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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 456 by ph10, Fri Oct 2 08:53:31 2009 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 backreference
# 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    Balinese,
517  Bengali,  Bengali,
518  Bopomofo,  Bopomofo,
519  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 523  Canadian_Aboriginal,
523  Cherokee,  Cherokee,
524  Common,  Common,
525  Coptic,  Coptic,
526    Cuneiform,
527  Cypriot,  Cypriot,
528  Cyrillic,  Cyrillic,
529  Deseret,  Deseret,
# Line 345  Malayalam, Line 553  Malayalam,
553  Mongolian,  Mongolian,
554  Myanmar,  Myanmar,
555  New_Tai_Lue,  New_Tai_Lue,
556    Nko,
557  Ogham,  Ogham,
558  Old_Italic,  Old_Italic,
559  Old_Persian,  Old_Persian,
560  Oriya,  Oriya,
561  Osmanya,  Osmanya,
562    Phags_Pa,
563    Phoenician,
564  Runic,  Runic,
565  Shavian,  Shavian,
566  Sinhala,  Sinhala,
# Line 430  The special property L& is also supporte Line 641  The special property L& is also supporte
641  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
642  a modifier or "other".  a modifier or "other".
643  .P  .P
644  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
645    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
646    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
647    (see the discussion of PCRE_NO_UTF8_CHECK in the
648    .\" HREF
649    \fBpcreapi\fP
650    .\"
651    page). Perl does not support the Cs property.
652    .P
653    The long synonyms for property names that Perl supports (such as \ep{Letter})
654  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
655  properties with "Is".  properties with "Is".
656  .P  .P
# Line 454  atomic group Line 674  atomic group
674  (see below).  (see below).
675  .\"  .\"
676  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
677  preceding character.  preceding character. None of them have codepoints less than 256, so in
678    non-UTF-8 mode \eX matches any one character.
679  .P  .P
680  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
681  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 683  why the traditional escape sequences suc
683  properties in PCRE.  properties in PCRE.
684  .  .
685  .  .
686    .\" HTML <a name="resetmatchstart"></a>
687    .SS "Resetting the match start"
688    .rs
689    .sp
690    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
691    matched characters not to be included in the final matched sequence. For
692    example, the pattern:
693    .sp
694      foo\eKbar
695    .sp
696    matches "foobar", but reports that it has matched "bar". This feature is
697    similar to a lookbehind assertion
698    .\" HTML <a href="#lookbehind">
699    .\" </a>
700    (described below).
701    .\"
702    However, in this case, the part of the subject before the real match does not
703    have to be of fixed length, as lookbehind assertions do. The use of \eK does
704    not interfere with the setting of
705    .\" HTML <a href="#subpattern">
706    .\" </a>
707    captured substrings.
708    .\"
709    For example, when the pattern
710    .sp
711      (foo)\eKbar
712    .sp
713    matches "foobar", the first substring is still set to "foo".
714    .
715    .
716  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
717  .SS "Simple assertions"  .SS "Simple assertions"
718  .rs  .rs
719  .sp  .sp
720  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
721  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,
722  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
723  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 729  The backslashed assertions are:
729  .sp  .sp
730    \eb     matches at a word boundary    \eb     matches at a word boundary
731    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
732    \eA     matches at start of subject    \eA     matches at the start of the subject
733    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
734    \ez     matches at end of subject            also matches before a newline at the end of the subject
735    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
736      \eG     matches at the first matching position in the subject
737  .sp  .sp
738  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
739  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 741  different meaning, namely the backspace
741  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
742  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
743  \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
744  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745    separte "start of word" or "end of word" metasequence. However, whatever
746    follows \eb normally determines which it is. For example, the fragment
747    \eba matches "a" at the start of a word.
748  .P  .P
749  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
750  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 833  end of the subject in both modes, and if
833  .sp  .sp
834  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
835  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
836  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.
837  a line ending is defined as a single character (CR or LF), dot never matches  .P
838  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
839  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
840  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
841    (including isolated CRs and LFs). When any Unicode line endings are being
842    recognized, dot does not match CR or LF or any of the other line ending
843    characters.
844  .P  .P
845  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
846  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
847  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
848    to match it.
849  .P  .P
850  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
851  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 856  special meaning in a character class.
856  .rs  .rs
857  .sp  .sp
858  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
859  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
860  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
861  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,
862  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,
863  sequence is best avoided.  the \eC escape sequence is best avoided.
