset" feature is to use
1217    duplicate named subpatterns, as described in the next section.
1218    .
1219    .
1220  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1221  .rs  .rs
1222  .sp  .sp
1223  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
1224  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1225  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
1226  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1227  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
1228    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1229    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1230    have different names, but PCRE does not.
1231    .P
1232    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1233    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1234  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1235  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1236  .\" </a>  .\" </a>
1237  backreferences,  back references,
1238  .\"  .\"
1239  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1240  .\" </a>  .\" </a>
# Line 890  conditions, Line 1248  conditions,
1248  can be made by name as well as by number.  can be made by name as well as by number.
1249  .P  .P
1250  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1251  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1252  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
1253  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
1254  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1255  .P  .P
1256  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
1257  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
1258  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
1259  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
1260  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
1261  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
1262  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1263    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1264    (?P<DN>Tue)(?:sday)?|  .sp
1265    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1266    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1267    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1268      (?<DN>Thu)(?:rsday)?|
1269      (?<DN>Sat)(?:urday)?
1270  .sp  .sp
1271  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.
1272    (An alternative way of solving this problem is to use a "branch reset"
1273    subpattern, as described in the previous section.)
1274    .P
1275  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1276  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
1277  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1278  make a reference to a non-unique named subpattern from elsewhere in the  .P
1279  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
1280  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1281    used. In the absence of duplicate numbers (see the previous section) this is
1282    the one with the lowest number. If you use a named reference in a condition
1283    test (see the
1284    .\"
1285    .\" HTML <a href="#conditions">
1286    .\" </a>
1287    section about conditions
1288    .\"
1289    below), either to check whether a subpattern has matched, or to check for
1290    recursion, all subpatterns with the same name are tested. If the condition is
1291    true for any one of them, the overall condition is true. This is the same
1292    behaviour as testing by number. For further details of the interfaces for
1293    handling named subpatterns, see the
1294  .\" HREF  .\" HREF
1295  \fBpcreapi\fP  \fBpcreapi\fP
1296  .\"  .\"
1297  documentation.  documentation.
1298    .P
1299    \fBWarning:\fP You cannot use different names to distinguish between two
1300    subpatterns with the same number because PCRE uses only the numbers when
1301    matching. For this reason, an error is given at compile time if different names
1302    are given to subpatterns with the same number. However, you can give the same
1303    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1304  .  .
1305  .  .
1306  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1310  Repetition is specified by quantifiers,
1310  items:  items:
1311  .sp  .sp
1312    a literal data character    a literal data character
1313    the . metacharacter    the dot metacharacter
1314    the \eC escape sequence    the \eC escape sequence
1315    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1316      the \eR escape sequence
1317    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1318    a character class    a character class
1319    a back reference (see next section)    a back reference (see next section)
1320    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1321      a recursive or "subroutine" call to a subpattern
1322  .sp  .sp
1323  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1324  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 1350  support is available, \eX{3} matches thr
1350  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).
1351  .P  .P
1352  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
1353  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1354    subpatterns that are referenced as
1355    .\" HTML <a href="#subpatternsassubroutines">
1356    .\" </a>
1357    subroutines
1358    .\"
1359    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1360    quantifier are omitted from the compiled pattern.
1361  .P  .P
1362  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1363  quantifiers have single-character abbreviations:  abbreviations:
1364  .sp  .sp
1365    *    is equivalent to {0,}    *    is equivalent to {0,}
1366    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1408  own right. Because it has two uses, it c
1408  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
1409  way the rest of the pattern matches.  way the rest of the pattern matches.
1410  .P  .P
1411  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),
1412  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
1413  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
1414  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1418  is greater than 1 or with a limited maxi
1418  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1419  .P  .P
1420  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
1421  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
1422  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1423  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
1424  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 1429  worth setting PCRE_DOTALL in order to ob
1429  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1430  .P  .P
1431  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1432  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1433  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
1434  succeed. Consider, for example:  succeeds. Consider, for example:
1435  .sp  .sp
1436    (.*)abc\e1    (.*)abc\e1
1437  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1457  matches "aba" the value of the second ca
1457  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1458  .rs  .rs
1459  .sp  .sp
1460  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1461  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
1462  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
1463  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
1464  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
1465  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1466  .P  .P
1467  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
1468  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1474  item, and then with 4, and so on, before
1474  (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
1475  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.
