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revision 77 by nigel, Sat Feb 24 21:40:45 2007 UTC revision 259 by ph10, Wed Sep 19 09:52:44 2007 UTC
# Line 1  Line 1 
1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. Perl's regular expressions are described in its own documentation, and
13    regular expressions in general are covered in a number of books, some of which
14    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15    published by O'Reilly, covers regular expressions in great detail. This
16    description of PCRE's regular expressions is intended as reference material.
17  .P  .P
18  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,
19  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this, you must
# Line 30  The remainder of this document discusses Line 34  The remainder of this document discusses
34  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
35  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
36  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
37  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
38  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
39    alternative function, and how it differs from the normal function, are
40    discussed in the
41  .\" HREF  .\" HREF
42  \fBpcrematching\fP  \fBpcrematching\fP
43  .\"  .\"
44  page.  page.
45    .
46    .
47    .SH "NEWLINE CONVENTIONS"
48    .rs
49    .sp
50    PCRE supports five different conventions for indicating line breaks in
51    strings: a single CR (carriage return) character, a single LF (linefeed)
52    character, the two-character sequence CRLF, any of the three preceding, or any
53    Unicode newline sequence. The
54    .\" HREF
55    \fBpcreapi\fP
56    .\"
57    page has
58    .\" HTML <a href="pcreapi.html#newlines">
59    .\" </a>
60    further discussion
61    .\"
62    about newlines, and shows how to set the newline convention in the
63    \fIoptions\fP arguments for the compiling and matching functions.
64  .P  .P
65    It is also possible to specify a newline convention by starting a pattern
66    string with one of the following five sequences:
67    .sp
68      (*CR)        carriage return
69      (*LF)        linefeed
70      (*CRLF)      carriage return, followed by linefeed
71      (*ANYCRLF)   any of the three above
72      (*ANY)       all Unicode newline sequences
73    .sp
74    These override the default and the options given to \fBpcre_compile()\fP. For
75    example, on a Unix system where LF is the default newline sequence, the pattern
76    .sp
77      (*CR)a.b
78    .sp
79    changes the convention to CR. That pattern matches "a\enb" because LF is no
80    longer a newline. Note that these special settings, which are not
81    Perl-compatible, are recognized only at the very start of a pattern, and that
82    they must be in upper case. If more than one of them is present, the last one
83    is used.
84    .P
85    The newline convention does not affect what the \eR escape sequence matches. By
86    default, this is any Unicode newline sequence, for Perl compatibility. However,
87    this can be changed; see the description of \eR in the section entitled
88    .\" HTML <a href="#newlineseq">
89    .\" </a>
90    "Newline sequences"
91    .\"
92    below. A change of \eR setting can be combined with a change of newline
93    convention.
94    .
95    .
96    .SH "CHARACTERS AND METACHARACTERS"
97    .rs
98    .sp
99  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
100  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
101  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 119  interpreted in some special way.
119  .P  .P
120  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
121  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
122  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
123  as follows:  are as follows:
124  .sp  .sp
125    \e      general escape character with several uses    \e      general escape character with several uses
126    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 151  a character class the only metacharacter
151  .sp  .sp
152  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
153  .  .
154    .
155  .SH BACKSLASH  .SH BACKSLASH
156  .rs  .rs
157  .sp  .sp
158  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
159  non-alphanumeric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
160  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
161  outside character classes.  outside character classes.
162  .P  .P
163  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
# Line 108  particular, if you want to match a backs Line 168  particular, if you want to match a backs
168  .P  .P
169  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
170  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
171  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
172  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
 pattern.  
173  .P  .P
174  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
175  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 143  represents: Line 202  represents:
202    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
203    \ee        escape (hex 1B)    \ee        escape (hex 1B)
204    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
205    \en        newline (hex 0A)    \en        linefeed (hex 0A)
206    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
207    \et        tab (hex 09)    \et        tab (hex 09)
208    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
209    \exhh      character with hex code hh    \exhh      character with hex code hh
210    \ex{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
211  .sp  .sp
212  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
213  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
# Line 156  Thus \ecz becomes hex 1A, but \ec{ becom Line 215  Thus \ecz becomes hex 1A, but \ec{ becom
215  7B.  7B.