864  .P  .P
865  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
866  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 617  the lookbehind. Line 876  the lookbehind.
876  .rs  .rs
877  .sp  .sp
878  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
879  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.
880  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
881  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
882  escaped with a backslash.  a member of the class, it should be the first data character in the class
883    (after an initial circumflex, if present) or escaped with a backslash.
884  .P  .P
885  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
886  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
887  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
888  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
889  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
890  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 894  For example, the character class [aeiou]
894  [^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
895  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
896  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
897  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
898  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
899  string.  string.
900  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 908  caseful version would. In UTF-8 mode, PC
908  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
909  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
910  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
911  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,
912  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
913  UTF-8 support.  with UTF-8 support.
914  .P  .P
915  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
916  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
917  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
918  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
919  .P  .P
920  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
921  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 939  example [\ex{100}-\ex{2ff}].
939  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
940  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
941  [][\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
942  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
943  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
944  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
945  property support.  property support.
# Line 769  alternative in the subpattern. Line 1029  alternative in the subpattern.
1029  .rs  .rs
1030  .sp  .sp
1031  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1032  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
1033  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1034    The option letters are
1035  .sp  .sp
1036    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1037    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1045  PCRE_MULTILINE while unsetting PCRE_DOTA
1045  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
1046  unset.  unset.
1047  .P  .P
1048  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
1049  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
1050  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
 the global options (and it will therefore show up in data extracted by the  
 \fBpcre_fullinfo()\fP function).  
1051  .P  .P
1052  An option change within a subpattern affects only that part of the current  When one of these option changes occurs at top level (that is, not inside
1053  pattern that follows it, so  subpattern parentheses), the change applies to the remainder of the pattern
1054    that follows. If the change is placed right at the start of a pattern, PCRE
1055    extracts it into the global options (and it will therefore show up in data
1056    extracted by the \fBpcre_fullinfo()\fP function).
1057    .P
1058    An option change within a subpattern (see below for a description of
1059    subpatterns) affects only that part of the current pattern that follows it, so
1060  .sp  .sp
1061    (a(?i)b)c    (a(?i)b)c
1062  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1072  branch is abandoned before the option se
1072  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1073  behaviour otherwise.  behaviour otherwise.
1074  .P  .P
1075  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
1076  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
1077  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1078    the application has set or what has been defaulted. Details are given in the
1079    section entitled
1080    .\" HTML <a href="#newlineseq">
1081    .\" </a>
1082    "Newline sequences"
1083    .\"
1084    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1085    mode; this is equivalent to setting the PCRE_UTF8 option.
1086  .  .
1087  .  .
1088  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1097  Turning part of a pattern into a subpatt
1097    cat(aract|erpillar|)    cat(aract|erpillar|)
1098  .sp  .sp
1099  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1100  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1101  .sp  .sp
1102  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
1103  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 1122  the string "the white queen" is matched
1122    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1123  .sp  .sp
1124  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
1125  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.  
1126  .P  .P
1127  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
1128  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 1137  is reached, an option setting in one bra
1137  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1138  .  .
1139  .  .
1140    .\" HTML <a name="dupsubpatternnumber"></a>
1141    .SH "DUPLICATE SUBPATTERN NUMBERS"
1142    .rs
1143    .sp
1144    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1145    the same numbers for its capturing parentheses. Such a subpattern starts with
1146    (?| and is itself a non-capturing subpattern. For example, consider this
1147    pattern:
1148    .sp
1149      (?|(Sat)ur|(Sun))day
1150    .sp
1151    Because the two alternatives are inside a (?| group, both sets of capturing
1152    parentheses are numbered one. Thus, when the pattern matches, you can look
1153    at captured substring number one, whichever alternative matched. This construct
1154    is useful when you want to capture part, but not all, of one of a number of
1155    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1156    number is reset at the start of each branch. The numbers of any capturing
1157    buffers that follow the subpattern start after the highest number used in any
1158    branch. The following example is taken from the Perl documentation.
1159    The numbers underneath show in which buffer the captured content will be
1160    stored.
1161    .sp
1162      # before  ---------------branch-reset----------- after
1163      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164      # 1            2         2  3        2     3     4
1165    .sp
1166    A backreference to a numbered subpattern uses the most recent value that is set
1167    for that number by any subpattern. The following pattern matches "abcabc" or
1168    "defdef":
1169    .sp
1170      /(?|(abc)|(def))\1/
1171    .sp
1172    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173    refers to the first one in the pattern with the given number. The following
1174    pattern matches "abcabc" or "defabc":
1175    .sp
1176      /(?|(abc)|(def))(?1)/
1177    .sp
1178    .P
1179    An alternative approach to using the "branch reset" feature is to use
1180    duplicate named subpatterns, as described in the next section.