1476  .P  .P
1477  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
1478  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
1479  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1480  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1504  previous example can be rewritten as
1504  .sp  .sp
1505    \ed++foo    \ed++foo
1506  .sp  .sp
1507    Note that a possessive quantifier can be used with an entire group, for
1508    example:
1509    .sp
1510      (abc|xyz){2,3}+
1511    .sp
1512  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1513  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
1514  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
1515  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1516  .P  difference; possessive quantifiers should be slightly faster.
1517  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1518  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.
1519  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
1520  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1521    package, and PCRE copied it from there. It ultimately found its way into Perl
1522    at release 5.10.
1523    .P
1524    PCRE has an optimization that automatically "possessifies" certain simple
1525    pattern constructs. For example, the sequence A+B is treated as A++B because
1526    there is no point in backtracking into a sequence of A's when B must follow.
1527  .P  .P
1528  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
1529  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 1569  numbers less than 10. A "forward back re
1569  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
1570  in an earlier iteration.  in an earlier iteration.
1571  .P  .P
1572  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
1573  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
1574  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1575  "Non-printing characters"  "Non-printing characters"
1576  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1577  .\" </a>  .\" </a>
1578  above  above
1579  .\"  .\"
1580  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1581    no such problem when named parentheses are used. A back reference to any
1582    subpattern is possible using named parentheses (see below).
1583    .P
1584    Another way of avoiding the ambiguity inherent in the use of digits following a
1585    backslash is to use the \eg escape sequence, which is a feature introduced in
1586    Perl 5.10. This escape must be followed by an unsigned number or a negative
1587    number, optionally enclosed in braces. These examples are all identical:
1588    .sp
1589      (ring), \e1
1590      (ring), \eg1
1591      (ring), \eg{1}
1592    .sp
1593    An unsigned number specifies an absolute reference without the ambiguity that
1594    is present in the older syntax. It is also useful when literal digits follow
1595    the reference. A negative number is a relative reference. Consider this
1596    example:
1597    .sp
1598      (abc(def)ghi)\eg{-1}
1599    .sp
1600    The sequence \eg{-1} is a reference to the most recently started capturing
1601    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1602    would be equivalent to \e1. The use of relative references can be helpful in
1603    long patterns, and also in patterns that are created by joining together
1604    fragments that contain references within themselves.
1605  .P  .P
1606  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1607  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 1623  back reference, the case of letters is r
1623  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
1624  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1625  .P  .P
1626  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1627  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1628    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1629    back reference syntax, in which \eg can be used for both numeric and named
1630    references, is also supported. We could rewrite the above example in any of
1631    the following ways:
1632  .sp  .sp
1633      (?<p1>(?i)rah)\es+\ek<p1>
1634      (?'p1'(?i)rah)\es+\ek{p1}
1635    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1636      (?<p1>(?i)rah)\es+\eg{p1}
1637  .sp  .sp
1638  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
1639  after the reference.  after the reference.
1640  .P  .P
1641  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
1642  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1643  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1644  .sp  .sp
1645    (a|(bc))\e2    (a|(bc))\e2
1646  .sp  .sp
1647  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
1648  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
1649  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1650  with a digit character, some delimiter must be used to terminate the back  .P
1651  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1652  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1653    If the pattern continues with a digit character, some delimiter must be used to
1654    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1655    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1656  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1657  .\" </a>  .\" </a>
1658  "Comments"  "Comments"
1659  .\"  .\"
1660  below) can be used.  below) can be used.
1661  .P  .
1662    .SS "Recursive back references"
1663    .rs
1664    .sp
1665  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
1666  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.
