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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 247 by ph10, Mon Sep 17 09:38:32 2007 UTC
# Line 14  man page, in case the conversion went wr Line 14  man page, in case the conversion went wr
14  <br>  <br>
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17  <li><a name="TOC2" href="#SEC2">BACKSLASH</a>  <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18  <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>  <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19  <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>  <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20  <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>  <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21  <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a>  <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
22  <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>  <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23  <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>  <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24  <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>  <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25  <li><a name="TOC10" href="#SEC10">SUBPATTERNS</a>  <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26  <li><a name="TOC11" href="#SEC11">NAMED SUBPATTERNS</a>  <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27  <li><a name="TOC12" href="#SEC12">REPETITION</a>  <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28  <li><a name="TOC13" href="#SEC13">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>  <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29  <li><a name="TOC14" href="#SEC14">BACK REFERENCES</a>  <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30  <li><a name="TOC15" href="#SEC15">ASSERTIONS</a>  <li><a name="TOC15" href="#SEC15">REPETITION</a>
31  <li><a name="TOC16" href="#SEC16">CONDITIONAL SUBPATTERNS</a>  <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32  <li><a name="TOC17" href="#SEC17">COMMENTS</a>  <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33  <li><a name="TOC18" href="#SEC18">RECURSIVE PATTERNS</a>  <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34  <li><a name="TOC19" href="#SEC19">SUBPATTERNS AS SUBROUTINES</a>  <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35  <li><a name="TOC20" href="#SEC20">CALLOUTS</a>  <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36    <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37    <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38    <li><a name="TOC23" href="#SEC23">CALLOUTS</a>
39    <li><a name="TOC24" href="#SEC24">BACKTRACKING CONTROL</a>
40    <li><a name="TOC25" href="#SEC25">SEE ALSO</a>
41    <li><a name="TOC26" href="#SEC26">AUTHOR</a>
42    <li><a name="TOC27" href="#SEC27">REVISION</a>
43  </ul>  </ul>
44  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
45  <P>  <P>
46  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
47  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
48  documentation and in a number of books, some of which have copious examples.  <a href="pcresyntax.html"><b>pcresyntax</b></a>
49  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  page. Perl's regular expressions are described in its own documentation, and
50  regular expressions in great detail. This description of PCRE's regular  regular expressions in general are covered in a number of books, some of which
51  expressions is intended as reference material.  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
52    published by O'Reilly, covers regular expressions in great detail. This
53    description of PCRE's regular expressions is intended as reference material.
54  </P>  </P>
55  <P>  <P>
56  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,
# Line 59  The remainder of this document discusses Line 68  The remainder of this document discusses
68  PCRE when its main matching function, <b>pcre_exec()</b>, is used.  PCRE when its main matching function, <b>pcre_exec()</b>, is used.
69  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
70  <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not  <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
71  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
72  and how it differs from the normal function, are discussed in the  <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
73    alternative function, and how it differs from the normal function, are
74    discussed in the
75  <a href="pcrematching.html"><b>pcrematching</b></a>  <a href="pcrematching.html"><b>pcrematching</b></a>
76  page.  page.
77  </P>  </P>
78    <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
79    <P>
80    PCRE supports five different conventions for indicating line breaks in
81    strings: a single CR (carriage return) character, a single LF (linefeed)
82    character, the two-character sequence CRLF, any of the three preceding, or any
83    Unicode newline sequence. The
84    <a href="pcreapi.html"><b>pcreapi</b></a>
85    page has
86    <a href="pcreapi.html#newlines">further discussion</a>
87    about newlines, and shows how to set the newline convention in the
88    <i>options</i> arguments for the compiling and matching functions.
89    </P>
90    <P>
91    It is also possible to specify a newline convention by starting a pattern
92    string with one of the following five sequences:
93    <pre>
94      (*CR)        carriage return
95      (*LF)        linefeed
96      (*CRLF)      carriage return, followed by linefeed
97      (*ANYCRLF)   any of the three above
98      (*ANY)       all Unicode newline sequences
99    </pre>
100    These override the default and the options given to <b>pcre_compile()</b>. For
101    example, on a Unix system where LF is the default newline sequence, the pattern
102    <pre>
103      (*CR)a.b
104    </pre>
105    changes the convention to CR. That pattern matches "a\nb" because LF is no
106    longer a newline. Note that these special settings, which are not
107    Perl-compatible, are recognized only at the very start of a pattern, and that
108    they must be in upper case. If more than one of them is present, the last one
109    is used.
110    </P>
111    <P>
112    The newline convention does not affect what the \R escape sequence matches. By
113    default, this is any Unicode newline sequence, for Perl compatibility. However,
114    this can be changed; see the description of \R in the section entitled
115    <a href="#newlineseq">"Newline sequences"</a>
116    below. A change of \R setting can be combined with a change of newline
117    convention.
118    </P>
119    <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
120  <P>  <P>
121  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
122  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
# Line 90  interpreted in some special way. Line 143  interpreted in some special way.
143  <P>  <P>
144  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
145  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
146  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
147  as follows:  are as follows:
148  <pre>  <pre>
149    \      general escape character with several uses    \      general escape character with several uses
150    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 120  a character class the only metacharacter Line 173  a character class the only metacharacter
173  </pre>  </pre>
174  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
175  </P>  </P>
176  <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>  <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
177  <P>  <P>
178  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
179  non-alphanumeric character, it takes away any special meaning that character  non-alphanumeric character, it takes away any special meaning that character
# Line 169  represents: Line 222  represents:
222    \cx       "control-x", where x is any character    \cx       "control-x", where x is any character
223    \e        escape (hex 1B)    \e        escape (hex 1B)
224    \f        formfeed (hex 0C)    \f        formfeed (hex 0C)
225    \n        newline (hex 0A)    \n        linefeed (hex 0A)
226    \r        carriage return (hex 0D)    \r        carriage return (hex 0D)
227    \t        tab (hex 09)    \t        tab (hex 09)
228    \ddd      character with octal code ddd, or backreference    \ddd      character with octal code ddd, or backreference
# Line 185  Thus \cz becomes hex 1A, but \c{ becomes Line 238  Thus \cz becomes hex 1A, but \c{ becomes
238  After \x, from zero to two hexadecimal digits are read (letters can be in  After \x, from zero to two hexadecimal digits are read (letters can be in
239  upper or lower case). Any number of hexadecimal digits may appear between \x{  upper or lower case). Any number of hexadecimal digits may appear between \x{
240  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
241  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
242  is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
243  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
244  Instead, the initial \x will be interpreted as a basic hexadecimal escape,  </P>
245  with no following digits, giving a character whose value is zero.  <P>
246    If characters other than hexadecimal digits appear between \x{ and }, or if
247    there is no terminating }, this form of escape is not recognized. Instead, the
248    initial \x will be interpreted as a basic hexadecimal escape, with no
249    following digits, giving a character whose value is zero.