1181    .
1182    .
1183  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1184  .rs  .rs
1185  .sp  .sp
1186  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
1187  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1188  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
1189  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1190  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
1191    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1192    the Perl and the Python syntax.
1193    .P
1194    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1195    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1196  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1197  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1198  .\" </a>  .\" </a>
# Line 890  conditions, Line 1210  conditions,
1210  can be made by name as well as by number.  can be made by name as well as by number.
1211  .P  .P
1212  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1213  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1214  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
1215  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
1216  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1217  .P  .P
1218  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
1219  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. This can
# Line 902  match. Suppose you want to match the nam Line 1222  match. Suppose you want to match the nam
1222  abbreviation or as the full name, and in both cases you want to extract the  abbreviation or as the full name, and in both cases you want to extract the
1223  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1224  .sp  .sp
1225    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1226    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
1227    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
1228    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
1229    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1230  .sp  .sp
1231  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.
1232    (An alternative way of solving this problem is to use a "branch reset"
1233    subpattern, as described in the previous section.)
1234    .P
1235  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1236  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
1237  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
1238  make a reference to a non-unique named subpattern from elsewhere in the  make a reference to a non-unique named subpattern from elsewhere in the
1239  pattern, the one that corresponds to the lowest number is used. For further  pattern, the one that corresponds to the lowest number is used. For further
# Line 919  details of the interfaces for handling n Line 1242  details of the interfaces for handling n
1242  \fBpcreapi\fP  \fBpcreapi\fP
1243  .\"  .\"
1244  documentation.  documentation.
1245    .P
1246    \fBWarning:\fP You cannot use different names to distinguish between two
1247    subpatterns with the same number (see the previous section) because PCRE uses
1248    only the numbers when matching.
1249  .  .
1250  .  .
1251  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1255  Repetition is specified by quantifiers,
1255  items:  items:
1256  .sp  .sp
1257    a literal data character    a literal data character
1258    the . metacharacter    the dot metacharacter
1259    the \eC escape sequence    the \eC escape sequence
1260    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1261      the \eR escape sequence
1262    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1263    a character class    a character class
1264    a back reference (see next section)    a back reference (see next section)
1265    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1266      a recursive or "subroutine" call to a subpattern
1267  .sp  .sp
1268  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1269  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 1295  support is available, \eX{3} matches thr
1295  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).
1296  .P  .P
1297  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
1298  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1299    subpatterns that are referenced as
1300    .\" HTML <a href="#subpatternsassubroutines">
1301    .\" </a>
1302    subroutines
1303    .\"
1304    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1305    quantifier are omitted from the compiled pattern.
1306  .P  .P
1307  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1308  quantifiers have single-character abbreviations:  abbreviations:
1309  .sp  .sp
1310    *    is equivalent to {0,}    *    is equivalent to {0,}
1311    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1353  own right. Because it has two uses, it c
1353  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
1354  way the rest of the pattern matches.  way the rest of the pattern matches.
1355  .P  .P
1356  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),
1357  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
1358  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
1359  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1363  is greater than 1 or with a limited maxi
1363  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1364  .P  .P
1365  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
1366  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
1367  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1368  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
1369  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 1039  alternatively using ^ to indicate anchor Line 1375  alternatively using ^ to indicate anchor
1375  .P  .P
1376  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1377  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1378  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
1379  succeed. Consider, for example:  succeeds. Consider, for example:
1380  .sp  .sp
1381    (.*)abc\e1    (.*)abc\e1
1382  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1402  matches "aba" the value of the second ca
1402  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1403  .rs  .rs
1404  .sp  .sp
1405  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1406  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
1407  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
1408  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
1409  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
1410  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1411  .P  .P
1412  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
1413  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1419  item, and then with 4, and so on, before
1419  (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
1420  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.
1421  .P  .P
1422  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
1423  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
1424  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1425  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1449  previous example can be rewritten as
1449  .sp  .sp
1450    \ed++foo    \ed++foo
1451  .sp  .sp
1452    Note that a possessive quantifier can be used with an entire group, for
1453    example:
1454    .sp
1455      (abc|xyz){2,3}+
1456    .sp
1457  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1458  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
1459  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
1460  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1461  .P  difference; possessive quantifiers should be slightly faster.