1667  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 1675  to the previous iteration. In order for
1675  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
1676  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
1677  minimum of zero.  minimum of zero.
1678    .P
1679    Back references of this type cause the group that they reference to be treated
1680    as an
1681    .\" HTML <a href="#atomicgroup">
1682    .\" </a>
1683    atomic group.
1684    .\"
1685    Once the whole group has been matched, a subsequent matching failure cannot
1686    cause backtracking into the middle of the group.
1687  .  .
1688  .  .
1689  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1289  lookbehind assertion is needed to achiev Line 1737  lookbehind assertion is needed to achiev
1737  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
1738  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
1739  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.
1740    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1741    synonym for (?!).
1742  .  .
1743  .  .
1744  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1763  is permitted, but
1763  .sp  .sp
1764  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1765  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
1766  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
1767  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1768  .sp  .sp
1769    (?<=ab(c|de))    (?<=ab(c|de))
1770  .sp  .sp
1771  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
1772  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
1773    branches:
1774  .sp  .sp
1775    (?<=abc|abde)    (?<=abc|abde)
1776  .sp  .sp
1777    In some cases, the Perl 5.10 escape sequence \eK
1778    .\" HTML <a href="#resetmatchstart">
1779    .\" </a>
1780    (see above)
1781    .\"
1782    can be used instead of a lookbehind assertion to get round the fixed-length
1783    restriction.
1784    .P
1785  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1786  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
1787  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1788  match is deemed to fail.  assertion fails.
1789  .P  .P
1790  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)
1791  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1792  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
1793  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1794    .P
1795    .\" HTML <a href="#subpatternsassubroutines">
1796    .\" </a>
1797    "Subroutine"
1798    .\"
1799    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1800    as the subpattern matches a fixed-length string.
1801    .\" HTML <a href="#recursion">
1802    .\" </a>
1803    Recursion,
1804    .\"
1805    however, is not supported.
1806  .P  .P
1807  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1808  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
1809  such as  strings. Consider a simple pattern such as
1810  .sp  .sp
1811    abcd$    abcd$
1812  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1822  then all but the last two characters, an
1822  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,
1823  if the pattern is written as  if the pattern is written as
1824  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1825    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1826  .sp  .sp
1827  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
1828  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
1829  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1830  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 1870  characters that are not "999".
1870  .sp  .sp
1871  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1872  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1873  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1874  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1875  .sp  .sp
1876    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1877    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1413  If the condition is satisfied, the yes-p Line 1880  If the condition is satisfied, the yes-p
1880  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
1881  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1882  .P  .P
1883  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1884  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1885  underscores, the condition is satisfied if the capturing subpattern of that  .
1886  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1887  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1888  named subpattern; if it cannot find one and the text consists entirely of  .sp
1889  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
1890  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
1891  recommended.  matched. If there is more than one capturing subpattern with the same number
1892    (see the earlier
1893    .\"
1894    .\" HTML <a href="#recursion">
1895    .\" </a>
1896    section about duplicate subpattern numbers),
1897    .\"
1898    the condition is true if any of them have been set. An alternative notation is
1899    to precede the digits with a plus or minus sign. In this case, the subpattern
1900    number is relative rather than absolute. The most recently opened parentheses
1901    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1902    looping constructs it can also make sense to refer to subsequent groups with
1903    constructs such as (?(+2).
1904  .P  .P
1905  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1906  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 1916  or not. If they did, that is, if subject
1916  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
1917  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1918  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1919  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1920  named subpattern gives this:  .P
1921    If you were embedding this pattern in a larger one, you could use a relative
1922    reference:
1923    .sp
1924      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1925    .sp
1926    This makes the fragment independent of the parentheses in the larger pattern.
1927    .