250  </P>  </P>
251  <P>  <P>
252  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
# Line 216  following the discussion of Line 273  following the discussion of
273  <P>  <P>
274  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
275  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
276  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
277  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
278  character specified in octal must be less than \400. In UTF-8 mode, values up  character specified in octal must be less than \400. In UTF-8 mode, values up
279  to \777 are permitted. For example:  to \777 are permitted. For example:
# Line 238  zero, because no more than three octal d Line 295  zero, because no more than three octal d
295  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
296  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
297  sequence \b is interpreted as the backspace character (hex 08), and the  sequence \b is interpreted as the backspace character (hex 08), and the
298  sequence \X is interpreted as the character "X". Outside a character class,  sequences \R and \X are interpreted as the characters "R" and "X",
299  these sequences have different meanings  respectively. Outside a character class, these sequences have different
300    meanings
301  <a href="#uniextseq">(see below).</a>  <a href="#uniextseq">(see below).</a>
302  </P>  </P>
303  <br><b>  <br><b>
304    Absolute and relative back references
305    </b><br>
306    <P>
307    The sequence \g followed by an unsigned or a negative number, optionally
308    enclosed in braces, is an absolute or relative back reference. A named back
309    reference can be coded as \g{name}. Back references are discussed
310    <a href="#backreferences">later,</a>
311    following the discussion of
312    <a href="#subpattern">parenthesized subpatterns.</a>
313    </P>
314    <br><b>
315  Generic character types  Generic character types
316  </b><br>  </b><br>
317  <P>  <P>
318  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
319  following are always recognized:  following are always recognized:
320  <pre>  <pre>
321    \d     any decimal digit    \d     any decimal digit
322    \D     any character that is not a decimal digit    \D     any character that is not a decimal digit
323      \h     any horizontal whitespace character
324      \H     any character that is not a horizontal whitespace character
325    \s     any whitespace character    \s     any whitespace character
326    \S     any character that is not a whitespace character    \S     any character that is not a whitespace character
327      \v     any vertical whitespace character
328      \V     any character that is not a vertical whitespace character
329    \w     any "word" character    \w     any "word" character
330    \W     any "non-word" character    \W     any "non-word" character
331  </pre>  </pre>
# Line 268  there is no character to match. Line 341  there is no character to match.
341  <P>  <P>
342  For compatibility with Perl, \s does not match the VT character (code 11).  For compatibility with Perl, \s does not match the VT character (code 11).
343  This makes it different from the the POSIX "space" class. The \s characters  This makes it different from the the POSIX "space" class. The \s characters
344  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
345  included in a Perl script, \s may match the VT character. In PCRE, it never  included in a Perl script, \s may match the VT character. In PCRE, it never
346  does.)  does.
347    </P>
348    <P>
349    In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
350    \w, and always match \D, \S, and \W. This is true even when Unicode
351    character property support is available. These sequences retain their original
352    meanings from before UTF-8 support was available, mainly for efficiency
353    reasons.
354    </P>
355    <P>
356    The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
357    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
358    The horizontal space characters are:
359    <pre>
360      U+0009     Horizontal tab
361      U+0020     Space
362      U+00A0     Non-break space
363      U+1680     Ogham space mark
364      U+180E     Mongolian vowel separator
365      U+2000     En quad
366      U+2001     Em quad
367      U+2002     En space
368      U+2003     Em space
369      U+2004     Three-per-em space
370      U+2005     Four-per-em space
371      U+2006     Six-per-em space
372      U+2007     Figure space
373      U+2008     Punctuation space
374      U+2009     Thin space
375      U+200A     Hair space
376      U+202F     Narrow no-break space
377      U+205F     Medium mathematical space
378      U+3000     Ideographic space
379    </pre>
380    The vertical space characters are:
381    <pre>
382      U+000A     Linefeed
383      U+000B     Vertical tab
384      U+000C     Formfeed
385      U+000D     Carriage return
386      U+0085     Next line
387      U+2028     Line separator
388      U+2029     Paragraph separator
389    </PRE>
390  </P>  </P>
391  <P>  <P>
392  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
# Line 280  place (see Line 396  place (see
396  <a href="pcreapi.html#localesupport">"Locale support"</a>  <a href="pcreapi.html#localesupport">"Locale support"</a>
397  in the  in the
398  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
399  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,
400  greater than 128 are used for accented letters, and these are matched by \w.  or "french" in Windows, some character codes greater than 128 are used for
401  </P>  accented letters, and these are matched by \w. The use of locales with Unicode
402    is discouraged.
403    <a name="newlineseq"></a></P>
404    <br><b>
405    Newline sequences
406    </b><br>
407  <P>  <P>
408  In UTF-8 mode, characters with values greater than 128 never match \d, \s, or  Outside a character class, by default, the escape sequence \R matches any
409  \w, and always match \D, \S, and \W. This is true even when Unicode  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
410  character property support is available. The use of locales with Unicode is  equivalent to the following:
411  discouraged.  <pre>
412      (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
413    </pre>
414    This is an example of an "atomic group", details of which are given
415    <a href="#atomicgroup">below.</a>
416    This particular group matches either the two-character sequence CR followed by
417    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
418    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
419    line, U+0085). The two-character sequence is treated as a single unit that
420    cannot be split.
421    </P>
422    <P>
423    In UTF-8 mode, two additional characters whose codepoints are greater than 255
424    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
425    Unicode character property support is not needed for these characters to be
426    recognized.
427    </P>
428    <P>
429    It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
430    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
431    either at compile time or when the pattern is matched. (BSR is an abbrevation
432    for "backslash R".) This can be made the default when PCRE is built; if this is
433    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
434    It is also possible to specify these settings by starting a pattern string with
435    one of the following sequences:
436    <pre>
437      (*BSR_ANYCRLF)   CR, LF, or CRLF only
438      (*BSR_UNICODE)   any Unicode newline sequence
439    </pre>
440    These override the default and the options given to <b>pcre_compile()</b>, but
441    they can be overridden by options given to <b>pcre_exec()</b>. Note that these
442    special settings, which are not Perl-compatible, are recognized only at the
443    very start of a pattern, and that they must be in upper case. If more than one
444    of them is present, the last one is used. They can be combined with a change of
445    newline convention, for example, a pattern can start with:
446    <pre>
447      (*ANY)(*BSR_ANYCRLF)
448    </pre>
449    Inside a character class, \R matches the letter "R".
450  <a name="uniextseq"></a></P>  <a name="uniextseq"></a></P>
451  <br><b>  <br><b>
452  Unicode character properties  Unicode character properties
453  </b><br>  </b><br>
454  <P>  <P>
455  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
456  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
457  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
458    characters whose codepoints are less than 256, but they do work in this mode.