1462  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1463  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.
1464  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
1465  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1466    package, and PCRE copied it from there. It ultimately found its way into Perl
1467    at release 5.10.
1468    .P
1469    PCRE has an optimization that automatically "possessifies" certain simple
1470    pattern constructs. For example, the sequence A+B is treated as A++B because
1471    there is no point in backtracking into a sequence of A's when B must follow.
1472  .P  .P
1473  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
1474  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 1514  numbers less than 10. A "forward back re
1514  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
1515  in an earlier iteration.  in an earlier iteration.
1516  .P  .P
1517  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
1518  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
1519  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1520  "Non-printing characters"  "Non-printing characters"
1521  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1522  .\" </a>  .\" </a>
1523  above  above
1524  .\"  .\"
1525  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1526    no such problem when named parentheses are used. A back reference to any
1527    subpattern is possible using named parentheses (see below).
1528    .P
1529    Another way of avoiding the ambiguity inherent in the use of digits following a
1530    backslash is to use the \eg escape sequence, which is a feature introduced in
1531    Perl 5.10. This escape must be followed by an unsigned number or a negative
1532    number, optionally enclosed in braces. These examples are all identical:
1533    .sp
1534      (ring), \e1
1535      (ring), \eg1
1536      (ring), \eg{1}
1537    .sp
1538    An unsigned number specifies an absolute reference without the ambiguity that
1539    is present in the older syntax. It is also useful when literal digits follow
1540    the reference. A negative number is a relative reference. Consider this
1541    example:
1542    .sp
1543      (abc(def)ghi)\eg{-1}
1544    .sp
1545    The sequence \eg{-1} is a reference to the most recently started capturing
1546    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1547    would be equivalent to \e1. The use of relative references can be helpful in
1548    long patterns, and also in patterns that are created by joining together
1549    fragments that contain references within themselves.
1550  .P  .P
1551  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1552  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 1568  back reference, the case of letters is r
1568  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
1569  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1570  .P  .P
1571  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1572  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1573    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1574    back reference syntax, in which \eg can be used for both numeric and named
1575    references, is also supported. We could rewrite the above example in any of
1576    the following ways:
1577  .sp  .sp
1578      (?<p1>(?i)rah)\es+\ek<p1>
1579      (?'p1'(?i)rah)\es+\ek{p1}
1580    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1581      (?<p1>(?i)rah)\es+\eg{p1}
1582  .sp  .sp
1583  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
1584  after the reference.  after the reference.
1585  .P  .P
1586  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
1587  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1588  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1589  .sp  .sp
1590    (a|(bc))\e2    (a|(bc))\e2
1591  .sp  .sp
1592  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
1593  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
1594  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1595  with a digit character, some delimiter must be used to terminate the back  .P
1596  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1597  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1598    If the pattern continues with a digit character, some delimiter must be used to
1599    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1600    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1601  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1602  .\" </a>  .\" </a>
1603  "Comments"  "Comments"
# Line 1289  lookbehind assertion is needed to achiev Line 1670  lookbehind assertion is needed to achiev
1670  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
1671  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
1672  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.
1673    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1674    synonym for (?!).
1675  .  .
1676  .  .
1677  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1696  is permitted, but
1696  .sp  .sp
1697  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1698  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
1699  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
1700  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1701  .sp  .sp
1702    (?<=ab(c|de))    (?<=ab(c|de))
1703  .sp  .sp
1704  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
1705  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
1706    branches:
1707  .sp  .sp
1708    (?<=abc|abde)    (?<=abc|abde)
1709  .sp  .sp
1710    In some cases, the Perl 5.10 escape sequence \eK
1711    .\" HTML <a href="#resetmatchstart">
1712    .\" </a>
1713    (see above)
1714    .\"
1715    can be used instead of a lookbehind assertion to get round the fixed-length
1716    restriction.
1717    .P
1718  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1719  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
1720  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1721  match is deemed to fail.  assertion fails.
1722  .P  .P
1723  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)
1724  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1725  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
1726  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1727    .P
1728    .\" HTML <a href="#subpatternsassubroutines">
1729    .\" </a>
1730    "Subroutine"
1731    .\"
1732    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1733    as the subpattern matches a fixed-length string.
1734    .\" HTML <a href="#recursion">
1735    .\" </a>
1736    Recursion,
1737    .\"
1738    however, is not supported.