1928    .SS "Checking for a used subpattern by name"
1929    .rs
1930    .sp
1931    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1932    subpattern by name. For compatibility with earlier versions of PCRE, which had
1933    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1934    there is a possible ambiguity with this syntax, because subpattern names may
1935    consist entirely of digits. PCRE looks first for a named subpattern; if it
1936    cannot find one and the name consists entirely of digits, PCRE looks for a
1937    subpattern of that number, which must be greater than zero. Using subpattern
1938    names that consist entirely of digits is not recommended.
1939    .P
1940    Rewriting the above example to use a named subpattern gives this:
1941    .sp
1942      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1943  .sp  .sp
1944    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )  If the name used in a condition of this kind is a duplicate, the test is
1945    applied to all subpatterns of the same name, and is true if any one of them has
1946    matched.
1947    .
1948    .SS "Checking for pattern recursion"
1949    .rs
1950  .sp  .sp
1951  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,
1952  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
1953  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
1954  Recursive patterns are described in the next section.  letter R, for example:
1955    .sp
1956      (?(R3)...) or (?(R&name)...)
1957    .sp
1958    the condition is true if the most recent recursion is into a subpattern whose
1959    number or name is given. This condition does not check the entire recursion
1960    stack. If the name used in a condition of this kind is a duplicate, the test is
1961    applied to all subpatterns of the same name, and is true if any one of them is
1962    the most recent recursion.
1963  .P  .P
1964  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.
1965    .\" HTML <a href="#recursion">
1966    .\" </a>
1967    The syntax for recursive patterns
1968    .\"
1969    is described below.
1970    .
1971    .SS "Defining subpatterns for use by reference only"
1972    .rs
1973    .sp
1974    If the condition is the string (DEFINE), and there is no subpattern with the
1975    name DEFINE, the condition is always false. In this case, there may be only one
1976    alternative in the subpattern. It is always skipped if control reaches this
1977    point in the pattern; the idea of DEFINE is that it can be used to define
1978    "subroutines" that can be referenced from elsewhere. (The use of
1979    .\" HTML <a href="#subpatternsassubroutines">
1980    .\" </a>
1981    "subroutines"
1982    .\"
1983    is described below.) For example, a pattern to match an IPv4 address could be
1984    written like this (ignore whitespace and line breaks):
1985    .sp
1986      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1987      \eb (?&byte) (\e.(?&byte)){3} \eb
1988    .sp
1989    The first part of the pattern is a DEFINE group inside which a another group
1990    named "byte" is defined. This matches an individual component of an IPv4
1991    address (a number less than 256). When matching takes place, this part of the
1992    pattern is skipped because DEFINE acts like a false condition. The rest of the
1993    pattern uses references to the named group to match the four dot-separated
1994    components of an IPv4 address, insisting on a word boundary at each end.
1995    .
1996    .SS "Assertion conditions"
1997    .rs
1998    .sp
1999    If the condition is not in any of the above formats, it must be an assertion.
2000  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2001  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2002  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 2032  next newline in the pattern.
2032  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2033  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2034  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
2035  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2036  that allows regular expressions to recurse (amongst other things). It does this  .P
2037  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
2038  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
2039  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2040    pattern using code interpolation to solve the parentheses problem can be
2041    created like this:
2042  .sp  .sp
2043    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2044  .sp  .sp
2045  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
2046  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2047  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2048  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2049  .P  supports special syntax for recursion of the entire pattern, and also for
2050  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2051  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.
2052  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2053  "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
2054  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2055  .P  provided that it occurs inside that subpattern. (If not, it is a
2056  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2057  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2058  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
2059    .\"
2060    call, which is described in the next section.) The special item (?R) or (?0) is
2061    a recursive call of the entire regular expression.
2062  .P  .P
2063  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2064  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2065  .sp  .sp
2066    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2067  .sp  .sp
2068  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2069  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
2070  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2071  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2072    to avoid backtracking into sequences of non-parentheses.
2073  .P  .P
2074  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
2075  pattern, so instead you could use this:  pattern, so instead you could use this:
2076  .sp  .sp
2077    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2078  .sp  .sp
2079  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
2080  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2081  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2082  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
2083  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).