216  .P  .P
217  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
218  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
219  appear between \ex{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
220  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
221  other than hexadecimal digits appear between \ex{ and }, or if there is no  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
222  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
223  \ex will be interpreted as a basic hexadecimal escape, with no following  .P
224  digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
225    there is no terminating }, this form of escape is not recognized. Instead, the
226    initial \ex will be interpreted as a basic hexadecimal escape, with no
227    following digits, giving a character whose value is zero.
228  .P  .P
229  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
230  syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
231  way they are handled. For example, \exdc is exactly the same as \ex{dc}.  example, \exdc is exactly the same as \ex{dc}.
232  .P  .P
233  After \e0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
234  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
235  sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
236  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
237  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
238  .P  .P
239  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
240  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
# Line 191  parenthesized subpatterns. Line 253  parenthesized subpatterns.
253  .P  .P
254  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
255  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
256  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
257  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
258  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
259    to \e777 are permitted. For example:
260  .sp  .sp
261    \e040   is another way of writing a space    \e040   is another way of writing a space
262  .\" JOIN  .\" JOIN
# Line 218  For example: Line 281  For example:
281  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
282  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
283  .P  .P
284  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
285  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
286  addition, inside a character class, the sequence \eb is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
287  backspace character (hex 08), and the sequence \eX is interpreted as the  sequences \eR and \eX are interpreted as the characters "R" and "X",
288  character "X". Outside a character class, these sequences have different  respectively. Outside a character class, these sequences have different
289  meanings  meanings
290  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
291  .\" </a>  .\" </a>
# Line 230  meanings Line 293  meanings
293  .\"  .\"
294  .  .
295  .  .
296    .SS "Absolute and relative back references"
297    .rs
298    .sp
299    The sequence \eg followed by an unsigned or a negative number, optionally
300    enclosed in braces, is an absolute or relative back reference. A named back
301    reference can be coded as \eg{name}. Back references are discussed
302    .\" HTML <a href="#backreferences">
303    .\" </a>
304    later,
305    .\"
306    following the discussion of
307    .\" HTML <a href="#subpattern">
308    .\" </a>
309    parenthesized subpatterns.
310    .\"
311    .
312    .
313  .SS "Generic character types"  .SS "Generic character types"
314  .rs  .rs
315  .sp  .sp
316  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
317  following are always recognized:  following are always recognized:
318  .sp  .sp
319    \ed     any decimal digit    \ed     any decimal digit
320    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
321      \eh     any horizontal whitespace character
322      \eH     any character that is not a horizontal whitespace character
323    \es     any whitespace character    \es     any whitespace character
324    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
325      \ev     any vertical whitespace character
326      \eV     any character that is not a vertical whitespace character
327    \ew     any "word" character    \ew     any "word" character
328    \eW     any "non-word" character    \eW     any "non-word" character
329  .sp  .sp
# Line 253  there is no character to match. Line 337  there is no character to match.
337  .P  .P
338  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
339  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
340  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
341    included in a Perl script, \es may match the VT character. In PCRE, it never
342    does.
343    .P
344    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
345    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
346    character property support is available. These sequences retain their original
347    meanings from before UTF-8 support was available, mainly for efficiency
348    reasons.