1739  .P  .P
1740  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1741  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
1742  such as  strings. Consider a simple pattern such as
1743  .sp  .sp
1744    abcd$    abcd$
1745  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1755  then all but the last two characters, an
1755  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,
1756  if the pattern is written as  if the pattern is written as
1757  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1758    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1759  .sp  .sp
1760  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
1761  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
1762  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1763  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 1803  characters that are not "999".
1803  .sp  .sp
1804  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1805  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1806  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1807  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1808  .sp  .sp
1809    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1810    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1413  If the condition is satisfied, the yes-p Line 1813  If the condition is satisfied, the yes-p
1813  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
1814  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1815  .P  .P
1816  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1817  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1818  underscores, the condition is satisfied if the capturing subpattern of that  .
1819  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1820  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1821  named subpattern; if it cannot find one and the text consists entirely of  .sp
1822  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
1823  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
1824  recommended.  matched. If there is more than one capturing subpattern with the same number
1825    (see the earlier
1826    .\"
1827    .\" HTML <a href="#recursion">
1828    .\" </a>
1829    section about duplicate subpattern numbers),
1830    .\"
1831    the condition is true if any of them have been set. An alternative notation is
1832    to precede the digits with a plus or minus sign. In this case, the subpattern
1833    number is relative rather than absolute. The most recently opened parentheses
1834    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1835    looping constructs it can also make sense to refer to subsequent groups with
1836    constructs such as (?(+2).
1837  .P  .P
1838  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1839  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 1849  or not. If they did, that is, if subject
1849  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
1850  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1851  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1852  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1853  named subpattern gives this:  .P
1854    If you were embedding this pattern in a larger one, you could use a relative
1855    reference:
1856    .sp
1857      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1858  .sp  .sp
1859    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )  This makes the fragment independent of the parentheses in the larger pattern.
1860    .
1861    .SS "Checking for a used subpattern by name"
1862    .rs
1863    .sp
1864    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1865    subpattern by name. For compatibility with earlier versions of PCRE, which had
1866    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1867    there is a possible ambiguity with this syntax, because subpattern names may
1868    consist entirely of digits. PCRE looks first for a named subpattern; if it
1869    cannot find one and the name consists entirely of digits, PCRE looks for a
1870    subpattern of that number, which must be greater than zero. Using subpattern
1871    names that consist entirely of digits is not recommended.
1872    .P
1873    Rewriting the above example to use a named subpattern gives this:
1874    .sp
1875      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1876    .sp
1877    .
1878    .SS "Checking for pattern recursion"
1879    .rs
1880  .sp  .sp
1881  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,
1882  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
1883  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
1884  Recursive patterns are described in the next section.  letter R, for example:
1885    .sp
1886      (?(R3)...) or (?(R&name)...)
1887    .sp
1888    the condition is true if the most recent recursion is into a subpattern whose
1889    number or name is given. This condition does not check the entire recursion
1890    stack.
1891  .P  .P
1892  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.
1893    .\" HTML <a href="#recursion">
1894    .\" </a>
1895    Recursive patterns
1896    .\"
1897    are described below.
1898    .
1899    .SS "Defining subpatterns for use by reference only"
1900    .rs
1901    .sp
1902    If the condition is the string (DEFINE), and there is no subpattern with the
1903    name DEFINE, the condition is always false. In this case, there may be only one
1904    alternative in the subpattern. It is always skipped if control reaches this
1905    point in the pattern; the idea of DEFINE is that it can be used to define
1906    "subroutines" that can be referenced from elsewhere. (The use of
1907    .\" HTML <a href="#subpatternsassubroutines">
1908    .\" </a>
1909    "subroutines"
1910    .\"
1911    is described below.) For example, a pattern to match an IPv4 address could be
1912    written like this (ignore whitespace and line breaks):
1913    .sp
1914      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1915      \eb (?&byte) (\e.(?&byte)){3} \eb
1916    .sp
1917    The first part of the pattern is a DEFINE group inside which a another group
1918    named "byte" is defined. This matches an individual component of an IPv4
1919    address (a number less than 256). When matching takes place, this part of the
1920    pattern is skipped because DEFINE acts like a false condition. The rest of the
1921    pattern uses references to the named group to match the four dot-separated
1922    components of an IPv4 address, insisting on a word boundary at each end.
1923    .
1924    .SS "Assertion conditions"
1925    .rs
1926    .sp
1927    If the condition is not in any of the above formats, it must be an assertion.
1928  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1929  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1930  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1960  next newline in the pattern.