459    The extra escape sequences are:
460  <pre>  <pre>
461    \p{<i>xx</i>}   a character with the <i>xx</i> property    \p{<i>xx</i>}   a character with the <i>xx</i> property
462    \P{<i>xx</i>}   a character without the <i>xx</i> property    \P{<i>xx</i>}   a character without the <i>xx</i> property
# Line 321  Those that are not part of an identified Line 482  Those that are not part of an identified
482  <P>  <P>
483  Arabic,  Arabic,
484  Armenian,  Armenian,
485    Balinese,
486  Bengali,  Bengali,
487  Bopomofo,  Bopomofo,
488  Braille,  Braille,
# Line 330  Canadian_Aboriginal, Line 492  Canadian_Aboriginal,
492  Cherokee,  Cherokee,
493  Common,  Common,
494  Coptic,  Coptic,
495    Cuneiform,
496  Cypriot,  Cypriot,
497  Cyrillic,  Cyrillic,
498  Deseret,  Deseret,
# Line 359  Malayalam, Line 522  Malayalam,
522  Mongolian,  Mongolian,
523  Myanmar,  Myanmar,
524  New_Tai_Lue,  New_Tai_Lue,
525    Nko,
526  Ogham,  Ogham,
527  Old_Italic,  Old_Italic,
528  Old_Persian,  Old_Persian,
529  Oriya,  Oriya,
530  Osmanya,  Osmanya,
531    Phags_Pa,
532    Phoenician,
533  Runic,  Runic,
534  Shavian,  Shavian,
535  Sinhala,  Sinhala,
# Line 447  the Lu, Ll, or Lt property, in other wor Line 613  the Lu, Ll, or Lt property, in other wor
613  a modifier or "other".  a modifier or "other".
614  </P>  </P>
615  <P>  <P>
616    The Cs (Surrogate) property applies only to characters in the range U+D800 to
617    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
618    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
619    (see the discussion of PCRE_NO_UTF8_CHECK in the
620    <a href="pcreapi.html"><b>pcreapi</b></a>
621    page).
622    </P>
623    <P>
624  The long synonyms for these properties that Perl supports (such as \p{Letter})  The long synonyms for these properties that Perl supports (such as \p{Letter})
625  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
626  properties with "Is".  properties with "Is".
# Line 471  or more characters with the "mark" prope Line 645  or more characters with the "mark" prope
645  atomic group  atomic group
646  <a href="#atomicgroup">(see below).</a>  <a href="#atomicgroup">(see below).</a>
647  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
648  preceding character.  preceding character. None of them have codepoints less than 256, so in
649    non-UTF-8 mode \X matches any one character.
650  </P>  </P>
651  <P>  <P>
652  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
653  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
654  why the traditional escape sequences such as \d and \w do not use Unicode  why the traditional escape sequences such as \d and \w do not use Unicode
655  properties in PCRE.  properties in PCRE.
656    <a name="resetmatchstart"></a></P>
657    <br><b>
658    Resetting the match start
659    </b><br>
660    <P>
661    The escape sequence \K, which is a Perl 5.10 feature, causes any previously
662    matched characters not to be included in the final matched sequence. For
663    example, the pattern:
664    <pre>
665      foo\Kbar
666    </pre>
667    matches "foobar", but reports that it has matched "bar". This feature is
668    similar to a lookbehind assertion
669    <a href="#lookbehind">(described below).</a>
670    However, in this case, the part of the subject before the real match does not
671    have to be of fixed length, as lookbehind assertions do. The use of \K does
672    not interfere with the setting of
673    <a href="#subpattern">captured substrings.</a>
674    For example, when the pattern
675    <pre>
676      (foo)\Kbar
677    </pre>
678    matches "foobar", the first substring is still set to "foo".
679  <a name="smallassertions"></a></P>  <a name="smallassertions"></a></P>
680  <br><b>  <br><b>
681  Simple assertions  Simple assertions
682  </b><br>  </b><br>
683  <P>  <P>
684  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
685  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,
686  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
687  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 492  The backslashed assertions are: Line 690  The backslashed assertions are:
690  <pre>  <pre>
691    \b     matches at a word boundary    \b     matches at a word boundary
692    \B     matches when not at a word boundary    \B     matches when not at a word boundary
693    \A     matches at start of subject    \A     matches at the start of the subject
694    \Z     matches at end of subject or before newline at end    \Z     matches at the end of the subject
695    \z     matches at end of subject            also matches before a newline at the end of the subject
696    \G     matches at first matching position in subject    \z     matches only at the end of the subject
697      \G     matches at the first matching position in the subject
698  </pre>  </pre>
699  These assertions may not appear in character classes (but note that \b has a  These assertions may not appear in character classes (but note that \b has a
700  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 538  If all the alternatives of a pattern beg Line 737  If all the alternatives of a pattern beg
737  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
738  regular expression.  regular expression.
739  </P>  </P>
740  <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>  <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
741  <P>  <P>
742  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
743  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is
# Line 592  Note that the sequences \A, \Z, and \z c Line 791  Note that the sequences \A, \Z, and \z c
791  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
792  \A it is always anchored, whether or not PCRE_MULTILINE is set.  \A it is always anchored, whether or not PCRE_MULTILINE is set.
793  </P>  </P>
794  <br><a name="SEC4" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>  <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
795  <P>  <P>
796  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
797  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
798  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.
799  a line ending is defined as a single character (CR or LF), dot never matches  </P>
800  that character; when the two-character sequence CRLF is used, dot does not  <P>
801  match CR if it is immediately followed by LF, but otherwise it matches all  When a line ending is defined as a single character, dot never matches that
802  characters (including isolated CRs and LFs).  character; when the two-character sequence CRLF is used, dot does not match CR
803    if it is immediately followed by LF, but otherwise it matches all characters
804    (including isolated CRs and LFs). When any Unicode line endings are being
805    recognized, dot does not match CR or LF or any of the other line ending
806    characters.
807  </P>  </P>
808  <P>  <P>
809  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
810  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
811  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
812    to match it.
813  </P>  </P>
814  <P>  <P>
815  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
816  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
817  special meaning in a character class.  special meaning in a character class.