2084  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
2085  .sp  most recently opened parentheses preceding the recursion. In other words, a
2086    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2087  .sp  it is encountered.
2088  This particular example pattern contains nested unlimited repeats, and so the  .P
2089  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2090  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
2091  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2092    .\" HTML <a href="#subpatternsassubroutines">
2093    .\" </a>
2094    "subroutine"
2095    .\"
2096    calls, as described in the next section.
2097    .P
2098    An alternative approach is to use named parentheses instead. The Perl syntax
2099    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2100    could rewrite the above example as follows:
2101    .sp
2102      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2103    .sp
2104    If there is more than one subpattern with the same name, the earliest one is
2105    used.
2106    .P
2107    This particular example pattern that we have been looking at contains nested
2108    unlimited repeats, and so the use of a possessive quantifier for matching
2109    strings of non-parentheses is important when applying the pattern to strings
2110    that do not match. For example, when this pattern is applied to
2111  .sp  .sp
2112    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2113  .sp  .sp
2114  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,
2115  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
2116  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
2117  before failure can be reported.  before failure can be reported.
2118  .P  .P
2119  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
2120  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
2121  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  
2122  .\" HREF  .\" HREF
2123  \fBpcrecallout\fP  \fBpcrecallout\fP
2124  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2126  documentation). If the pattern above is
2126  .sp  .sp
2127    (ab(cd)ef)    (ab(cd)ef)
2128  .sp  .sp
2129  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
2130  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
2131  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2132    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2133       ^                        ^  .P
2134       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2135  .sp  obtain extra memory to store data during a recursion, which it does by using
2136  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
2137  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.  
2138  .P  .P
2139  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.
2140  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 2148  different alternatives for the recursive
2148  is the actual recursive call.  is the actual recursive call.
2149  .  .
2150  .  .
2151    .\" HTML <a name="recursiondifference"></a>
2152    .SS "Recursion difference from Perl"
2153    .rs
2154    .sp
2155    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2156    treated as an atomic group. That is, once it has matched some of the subject
2157    string, it is never re-entered, even if it contains untried alternatives and
2158    there is a subsequent matching failure. This can be illustrated by the
2159    following pattern, which purports to match a palindromic string that contains
2160    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2161    .sp
2162      ^(.|(.)(?1)\e2)$
2163    .sp
2164    The idea is that it either matches a single character, or two identical
2165    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2166    it does not if the pattern is longer than three characters. Consider the
2167    subject string "abcba":
2168    .P
2169    At the top level, the first character is matched, but as it is not at the end
2170    of the string, the first alternative fails; the second alternative is taken
2171    and the recursion kicks in. The recursive call to subpattern 1 successfully
2172    matches the next character ("b"). (Note that the beginning and end of line
2173    tests are not part of the recursion).
2174    .P
2175    Back at the top level, the next character ("c") is compared with what
2176    subpattern 2 matched, which was "a". This fails. Because the recursion is
2177    treated as an atomic group, there are now no backtracking points, and so the
2178    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2179    try the second alternative.) However, if the pattern is written with the
2180    alternatives in the other order, things are different:
2181    .sp
2182      ^((.)(?1)\e2|.)$
2183    .sp
2184    This time, the recursing alternative is tried first, and continues to recurse
2185    until it runs out of characters, at which point the recursion fails. But this
2186    time we do have another alternative to try at the higher level. That is the big
2187    difference: in the previous case the remaining alternative is at a deeper
2188    recursion level, which PCRE cannot use.
2189    .P
2190    To change the pattern so that matches all palindromic strings, not just those
2191    with an odd number of characters, it is tempting to change the pattern to this:
2192    .sp
2193      ^((.)(?1)\e2|.?)$
2194    .sp
2195    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2196    deeper recursion has matched a single character, it cannot be entered again in
2197    order to match an empty string. The solution is to separate the two cases, and
2198    write out the odd and even cases as alternatives at the higher level:
2199    .sp
2200      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2201    .sp
2202    If you want to match typical palindromic phrases, the pattern has to ignore all
2203    non-word characters, which can be done like this:
2204    .sp
2205      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2206    .sp
2207    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2208    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2209    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2210    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2211    more) to match typical phrases, and Perl takes so long that you think it has
2212    gone into a loop.