1960  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1961  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1962  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
1963  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1964  that allows regular expressions to recurse (amongst other things). It does this  .P
1965  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
1966  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
1967  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1968    pattern using code interpolation to solve the parentheses problem can be
1969    created like this:
1970  .sp  .sp
1971    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1972  .sp  .sp
1973  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
1974  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1975  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1976  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1977  .P  supports special syntax for recursion of the entire pattern, and also for
1978  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1979  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.
1980  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1981  "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
1982  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1983  .P  provided that it occurs inside that subpattern. (If not, it is a
1984  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
1985  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
1986  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
1987    .\"
1988    call, which is described in the next section.) The special item (?R) or (?0) is
1989    a recursive call of the entire regular expression.
1990  .P  .P
1991  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1992  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1993  .sp  .sp
1994    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
1995  .sp  .sp
1996  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1997  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
1998  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1999  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2000    to avoid backtracking into sequences of non-parentheses.
2001  .P  .P
2002  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
2003  pattern, so instead you could use this:  pattern, so instead you could use this:
2004  .sp  .sp
2005    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2006  .sp  .sp
2007  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
2008  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2009  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2010  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
2011  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).
2012  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
2013  .sp  most recently opened parentheses preceding the recursion. In other words, a
2014    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2015  .sp  it is encountered.
2016  This particular example pattern contains nested unlimited repeats, and so the  .P
2017  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2018  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
2019  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2020    .\" HTML <a href="#subpatternsassubroutines">
2021    .\" </a>
2022    "subroutine"
2023    .\"
2024    calls, as described in the next section.
2025    .P
2026    An alternative approach is to use named parentheses instead. The Perl syntax
2027    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2028    could rewrite the above example as follows:
2029    .sp
2030      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2031    .sp
2032    If there is more than one subpattern with the same name, the earliest one is
2033    used.
2034    .P
2035    This particular example pattern that we have been looking at contains nested
2036    unlimited repeats, and so the use of a possessive quantifier for matching
2037    strings of non-parentheses is important when applying the pattern to strings
2038    that do not match. For example, when this pattern is applied to
2039  .sp  .sp
2040    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2041  .sp  .sp
2042  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,
2043  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
2044  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
2045  before failure can be reported.  before failure can be reported.
# Line 1545  before failure can be reported. Line 2047  before failure can be reported.
2047  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
2048  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
2049  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
2050  the next section and the  below and the
2051  .\" HREF  .\" HREF
2052  \fBpcrecallout\fP  \fBpcrecallout\fP
2053  .\"  .\"
# Line 1556  documentation). If the pattern above is Line 2058  documentation). If the pattern above is
2058  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
2059  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
2060  .sp  .sp
2061    \e( ( ( (?>[^()]+) | (?R) )* ) \e)    \e( ( ( [^()]++ | (?R) )* ) \e)
2062       ^                        ^       ^                        ^
2063       ^                        ^       ^                        ^
2064  .sp  .sp
# Line 1578  different alternatives for the recursive Line 2080  different alternatives for the recursive
2080  is the actual recursive call.  is the actual recursive call.
2081  .  .
2082  .  .
2083    .\" HTML <a name="recursiondifference"></a>
2084    .SS "Recursion difference from Perl"
2085    .rs
2086    .sp
2087    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2088    treated as an atomic group. That is, once it has matched some of the subject
2089    string, it is never re-entered, even if it contains untried alternatives and
2090    there is a subsequent matching failure. This can be illustrated by the
2091    following pattern, which purports to match a palindromic string that contains
2092    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2093    .sp
2094      ^(.|(.)(?1)\e2)$
2095    .sp
2096    The idea is that it either matches a single character, or two identical
2097    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2098    it does not if the pattern is longer than three characters. Consider the
2099    subject string "abcba":
2100    .P
2101    At the top level, the first character is matched, but as it is not at the end
2102    of the string, the first alternative fails; the second alternative is taken
2103    and the recursion kicks in. The recursive call to subpattern 1 successfully
2104    matches the next character ("b"). (Note that the beginning and end of line
2105    tests are not part of the recursion).