818  </P>  </P>
819  <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>  <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
820  <P>  <P>
821  Outside a character class, the escape sequence \C matches any one byte, both  Outside a character class, the escape sequence \C matches any one byte, both
822  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
823  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
824  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,
825  the string may be a malformed UTF-8 string. For this reason, the \C escape  what remains in the string may be a malformed UTF-8 string. For this reason,
826  sequence is best avoided.  the \C escape sequence is best avoided.
827  </P>  </P>
828  <P>  <P>
829  PCRE does not allow \C to appear in lookbehind assertions  PCRE does not allow \C to appear in lookbehind assertions
# Line 627  PCRE does not allow \C to appear in look Line 831  PCRE does not allow \C to appear in look
831  because in UTF-8 mode this would make it impossible to calculate the length of  because in UTF-8 mode this would make it impossible to calculate the length of
832  the lookbehind.  the lookbehind.
833  <a name="characterclass"></a></P>  <a name="characterclass"></a></P>
834  <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>  <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
835  <P>  <P>
836  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
837  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. If a
# Line 670  ensure that PCRE is compiled with Unicod Line 874  ensure that PCRE is compiled with Unicod
874  UTF-8 support.  UTF-8 support.
875  </P>  </P>
876  <P>  <P>
877  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
878  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
879  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
880  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
881  </P>  </P>
882  <P>  <P>
883  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
# Line 701  example [\x{100}-\x{2ff}]. Line 905  example [\x{100}-\x{2ff}].
905  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
906  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
907  [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
908  tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E  tables for a French locale are in use, [\xc8-\xcb] matches accented E
909  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
910  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
911  property support.  property support.
# Line 722  introducing a POSIX class name - see the Line 926  introducing a POSIX class name - see the
926  closing square bracket. However, escaping other non-alphanumeric characters  closing square bracket. However, escaping other non-alphanumeric characters
927  does no harm.  does no harm.
928  </P>  </P>
929  <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>  <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
930  <P>  <P>
931  Perl supports the POSIX notation for character classes. This uses names  Perl supports the POSIX notation for character classes. This uses names
932  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
# Line 768  supported, and an error is given if they Line 972  supported, and an error is given if they
972  In UTF-8 mode, characters with values greater than 128 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
973  the POSIX character classes.  the POSIX character classes.
974  </P>  </P>
975  <br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br>  <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
976  <P>  <P>
977  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
978  the pattern  the pattern
# Line 783  that succeeds is used. If the alternativ Line 987  that succeeds is used. If the alternativ
987  "succeeds" means matching the rest of the main pattern as well as the  "succeeds" means matching the rest of the main pattern as well as the
988  alternative in the subpattern.  alternative in the subpattern.
989  </P>  </P>
990  <br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br>  <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
991  <P>  <P>
992  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
993  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
994  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
995    The option letters are
996  <pre>  <pre>
997    i  for PCRE_CASELESS    i  for PCRE_CASELESS
998    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 802  permitted. If a letter appears both befo Line 1007  permitted. If a letter appears both befo
1007  unset.  unset.
1008  </P>  </P>
1009  <P>  <P>
1010    The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1011    changed in the same way as the Perl-compatible options by using the characters
1012    J, U and X respectively.
1013    </P>
1014    <P>
1015  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
1016  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
1017  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
# Line 809  the global options (and it will therefor Line 1019  the global options (and it will therefor
1019  <b>pcre_fullinfo()</b> function).  <b>pcre_fullinfo()</b> function).
1020  </P>  </P>
1021  <P>  <P>
1022  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1023  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1024  <pre>  <pre>
1025    (a(?i)b)c    (a(?i)b)c
1026  </pre>  </pre>
# Line 825  matches "ab", "aB", "c", and "C", even t Line 1035  matches "ab", "aB", "c", and "C", even t
1035  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1036  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1037  behaviour otherwise.  behaviour otherwise.
 </P>  
 <P>  
 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  
 changed in the same way as the Perl-compatible options by using the characters  
 J, U and X respectively.  
1038  <a name="subpattern"></a></P>  <a name="subpattern"></a></P>
1039  <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>  <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1040  <P>  <P>
1041  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1042  Turning part of a pattern into a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
# Line 842  Turning part of a pattern into a subpatt Line 1047  Turning part of a pattern into a subpatt
1047    cat(aract|erpillar|)    cat(aract|erpillar|)
1048  </pre>  </pre>
1049  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1050  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1051  <br>  <br>
1052  <br>  <br>
1053  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
# Line 870  the string "the white queen" is matched Line 1075  the string "the white queen" is matched
1075    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1076  </pre>  </pre>
1077  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
1078  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.  
1079  </P>  </P>
1080  <P>  <P>
1081  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
# Line 886  from left to right, and options are not Line 1090  from left to right, and options are not
1090  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1091  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1092  </P>  </P>
1093  <br><a name="SEC11" href="#TOC1">NAMED SUBPATTERNS</a><br>  <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1094    <P>
1095    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1096    the same numbers for its capturing parentheses. Such a subpattern starts with
1097    (?| and is itself a non-capturing subpattern. For example, consider this
1098    pattern:
1099    <pre>
1100      (?|(Sat)ur|(Sun))day
1101    </pre>
1102    Because the two alternatives are inside a (?| group, both sets of capturing
1103    parentheses are numbered one. Thus, when the pattern matches, you can look
1104    at captured substring number one, whichever alternative matched. This construct
1105    is useful when you want to capture part, but not all, of one of a number of
1106    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1107    number is reset at the start of each branch. The numbers of any capturing
1108    buffers that follow the subpattern start after the highest number used in any
1109    branch. The following example is taken from the Perl documentation.
1110    The numbers underneath show in which buffer the captured content will be
1111    stored.
1112    <pre>
1113      # before  ---------------branch-reset----------- after
1114      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1115      # 1            2         2  3        2     3     4
1116    </pre>
1117    A backreference or a recursive call to a numbered subpattern always refers to
1118    the first one in the pattern with the given number.
1119    </P>
1120    <P>
1121    An alternative approach to using this "branch reset" feature is to use
1122    duplicate named subpatterns, as described in the next section.
1123    </P>
1124    <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1125  <P>  <P>
1126  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
1127  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1128  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
1129  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1130  not provide. The Python syntax (?P&#60;name&#62;...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1131    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1132    the Perl and the Python syntax.