349    .P
350    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
351    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
352    The horizontal space characters are:
353    .sp
354      U+0009     Horizontal tab
355      U+0020     Space
356      U+00A0     Non-break space
357      U+1680     Ogham space mark
358      U+180E     Mongolian vowel separator
359      U+2000     En quad
360      U+2001     Em quad
361      U+2002     En space
362      U+2003     Em space
363      U+2004     Three-per-em space
364      U+2005     Four-per-em space
365      U+2006     Six-per-em space
366      U+2007     Figure space
367      U+2008     Punctuation space
368      U+2009     Thin space
369      U+200A     Hair space
370      U+202F     Narrow no-break space
371      U+205F     Medium mathematical space
372      U+3000     Ideographic space
373    .sp
374    The vertical space characters are:
375    .sp
376      U+000A     Linefeed
377      U+000B     Vertical tab
378      U+000C     Formfeed
379      U+000D     Carriage return
380      U+0085     Next line
381      U+2028     Line separator
382      U+2029     Paragraph separator
383  .P  .P
384  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
385  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 267  in the Line 393  in the
393  .\" HREF  .\" HREF
394  \fBpcreapi\fP  \fBpcreapi\fP
395  .\"  .\"
396  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,
397  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
398  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
399  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
400  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
401  character property support is available.  .
402    .\" HTML <a name="newlineseq"></a>
403    .SS "Newline sequences"
404    .rs
405    .sp
406    Outside a character class, by default, the escape sequence \eR matches any
407    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
408    equivalent to the following:
409    .sp
410      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
411    .sp
412    This is an example of an "atomic group", details of which are given
413    .\" HTML <a href="#atomicgroup">
414    .\" </a>
415    below.
416    .\"
417    This particular group matches either the two-character sequence CR followed by
418    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
419    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
420    line, U+0085). The two-character sequence is treated as a single unit that
421    cannot be split.
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    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
429    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
430    either at compile time or when the pattern is matched. (BSR is an abbrevation
431    for "backslash R".) This can be made the default when PCRE is built; if this is
432    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
433    It is also possible to specify these settings by starting a pattern string with
434    one of the following sequences:
435    .sp
436      (*BSR_ANYCRLF)   CR, LF, or CRLF only
437      (*BSR_UNICODE)   any Unicode newline sequence
438    .sp
439    These override the default and the options given to \fBpcre_compile()\fP, but
440    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
441    special settings, which are not Perl-compatible, are recognized only at the
442    very start of a pattern, and that they must be in upper case. If more than one
443    of them is present, the last one is used. They can be combined with a change of
444    newline convention, for example, a pattern can start with:
445    .sp
446      (*ANY)(*BSR_ANYCRLF)
447    .sp
448    Inside a character class, \eR matches the letter "R".
449  .  .
450  .  .
451  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 280  character property support is available. Line 453  character property support is available.
453  .rs  .rs
454  .sp  .sp
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 generic character types 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  .sp  characters whose codepoints are less than 256, but they do work in this mode.
459   \ep{\fIxx\fP}   a character with the \fIxx\fP property  The extra escape sequences are:
460   \eP{\fIxx\fP}   a character without the \fIxx\fP property  .sp
461   \eX       an extended Unicode sequence    \ep{\fIxx\fP}   a character with the \fIxx\fP property
462  .sp    \eP{\fIxx\fP}   a character without the \fIxx\fP property
463  The property names represented by \fIxx\fP above are limited to the    \eX       an extended Unicode sequence
464  Unicode general category properties. Each character has exactly one such  .sp
465  property, specified by a two-letter abbreviation. For compatibility with Perl,  The property names represented by \fIxx\fP above are limited to the Unicode
466  negation can be specified by including a circumflex between the opening brace  script names, the general category properties, and "Any", which matches any
467  and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.  character (including newline). Other properties such as "InMusicalSymbols" are
468  .P  not currently supported by PCRE. Note that \eP{Any} does not match any
469  If only one letter is specified with \ep or \eP, it includes all the properties  characters, so always causes a match failure.