2106    .P
2107    Back at the top level, the next character ("c") is compared with what
2108    subpattern 2 matched, which was "a". This fails. Because the recursion is
2109    treated as an atomic group, there are now no backtracking points, and so the
2110    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2111    try the second alternative.) However, if the pattern is written with the
2112    alternatives in the other order, things are different:
2113    .sp
2114      ^((.)(?1)\e2|.)$
2115    .sp
2116    This time, the recursing alternative is tried first, and continues to recurse
2117    until it runs out of characters, at which point the recursion fails. But this
2118    time we do have another alternative to try at the higher level. That is the big
2119    difference: in the previous case the remaining alternative is at a deeper
2120    recursion level, which PCRE cannot use.
2121    .P
2122    To change the pattern so that matches all palindromic strings, not just those
2123    with an odd number of characters, it is tempting to change the pattern to this:
2124    .sp
2125      ^((.)(?1)\e2|.?)$
2126    .sp
2127    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2128    deeper recursion has matched a single character, it cannot be entered again in
2129    order to match an empty string. The solution is to separate the two cases, and
2130    write out the odd and even cases as alternatives at the higher level:
2131    .sp
2132      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2133    .sp
2134    If you want to match typical palindromic phrases, the pattern has to ignore all
2135    non-word characters, which can be done like this:
2136    .sp
2137      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2138    .sp
2139    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2140    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2141    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2142    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2143    more) to match typical phrases, and Perl takes so long that you think it has
2144    gone into a loop.
2145    .P
2146    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2147    string does not start with a palindrome that is shorter than the entire string.
2148    For example, although "abcba" is correctly matched, if the subject is "ababa",
2149    PCRE finds the palindrome "aba" at the start, then fails at top level because
2150    the end of the string does not follow. Once again, it cannot jump back into the
2151    recursion to try other alternatives, so the entire match fails.
2152    .
2153    .
2154  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2155  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2156  .rs  .rs
2157  .sp  .sp
2158  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
2159  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
2160  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2161  pattern  before or after the reference. A numbered reference can be absolute or
2162    relative, as in these examples:
2163    .sp
2164      (...(absolute)...)...(?2)...
2165      (...(relative)...)...(?-1)...
2166      (...(?+1)...(relative)...
2167    .sp
2168    An earlier example pointed out that the pattern
2169  .sp  .sp
2170    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2171  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2175  matches "sense and sensibility" and "res
2175    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2176  .sp  .sp
2177  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
2178  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.  
2179  .P  .P
2180  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2181  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
2182  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
2183  matching failure.  matching failure.
2184    .P
2185    When a subpattern is used as a subroutine, processing options such as
2186    case-independence are fixed when the subpattern is defined. They cannot be
2187    changed for different calls. For example, consider this pattern:
2188    .sp
2189      (abc)(?i:(?-1))
2190    .sp
2191    It matches "abcabc". It does not match "abcABC" because the change of
2192    processing option does not affect the called subpattern.
2193    .
2194    .
2195    .\" HTML <a name="onigurumasubroutines"></a>
2196    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2197    .rs
2198    .sp
2199    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2200    a number enclosed either in angle brackets or single quotes, is an alternative
2201    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2202    are two of the examples used above, rewritten using this syntax:
2203    .sp
2204      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2205      (sens|respons)e and \eg'1'ibility
2206    .sp
2207    PCRE supports an extension to Oniguruma: if a number is preceded by a
2208    plus or a minus sign it is taken as a relative reference. For example:
2209    .sp
2210      (abc)(?i:\eg<-1>)
2211    .sp
2212    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2213    synonymous. The former is a back reference; the latter is a subroutine call.
2214  .  .
2215  .  .
2216  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2231  function is to be called. If you want to
2231  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.
2232  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2233  .sp  .sp
2234    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2235  .sp  .sp
2236  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
2237  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 2247  description of the interface to the call
2247  \fBpcrecallout\fP  \fBpcrecallout\fP
2248  .\"  .\"
2249  documentation.  documentation.
2250    .
2251    .
2252    .SH "BACKTRACKING CONTROL"
2253    .rs
2254    .sp
2255    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2256    are described in the Perl documentation as "experimental and subject to change
2257    or removal in a future version of Perl". It goes on to say: "Their usage in
2258    production code should be noted to avoid problems during upgrades." The same
2259    remarks apply to the PCRE features described in this section.
2260    .P
2261    Since these verbs are specifically related to backtracking, most of them can be
2262    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2263    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2264    failing negative assertion, they cause an error if encountered by
2265    \fBpcre_dfa_exec()\fP.
2266    .P
2267    If any of these verbs are used in an assertion subpattern, their effect is
2268    confined to that subpattern; it does not extend to the surrounding pattern.