1133    </P>
1134    <P>
1135    In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1136    (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1137  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1138  <a href="#backreferences">backreferences,</a>  <a href="#backreferences">backreferences,</a>
1139  <a href="#recursion">recursion,</a>  <a href="#recursion">recursion,</a>
# Line 902  can be made by name as well as by number Line 1143  can be made by name as well as by number
1143  </P>  </P>
1144  <P>  <P>
1145  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1146  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1147  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
1148  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
1149  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1150  </P>  </P>
1151  <P>  <P>
1152  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
# Line 915  match. Suppose you want to match the nam Line 1156  match. Suppose you want to match the nam
1156  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
1157  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1158  <pre>  <pre>
1159    (?P&#60;DN&#62;Mon|Fri|Sun)(?:day)?|    (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1160    (?P&#60;DN&#62;Tue)(?:sday)?|    (?&#60;DN&#62;Tue)(?:sday)?|
1161    (?P&#60;DN&#62;Wed)(?:nesday)?|    (?&#60;DN&#62;Wed)(?:nesday)?|
1162    (?P&#60;DN&#62;Thu)(?:rsday)?|    (?&#60;DN&#62;Thu)(?:rsday)?|
1163    (?P&#60;DN&#62;Sat)(?:urday)?    (?&#60;DN&#62;Sat)(?:urday)?
1164  </pre>  </pre>
1165  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.
1166    (An alternative way of solving this problem is to use a "branch reset"
1167    subpattern, as described in the previous section.)
1168    </P>
1169    <P>
1170  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1171  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
1172  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
1173  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
1174  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 931  details of the interfaces for handling n Line 1176  details of the interfaces for handling n
1176  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
1177  documentation.  documentation.
1178  </P>  </P>
1179  <br><a name="SEC12" href="#TOC1">REPETITION</a><br>  <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1180  <P>  <P>
1181  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1182  items:  items:
1183  <pre>  <pre>
1184    a literal data character    a literal data character
1185    the . metacharacter    the dot metacharacter
1186    the \C escape sequence    the \C escape sequence
1187    the \X escape sequence (in UTF-8 mode with Unicode properties)    the \X escape sequence (in UTF-8 mode with Unicode properties)
1188      the \R escape sequence
1189    an escape such as \d that matches a single character    an escape such as \d that matches a single character
1190    a character class    a character class
1191    a back reference (see next section)    a back reference (see next section)
# Line 980  The quantifier {0} is permitted, causing Line 1226  The quantifier {0} is permitted, causing
1226  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1227  </P>  </P>
1228  <P>  <P>
1229  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1230  quantifiers have single-character abbreviations:  abbreviations:
1231  <pre>  <pre>
1232    *    is equivalent to {0,}    *    is equivalent to {0,}
1233    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1032  which matches one digit by preference, b Line 1278  which matches one digit by preference, b
1278  way the rest of the pattern matches.  way the rest of the pattern matches.
1279  </P>  </P>
1280  <P>  <P>
1281  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),
1282  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
1283  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
1284  default behaviour.  default behaviour.
# Line 1044  compiled pattern, in proportion to the s Line 1290  compiled pattern, in proportion to the s
1290  </P>  </P>
1291  <P>  <P>
1292  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
1293  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
1294  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1295  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
1296  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 1058  alternatively using ^ to indicate anchor Line 1304  alternatively using ^ to indicate anchor
1304  <P>  <P>
1305  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1306  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1307  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
1308  succeed. Consider, for example:  succeeds. Consider, for example:
1309  <pre>  <pre>
1310    (.*)abc\1    (.*)abc\1
1311  </pre>  </pre>
# Line 1081  example, after Line 1327  example, after
1327  </pre>  </pre>
1328  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1329  <a name="atomicgroup"></a></P>  <a name="atomicgroup"></a></P>
1330  <br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>  <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1331  <P>  <P>
1332  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1333  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
1334  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
1335  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
1336  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
1337  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1338  </P>  </P>
1339  <P>  <P>
1340  Consider, for example, the pattern \d+foo when applied to the subject line  Consider, for example, the pattern \d+foo when applied to the subject line
# Line 1102  item, and then with 4, and so on, before Line 1348  item, and then with 4, and so on, before
1348  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.
1349  </P>  </P>
1350  <P>  <P>
1351  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
1352  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
1353  special parenthesis, starting with (?&#62; as in this example:  special parenthesis, starting with (?&#62; as in this example:
1354  <pre>  <pre>
# Line 1135  previous example can be rewritten as Line 1381  previous example can be rewritten as
1381  <pre>  <pre>
1382    \d++foo    \d++foo
1383  </pre>  </pre>
1384    Note that a possessive quantifier can be used with an entire group, for
1385    example:
1386    <pre>
1387      (abc|xyz){2,3}+
1388    </pre>
1389  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1390  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
1391  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
1392  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1393    difference; possessive quantifiers should be slightly faster.
1394    </P>
1395    <P>
1396    The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1397    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1398    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1399    package, and PCRE copied it from there. It ultimately found its way into Perl
1400    at release 5.10.
1401  </P>  </P>
1402  <P>  <P>
1403  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  PCRE has an optimization that automatically "possessifies" certain simple
1404  Friedl originated the idea (and the name) in the first edition of his book.  pattern constructs. For example, the sequence A+B is treated as A++B because
1405  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  there is no point in backtracking into a sequence of A's when B must follow.
 and PCRE copied it from there.  
1406  </P>  </P>
1407  <P>  <P>
1408  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
# Line 1173  an atomic group, like this: Line 1431  an atomic group, like this:
1431  </pre>  </pre>
1432  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1433  <a name="backreferences"></a></P>  <a name="backreferences"></a></P>
1434  <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>  <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1435  <P>  <P>
1436  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1437  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
# Line 1190  when a repetition is involved and the su Line 1448  when a repetition is involved and the su
1448  in an earlier iteration.  in an earlier iteration.
1449  </P>  </P>
1450  <P>  <P>
1451  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
1452  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 \50 is
1453  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1454  "Non-printing characters"  "Non-printing characters"
1455  <a href="#digitsafterbackslash">above</a>  <a href="#digitsafterbackslash">above</a>
1456  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1457    no such problem when named parentheses are used. A back reference to any
1458    subpattern is possible using named parentheses (see below).
1459    </P>
1460    <P>
1461    Another way of avoiding the ambiguity inherent in the use of digits following a
1462    backslash is to use the \g escape sequence, which is a feature introduced in
1463    Perl 5.10. This escape must be followed by an unsigned number or a negative
1464    number, optionally enclosed in braces. These examples are all identical:
1465    <pre>
1466      (ring), \1
1467      (ring), \g1
1468      (ring), \g{1}
1469    </pre>
1470    An unsigned number specifies an absolute reference without the ambiguity that
1471    is present in the older syntax. It is also useful when literal digits follow
1472    the reference. A negative number is a relative reference. Consider this
1473    example:
1474    <pre>
1475      (abc(def)ghi)\g{-1}
1476    </pre>
1477    The sequence \g{-1} is a reference to the most recently started capturing
1478    subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1479    would be equivalent to \1. The use of relative references can be helpful in
1480    long patterns, and also in patterns that are created by joining together
1481    fragments that contain references within themselves.