2213    .P
2214    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2215    string does not start with a palindrome that is shorter than the entire string.
2216    For example, although "abcba" is correctly matched, if the subject is "ababa",
2217    PCRE finds the palindrome "aba" at the start, then fails at top level because
2218    the end of the string does not follow. Once again, it cannot jump back into the
2219    recursion to try other alternatives, so the entire match fails.
2220    .
2221    .
2222  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2223  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2224  .rs  .rs
2225  .sp  .sp
2226  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
2227  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
2228  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2229  pattern  before or after the reference. A numbered reference can be absolute or
2230    relative, as in these examples:
2231    .sp
2232      (...(absolute)...)...(?2)...
2233      (...(relative)...)...(?-1)...
2234      (...(?+1)...(relative)...
2235    .sp
2236    An earlier example pointed out that the pattern
2237  .sp  .sp
2238    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2239  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2243  matches "sense and sensibility" and "res
2243    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2244  .sp  .sp
2245  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
2246  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.  
2247  .P  .P
2248  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2249  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
2250  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
2251  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2252    call revert to their previous values afterwards.
2253    .P
2254    When a subpattern is used as a subroutine, processing options such as
2255    case-independence are fixed when the subpattern is defined. They cannot be
2256    changed for different calls. For example, consider this pattern:
2257    .sp
2258      (abc)(?i:(?-1))
2259    .sp
2260    It matches "abcabc". It does not match "abcABC" because the change of
2261    processing option does not affect the called subpattern.
2262    .
2263    .
2264    .\" HTML <a name="onigurumasubroutines"></a>
2265    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2266    .rs
2267    .sp
2268    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2269    a number enclosed either in angle brackets or single quotes, is an alternative
2270    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2271    are two of the examples used above, rewritten using this syntax:
2272    .sp
2273      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2274      (sens|respons)e and \eg'1'ibility
2275    .sp
2276    PCRE supports an extension to Oniguruma: if a number is preceded by a
2277    plus or a minus sign it is taken as a relative reference. For example:
2278    .sp
2279      (abc)(?i:\eg<-1>)
2280    .sp
2281    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2282    synonymous. The former is a back reference; the latter is a subroutine call.
2283  .  .
2284  .  .
2285  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2300  function is to be called. If you want to
2300  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.
2301  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2302  .sp  .sp
2303    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2304  .sp  .sp
2305  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
2306  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 2316  description of the interface to the call
2316  \fBpcrecallout\fP  \fBpcrecallout\fP
2317  .\"  .\"
2318  documentation.  documentation.
2319    .
2320    .
2321    .SH "BACKTRACKING CONTROL"
2322    .rs
2323    .sp
2324    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2325    are described in the Perl documentation as "experimental and subject to change
2326    or removal in a future version of Perl". It goes on to say: "Their usage in
2327    production code should be noted to avoid problems during upgrades." The same
2328    remarks apply to the PCRE features described in this section.
2329    .P
2330    Since these verbs are specifically related to backtracking, most of them can be
2331    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2332    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2333    failing negative assertion, they cause an error if encountered by
2334    \fBpcre_dfa_exec()\fP.
2335  .P  .P
2336  .in 0  If any of these verbs are used in an assertion or subroutine subpattern
2337  Last updated: 06 June 2006  (including recursive subpatterns), their effect is confined to that subpattern;
2338  .br  it does not extend to the surrounding pattern. Note that such subpatterns are
2339  Copyright (c) 1997-2006 University of Cambridge.  processed as anchored at the point where they are tested.
2340    .P
2341    The new verbs make use of what was previously invalid syntax: an opening
2342    parenthesis followed by an asterisk. In Perl, they are generally of the form
2343    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2344    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2345    are two kinds:
2346    .