2269    Note that assertion subpatterns are processed as anchored at the point where
2270    they are tested.
2271  .P  .P
2272  .in 0  The new verbs make use of what was previously invalid syntax: an opening
2273  Last updated: 06 June 2006  parenthesis followed by an asterisk. In Perl, they are generally of the form
2274  .br  (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2275  Copyright (c) 1997-2006 University of Cambridge.  form is just (*VERB). Any number of these verbs may occur in a pattern. There
2276    are two kinds:
2277    .
2278    .SS "Verbs that act immediately"
2279    .rs
2280    .sp
2281    The following verbs act as soon as they are encountered:
2282    .sp
2283       (*ACCEPT)
2284    .sp
2285    This verb causes the match to end successfully, skipping the remainder of the
2286    pattern. When inside a recursion, only the innermost pattern is ended
2287    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2288    captured. (This feature was added to PCRE at release 8.00.) For example:
2289    .sp
2290      A((?:A|B(*ACCEPT)|C)D)
2291    .sp
2292    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2293    the outer parentheses.
2294    .sp
2295      (*FAIL) or (*F)
2296    .sp
2297    This verb causes the match to fail, forcing backtracking to occur. It is
2298    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2299    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2300    Perl features that are not present in PCRE. The nearest equivalent is the
2301    callout feature, as for example in this pattern:
2302    .sp
2303      a+(?C)(*FAIL)
2304    .sp
2305    A match with the string "aaaa" always fails, but the callout is taken before
2306    each backtrack happens (in this example, 10 times).
2307    .
2308    .SS "Verbs that act after backtracking"
2309    .rs
2310    .sp
2311    The following verbs do nothing when they are encountered. Matching continues
2312    with what follows, but if there is no subsequent match, a failure is forced.
2313    The verbs differ in exactly what kind of failure occurs.
2314    .sp
2315      (*COMMIT)
2316    .sp
2317    This verb causes the whole match to fail outright if the rest of the pattern
2318    does not match. Even if the pattern is unanchored, no further attempts to find
2319    a match by advancing the starting point take place. Once (*COMMIT) has been
2320    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2321    starting point, or not at all. For example:
2322    .sp
2323      a+(*COMMIT)b
2324    .sp
2325    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2326    dynamic anchor, or "I've started, so I must finish."
2327    .sp
2328      (*PRUNE)
2329    .sp
2330    This verb causes the match to fail at the current position if the rest of the
2331    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2332    advance to the next starting character then happens. Backtracking can occur as
2333    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2334    if there is no match to the right, backtracking cannot cross (*PRUNE).
2335    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2336    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2337    be expressed in any other way.
2338    .sp
2339      (*SKIP)
2340    .sp
2341    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2342    "bumpalong" advance is not to the next character, but to the position in the
2343    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2344    was matched leading up to it cannot be part of a successful match. Consider:
2345    .sp
2346      a+(*SKIP)b
2347    .sp
2348    If the subject is "aaaac...", after the first match attempt fails (starting at
2349    the first character in the string), the starting point skips on to start the
2350    next attempt at "c". Note that a possessive quantifer does not have the same
2351    effect as this example; although it would suppress backtracking during the
2352    first match attempt, the second attempt would start at the second character
2353    instead of skipping on to "c".
2354    .sp
2355      (*THEN)
2356    .sp
2357    This verb causes a skip to the next alternation if the rest of the pattern does
2358    not match. That is, it cancels pending backtracking, but only within the
2359    current alternation. Its name comes from the observation that it can be used
2360    for a pattern-based if-then-else block:
2361    .sp
2362      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2363    .sp
2364    If the COND1 pattern matches, FOO is tried (and possibly further items after
2365    the end of the group if FOO succeeds); on failure the matcher skips to the
2366    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2367    is used outside of any alternation, it acts exactly like (*PRUNE).
2368    .
2369    .
2370    .SH "SEE ALSO"
2371    .rs
2372    .sp
2373    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2374    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2375    .
2376    .
2377    .SH AUTHOR
2378    .rs
2379    .sp
2380    .nf
2381    Philip Hazel
2382    University Computing Service
2383    Cambridge CB2 3QH, England.
2384    .fi
2385    .
2386    .
2387    .SH REVISION
2388    .rs
2389    .sp
2390    .nf
2391    Last updated: 30 September 2009
2392    Copyright (c) 1997-2009 University of Cambridge.
2393    .fi

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