1482  </P>  </P>
1483  <P>  <P>
1484  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
# Line 1216  matches "rah rah" and "RAH RAH", but not Line 1499  matches "rah rah" and "RAH RAH", but not
1499  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1500  </P>  </P>
1501  <P>  <P>
1502  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1503  rewrite the above example as follows:  subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1504    \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1505    back reference syntax, in which \g can be used for both numeric and named
1506    references, is also supported. We could rewrite the above example in any of
1507    the following ways:
1508  <pre>  <pre>
1509      (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1510      (?'p1'(?i)rah)\s+\k{p1}
1511    (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)    (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1512      (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1513  </pre>  </pre>
1514  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
1515  after the reference.  after the reference.
# Line 1255  that the first iteration does not need t Line 1545  that the first iteration does not need t
1545  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1546  minimum of zero.  minimum of zero.
1547  <a name="bigassertions"></a></P>  <a name="bigassertions"></a></P>
1548  <br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br>  <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1549  <P>  <P>
1550  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1551  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
# Line 1337  lengths, but it is acceptable if rewritt Line 1627  lengths, but it is acceptable if rewritt
1627  <pre>  <pre>
1628    (?&#60;=abc|abde)    (?&#60;=abc|abde)
1629  </pre>  </pre>
1630    In some cases, the Perl 5.10 escape sequence \K
1631    <a href="#resetmatchstart">(see above)</a>
1632    can be used instead of a lookbehind assertion; this is not restricted to a
1633    fixed-length.
1634    </P>
1635    <P>
1636  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1637  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
1638  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1639  match is deemed to fail.  assertion fails.
1640  </P>  </P>
1641  <P>  <P>
1642  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1643  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1644  the length of the lookbehind. The \X escape, which can match different numbers  the length of the lookbehind. The \X and \R escapes, which can match
1645  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1646  </P>  </P>
1647  <P>  <P>
1648  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1649  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching at the end of the subject string. Consider a simple
1650  such as  pattern such as
1651  <pre>  <pre>
1652    abcd$    abcd$
1653  </pre>  </pre>
# Line 1367  then all but the last two characters, an Line 1663  then all but the last two characters, an
1663  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,
1664  if the pattern is written as  if the pattern is written as
1665  <pre>  <pre>
   ^(?&#62;.*)(?&#60;=abcd)  
 </pre>  
 or, equivalently, using the possessive quantifier syntax,  
 <pre>  
1666    ^.*+(?&#60;=abcd)    ^.*+(?&#60;=abcd)
1667  </pre>  </pre>
1668  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
1669  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
1670  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1671  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1413  preceded by "foo", while Line 1705  preceded by "foo", while
1705  is another pattern that matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
1706  characters that are not "999".  characters that are not "999".
1707  <a name="conditions"></a></P>  <a name="conditions"></a></P>
1708  <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>  <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1709  <P>  <P>
1710  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1711  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
# Line 1428  no-pattern (if present) is used. If ther Line 1720  no-pattern (if present) is used. If ther
1720  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1721  </P>  </P>
1722  <P>  <P>
1723  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1724  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1725  underscores, the condition is satisfied if the capturing subpattern of that  </P>
1726  number or name has previously matched. There is a possible ambiguity here,  <br><b>
1727  because subpattern names may consist entirely of digits. PCRE looks first for a  Checking for a used subpattern by number
1728  named subpattern; if it cannot find one and the text consists entirely of  </b><br>
1729  digits, it looks for a subpattern of that number, which must be greater than  <P>
1730  zero. Using subpattern names that consist entirely of digits is not  If the text between the parentheses consists of a sequence of digits, the
1731  recommended.  condition is true if the capturing subpattern of that number has previously
1732    matched. An alternative notation is to precede the digits with a plus or minus
1733    sign. In this case, the subpattern number is relative rather than absolute.
1734    The most recently opened parentheses can be referenced by (?(-1), the next most
1735    recent by (?(-2), and so on. In looping constructs it can also make sense to
1736    refer to subsequent groups with constructs such as (?(+2).
1737  </P>  </P>
1738  <P>  <P>
1739  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
# Line 1453  or not. If they did, that is, if subject Line 1750  or not. If they did, that is, if subject
1750  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
1751  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1752  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1753  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1754  named subpattern gives this:  </P>
1755    <P>
1756    If you were embedding this pattern in a larger one, you could use a relative
1757    reference:
1758  <pre>  <pre>
1759    (?P&#60;OPEN&#62; \( )?    [^()]+    (?(OPEN) \) )    ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
1760  </pre>  </pre>
1761    This makes the fragment independent of the parentheses in the larger pattern.
1762    </P>
1763    <br><b>
1764    Checking for a used subpattern by name
1765    </b><br>
1766    <P>
1767    Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1768    subpattern by name. For compatibility with earlier versions of PCRE, which had
1769    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1770    there is a possible ambiguity with this syntax, because subpattern names may
1771    consist entirely of digits. PCRE looks first for a named subpattern; if it
1772    cannot find one and the name consists entirely of digits, PCRE looks for a
1773    subpattern of that number, which must be greater than zero. Using subpattern
1774    names that consist entirely of digits is not recommended.
1775    </P>
1776    <P>
1777    Rewriting the above example to use a named subpattern gives this:
1778    <pre>
1779      (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
1780    
1781    </PRE>
1782    </P>
1783    <br><b>
1784    Checking for pattern recursion
1785    </b><br>
1786    <P>
1787  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,
1788  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
1789  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
1790  Recursive patterns are described in the next section.  letter R, for example:
1791    <pre>
1792      (?(R3)...) or (?(R&name)...)
1793    </pre>
1794    the condition is true if the most recent recursion is into the subpattern whose
1795    number or name is given. This condition does not check the entire recursion
1796    stack.
1797    </P>
1798    <P>
1799    At "top level", all these recursion test conditions are false. Recursive
1800    patterns are described below.
1801    </P>
1802    <br><b>
1803    Defining subpatterns for use by reference only
1804    </b><br>
1805    <P>
1806    If the condition is the string (DEFINE), and there is no subpattern with the
1807    name DEFINE, the condition is always false. In this case, there may be only one
1808    alternative in the subpattern. It is always skipped if control reaches this
1809    point in the pattern; the idea of DEFINE is that it can be used to define
1810    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1811    is described below.) For example, a pattern to match an IPv4 address could be
1812    written like this (ignore whitespace and line breaks):
1813    <pre>
1814      (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1815      \b (?&byte) (\.(?&byte)){3} \b
1816    </pre>
1817    The first part of the pattern is a DEFINE group inside which a another group
1818    named "byte" is defined. This matches an individual component of an IPv4
1819    address (a number less than 256). When matching takes place, this part of the
1820    pattern is skipped because DEFINE acts like a false condition.