2347    .SS "Verbs that act immediately"
2348    .rs
2349    .sp
2350    The following verbs act as soon as they are encountered:
2351    .sp
2352       (*ACCEPT)
2353    .sp
2354    This verb causes the match to end successfully, skipping the remainder of the
2355    pattern. When inside a recursion, only the innermost pattern is ended
2356    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2357    captured. (This feature was added to PCRE at release 8.00.) For example:
2358    .sp
2359      A((?:A|B(*ACCEPT)|C)D)
2360    .sp
2361    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2362    the outer parentheses.
2363    .sp
2364      (*FAIL) or (*F)
2365    .sp
2366    This verb causes the match to fail, forcing backtracking to occur. It is
2367    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2368    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2369    Perl features that are not present in PCRE. The nearest equivalent is the
2370    callout feature, as for example in this pattern:
2371    .sp
2372      a+(?C)(*FAIL)
2373    .sp
2374    A match with the string "aaaa" always fails, but the callout is taken before
2375    each backtrack happens (in this example, 10 times).
2376    .
2377    .SS "Verbs that act after backtracking"
2378    .rs
2379    .sp
2380    The following verbs do nothing when they are encountered. Matching continues
2381    with what follows, but if there is no subsequent match, a failure is forced.
2382    The verbs differ in exactly what kind of failure occurs.
2383    .sp
2384      (*COMMIT)
2385    .sp
2386    This verb causes the whole match to fail outright if the rest of the pattern
2387    does not match. Even if the pattern is unanchored, no further attempts to find
2388    a match by advancing the starting point take place. Once (*COMMIT) has been
2389    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2390    starting point, or not at all. For example:
2391    .sp
2392      a+(*COMMIT)b
2393    .sp
2394    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2395    dynamic anchor, or "I've started, so I must finish."
2396    .sp
2397      (*PRUNE)
2398    .sp
2399    This verb causes the match to fail at the current position if the rest of the
2400    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2401    advance to the next starting character then happens. Backtracking can occur as
2402    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2403    if there is no match to the right, backtracking cannot cross (*PRUNE).
2404    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2405    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2406    be expressed in any other way.
2407    .sp
2408      (*SKIP)
2409    .sp
2410    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2411    "bumpalong" advance is not to the next character, but to the position in the
2412    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2413    was matched leading up to it cannot be part of a successful match. Consider:
2414    .sp
2415      a+(*SKIP)b
2416    .sp
2417    If the subject is "aaaac...", after the first match attempt fails (starting at
2418    the first character in the string), the starting point skips on to start the
2419    next attempt at "c". Note that a possessive quantifer does not have the same
2420    effect as this example; although it would suppress backtracking during the
2421    first match attempt, the second attempt would start at the second character
2422    instead of skipping on to "c".
2423    .sp
2424      (*THEN)
2425    .sp
2426    This verb causes a skip to the next alternation if the rest of the pattern does
2427    not match. That is, it cancels pending backtracking, but only within the
2428    current alternation. Its name comes from the observation that it can be used
2429    for a pattern-based if-then-else block:
2430    .sp
2431      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2432    .sp
2433    If the COND1 pattern matches, FOO is tried (and possibly further items after
2434    the end of the group if FOO succeeds); on failure the matcher skips to the
2435    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2436    is used outside of any alternation, it acts exactly like (*PRUNE).
2437    .
2438    .
2439    .SH "SEE ALSO"
2440    .rs
2441    .sp
2442    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2443    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2444    .
2445    .
2446    .SH AUTHOR
2447    .rs
2448    .sp
2449    .nf
2450    Philip Hazel
2451    University Computing Service
2452    Cambridge CB2 3QH, England.
2453    .fi
2454    .
2455    .
2456    .SH REVISION
2457    .rs
2458    .sp
2459    .nf
2460    Last updated: 06 March 2010
2461    Copyright (c) 1997-2010 University of Cambridge.
2462    .fi

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