1821    </P>
1822    <P>
1823    The rest of the pattern uses references to the named group to match the four
1824    dot-separated components of an IPv4 address, insisting on a word boundary at
1825    each end.
1826  </P>  </P>
1827    <br><b>
1828    Assertion conditions
1829    </b><br>
1830  <P>  <P>
1831  If the condition is not a sequence of digits or (R), it must be an assertion.  If the condition is not in any of the above formats, it must be an assertion.
1832  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1833  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1834  alternatives on the second line:  alternatives on the second line:
# Line 1479  subject is matched against the first alt Line 1843  subject is matched against the first alt
1843  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1844  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1845  <a name="comments"></a></P>  <a name="comments"></a></P>
1846  <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>  <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1847  <P>  <P>
1848  The sequence (?# marks the start of a comment that continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1849  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
# Line 1490  If the PCRE_EXTENDED option is set, an u Line 1854  If the PCRE_EXTENDED option is set, an u
1854  character class introduces a comment that continues to immediately after the  character class introduces a comment that continues to immediately after the
1855  next newline in the pattern.  next newline in the pattern.
1856  <a name="recursion"></a></P>  <a name="recursion"></a></P>
1857  <br><a name="SEC18" href="#TOC1">RECURSIVE PATTERNS</a><br>  <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
1858  <P>  <P>
1859  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1860  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1861  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
1862  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1863  that allows regular expressions to recurse (amongst other things). It does this  </P>
1864  by interpolating Perl code in the expression at run time, and the code can  <P>
1865  refer to the expression itself. A Perl pattern to solve the parentheses problem  For some time, Perl has provided a facility that allows regular expressions to
1866  can be created like this:  recurse (amongst other things). It does this by interpolating Perl code in the
1867    expression at run time, and the code can refer to the expression itself. A Perl
1868    pattern using code interpolation to solve the parentheses problem can be
1869    created like this:
1870  <pre>  <pre>
1871    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1872  </pre>  </pre>
1873  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
1874  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
 the interpolation of Perl code. Instead, it supports some special syntax for  
 recursion of the entire pattern, and also for individual subpattern recursion.  
1875  </P>  </P>
1876  <P>  <P>
1877  The special item that consists of (? followed by a number greater than zero and  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1878  a closing parenthesis is a recursive call of the subpattern of the given  supports special syntax for recursion of the entire pattern, and also for
1879  number, provided that it occurs inside that subpattern. (If not, it is a  individual subpattern recursion. After its introduction in PCRE and Python,
1880  "subroutine" call, which is described in the next section.) The special item  this kind of recursion was introduced into Perl at release 5.10.
 (?R) is a recursive call of the entire regular expression.  
1881  </P>  </P>
1882  <P>  <P>
1883  A recursive subpattern call is always treated as an atomic group. That is, once  A special item that consists of (? followed by a number greater than zero and a
1884  it has matched some of the subject string, it is never re-entered, even if  closing parenthesis is a recursive call of the subpattern of the given number,
1885  it contains untried alternatives and there is a subsequent matching failure.  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1886    call, which is described in the next section.) The special item (?R) or (?0) is
1887    a recursive call of the entire regular expression.
1888    </P>
1889    <P>
1890    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1891    treated as an atomic group. That is, once it has matched some of the subject
1892    string, it is never re-entered, even if it contains untried alternatives and
1893    there is a subsequent matching failure.
1894  </P>  </P>
1895  <P>  <P>
1896  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
# Line 1538  pattern, so instead you could use this: Line 1910  pattern, so instead you could use this:
1910    ( \( ( (?&#62;[^()]+) | (?1) )* \) )    ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1911  </pre>  </pre>
1912  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
1913  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1914  parenthesis numbers can be tricky. It may be more convenient to use named  </P>
1915  parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to  <P>
1916  the Python syntax that PCRE uses for named parentheses (Perl does not provide  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1917  named parentheses). We could rewrite the above example as follows:  is made easier by the use of relative references. (A Perl 5.10 feature.)
1918  <pre>  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1919    (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )  most recently opened parentheses preceding the recursion. In other words, a
1920  </pre>  negative number counts capturing parentheses leftwards from the point at which
1921  This particular example pattern contains nested unlimited repeats, and so the  it is encountered.
1922  use of atomic grouping for matching strings of non-parentheses is important  </P>
1923  when applying the pattern to strings that do not match. For example, when this  <P>
1924  pattern is applied to  It is also possible to refer to subsequently opened parentheses, by writing
1925    references such as (?+2). However, these cannot be recursive because the
1926    reference is not inside the parentheses that are referenced. They are always
1927    "subroutine" calls, as described in the next section.
1928    </P>
1929    <P>
1930    An alternative approach is to use named parentheses instead. The Perl syntax
1931    for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
1932    could rewrite the above example as follows:
1933    <pre>
1934      (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1935    </pre>
1936    If there is more than one subpattern with the same name, the earliest one is
1937    used.
1938    </P>
1939    <P>
1940    This particular example pattern that we have been looking at contains nested
1941    unlimited repeats, and so the use of atomic grouping for matching strings of
1942    non-parentheses is important when applying the pattern to strings that do not
1943    match. For example, when this pattern is applied to
1944  <pre>  <pre>
1945    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1946  </pre>  </pre>
# Line 1562  before failure can be reported. Line 1953  before failure can be reported.
1953  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
1954  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.
1955  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
1956  the next section and the  below and the
1957  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
1958  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1959  <pre>  <pre>
# Line 1593  In this pattern, (?(R) is the start of a Line 1984  In this pattern, (?(R) is the start of a
1984  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1985  is the actual recursive call.  is the actual recursive call.
1986  <a name="subpatternsassubroutines"></a></P>  <a name="subpatternsassubroutines"></a></P>
1987  <br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>  <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1988  <P>  <P>
1989  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
1990  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
1991  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1992  pattern  before or after the reference. A numbered reference can be absolute or
1993    relative, as in these examples:
1994    <pre>
1995      (...(absolute)...)...(?2)...
1996      (...(relative)...)...(?-1)...
1997      (...(?+1)...(relative)...
1998    </pre>
1999    An earlier example pointed out that the pattern
2000  <pre>  <pre>
2001    (sens|respons)e and \1ibility    (sens|respons)e and \1ibility
2002  </pre>  </pre>
# Line 1608  matches "sense and sensibility" and "res Line 2006  matches "sense and sensibility" and "res
2006    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2007  </pre>  </pre>
2008  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
2009  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.  
2010  </P>  </P>
2011  <P>  <P>
2012  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
# Line 1617  group. That is, once it has matched some Line 2014  group. That is, once it has matched some
2014  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
2015  matching failure.  matching failure.
2016  </P>  </P>
2017  <br><a name="SEC20" href="#TOC1">CALLOUTS</a><br>  <P>
2018    When a subpattern is used as a subroutine, processing options such as
2019    case-independence are fixed when the subpattern is defined. They cannot be
2020    changed for different calls. For example, consider this pattern:
2021    <pre>
2022      (abc)(?i:(?-1))
2023    </pre>
2024    It matches "abcabc". It does not match "abcABC" because the change of
2025    processing option does not affect the called subpattern.
2026    </P>
2027    <br><a name="SEC23" href="#TOC1">CALLOUTS</a><br>
2028  <P>  <P>
2029  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl  Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2030  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
# Line 1636  function is to be called. If you want to Line 2043  function is to be called. If you want to
2043  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.
2044  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2045  <pre>  <pre>
2046    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2047  </pre>  </pre>
2048  If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2049  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 1652  description of the interface to the call Line 2059  description of the interface to the call
2059  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
2060  documentation.  documentation.
2061  </P>  </P>
2062    <br><a name="SEC24" href="#TOC1">BACKTRACKING CONTROL</a><br>
2063  <P>  <P>
2064  Last updated: 06 June 2006  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2065    are described in the Perl documentation as "experimental and subject to change
2066    or removal in a future version of Perl". It goes on to say: "Their usage in
2067    production code should be noted to avoid problems during upgrades." The same
2068    remarks apply to the PCRE features described in this section.
2069    </P>
2070    <P>
2071    Since these verbs are specifically related to backtracking, they can be used
2072    only when the pattern is to be matched using <b>pcre_exec()</b>, which uses a
2073    backtracking algorithm. They cause an error if encountered by
2074    <b>pcre_dfa_exec()</b>.
2075    </P>
2076    <P>
2077    The new verbs make use of what was previously invalid syntax: an opening
2078    parenthesis followed by an asterisk. In Perl, they are generally of the form
2079    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2080    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2081    are two kinds:
2082    </P>
2083    <br><b>
2084    Verbs that act immediately
2085    </b><br>
2086    <P>
2087    The following verbs act as soon as they are encountered:
2088    <pre>
2089       (*ACCEPT)
2090    </pre>
2091    This verb causes the match to end successfully, skipping the remainder of the
2092    pattern. When inside a recursion, only the innermost pattern is ended
2093    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2094    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2095    captured. For example:
2096    <pre>
2097      A(A|B(*ACCEPT)|C)D
2098    </pre>
2099    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2100    captured.
2101    <pre>
2102      (*FAIL) or (*F)
2103    </pre>
2104    This verb causes the match to fail, forcing backtracking to occur. It is
2105    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2106    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2107    Perl features that are not present in PCRE. The nearest equivalent is the
2108    callout feature, as for example in this pattern:
2109    <pre>
2110      a+(?C)(*FAIL)
2111    </pre>
2112    A match with the string "aaaa" always fails, but the callout is taken before
2113    each backtrack happens (in this example, 10 times).
2114    </P>
2115    <br><b>
2116    Verbs that act after backtracking
2117    </b><br>
2118    <P>
2119    The following verbs do nothing when they are encountered. Matching continues
2120    with what follows, but if there is no subsequent match, a failure is forced.
2121    The verbs differ in exactly what kind of failure occurs.
2122    <pre>
2123      (*COMMIT)
2124    </pre>
2125    This verb causes the whole match to fail outright if the rest of the pattern
2126    does not match. Even if the pattern is unanchored, no further attempts to find
2127    a match by advancing the start point take place. Once (*COMMIT) has been
2128    passed, <b>pcre_exec()</b> is committed to finding a match at the current
2129    starting point, or not at all. For example:
2130    <pre>
2131      a+(*COMMIT)b
2132    </pre>
2133    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2134    dynamic anchor, or "I've started, so I must finish."
2135    <pre>
2136      (*PRUNE)
2137    </pre>
2138    This verb causes the match to fail at the current position if the rest of the
2139    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2140    advance to the next starting character then happens. Backtracking can occur as
2141    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2142    if there is no match to the right, backtracking cannot cross (*PRUNE).
2143    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2144    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2145    be expressed in any other way.
2146    <pre>
2147      (*SKIP)
2148    </pre>
2149    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2150    "bumpalong" advance is not to the next character, but to the position in the
2151    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2152    was matched leading up to it cannot be part of a successful match. Consider:
2153    <pre>
2154      a+(*SKIP)b
2155    </pre>
2156    If the subject is "aaaac...", after the first match attempt fails (starting at
2157    the first character in the string), the starting point skips on to start the
2158    next attempt at "c". Note that a possessive quantifer does not have the same
2159    effect in this example; although it would suppress backtracking during the
2160    first match attempt, the second attempt would start at the second character
2161    instead of skipping on to "c".
2162    <pre>
2163      (*THEN)
2164    </pre>
2165    This verb causes a skip to the next alternation if the rest of the pattern does
2166    not match. That is, it cancels pending backtracking, but only within the
2167    current alternation. Its name comes from the observation that it can be used
2168    for a pattern-based if-then-else block:
2169    <pre>
2170      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2171    </pre>
2172    If the COND1 pattern matches, FOO is tried (and possibly further items after
2173    the end of the group if FOO succeeds); on failure the matcher skips to the
2174    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2175    is used outside of any alternation, it acts exactly like (*PRUNE).
2176    </P>
2177    <br><a name="SEC25" href="#TOC1">SEE ALSO</a><br>
2178    <P>
2179    <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
2180    </P>
2181    <br><a name="SEC26" href="#TOC1">AUTHOR</a><br>
2182    <P>
2183    Philip Hazel
2184    <br>
2185    University Computing Service
2186    <br>
2187    Cambridge CB2 3QH, England.
2188    <br>
2189    </P>
2190    <br><a name="SEC27" href="#TOC1">REVISION</a><br>
2191    <P>
2192    Last updated: 14 September 2007
2193    <br>
2194    Copyright &copy; 1997-2007 University of Cambridge.
2195  <br>  <br>
 Copyright &copy; 1997-2006 University of Cambridge.  
2196  <p>  <p>
2197  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.
2198  </p>  </p>

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