470  that start with that letter. In this case, in the absence of negation, the  .P
471  curly brackets in the escape sequence are optional; these two examples have  Sets of Unicode characters are defined as belonging to certain scripts. A
472  the same effect:  character from one of these sets can be matched using a script name. For
473    example:
474    .sp
475      \ep{Greek}
476      \eP{Han}
477    .sp
478    Those that are not part of an identified script are lumped together as
479    "Common". The current list of scripts is:
480    .P
481    Arabic,
482    Armenian,
483    Balinese,
484    Bengali,
485    Bopomofo,
486    Braille,
487    Buginese,
488    Buhid,
489    Canadian_Aboriginal,
490    Cherokee,
491    Common,
492    Coptic,
493    Cuneiform,
494    Cypriot,
495    Cyrillic,
496    Deseret,
497    Devanagari,
498    Ethiopic,
499    Georgian,
500    Glagolitic,
501    Gothic,
502    Greek,
503    Gujarati,
504    Gurmukhi,
505    Han,
506    Hangul,
507    Hanunoo,
508    Hebrew,
509    Hiragana,
510    Inherited,
511    Kannada,
512    Katakana,
513    Kharoshthi,
514    Khmer,
515    Lao,
516    Latin,
517    Limbu,
518    Linear_B,
519    Malayalam,
520    Mongolian,
521    Myanmar,
522    New_Tai_Lue,
523    Nko,
524    Ogham,
525    Old_Italic,
526    Old_Persian,
527    Oriya,
528    Osmanya,
529    Phags_Pa,
530    Phoenician,
531    Runic,
532    Shavian,
533    Sinhala,
534    Syloti_Nagri,
535    Syriac,
536    Tagalog,
537    Tagbanwa,
538    Tai_Le,
539    Tamil,
540    Telugu,
541    Thaana,
542    Thai,
543    Tibetan,
544    Tifinagh,
545    Ugaritic,
546    Yi.
547    .P
548    Each character has exactly one general category property, specified by a
549    two-letter abbreviation. For compatibility with Perl, negation can be specified
550    by including a circumflex between the opening brace and the property name. For
551    example, \ep{^Lu} is the same as \eP{Lu}.
552    .P
553    If only one letter is specified with \ep or \eP, it includes all the general
554    category properties that start with that letter. In this case, in the absence
555    of negation, the curly brackets in the escape sequence are optional; these two
556    examples have the same effect:
557  .sp  .sp
558    \ep{L}    \ep{L}
559    \epL    \epL
560  .sp  .sp
561  The following property codes are supported:  The following general category property codes are supported:
562  .sp  .sp
563    C     Other    C     Other
564    Cc    Control    Cc    Control
# Line 347  The following property codes are support Line 604  The following property codes are support
604    Zp    Paragraph separator    Zp    Paragraph separator
605    Zs    Space separator    Zs    Space separator
606  .sp  .sp
607  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
608  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
609    a modifier or "other".
610    .P
611    The Cs (Surrogate) property applies only to characters in the range U+D800 to
612    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
613    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
614    (see the discussion of PCRE_NO_UTF8_CHECK in the
615    .\" HREF
616    \fBpcreapi\fP
617    .\"
618    page).
619    .P
620    The long synonyms for these properties that Perl supports (such as \ep{Letter})
621    are not supported by PCRE, nor is it permitted to prefix any of these
622    properties with "Is".
623    .P
624    No character that is in the Unicode table has the Cn (unassigned) property.
625    Instead, this property is assumed for any code point that is not in the
626    Unicode table.
627  .P  .P
628  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
629  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
# Line 366  atomic group Line 641  atomic group
641  (see below).  (see below).
642  .\"  .\"
643  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
644  preceding character.  preceding character. None of them have codepoints less than 256, so in
645    non-UTF-8 mode \eX matches any one character.
646  .P  .P
647  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
648  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 374  why the traditional escape sequences suc Line 650  why the traditional escape sequences suc
650  properties in PCRE.  properties in PCRE.
651  .  .
652  .  .
653    .\" HTML <a name="resetmatchstart"></a>
654    .SS "Resetting the match start"
655    .rs
656    .sp
657    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
658    matched characters not to be included in the final matched sequence. For
659    example, the pattern:
660    .sp
661      foo\eKbar
662    .sp
663    matches "foobar", but reports that it has matched "bar". This feature is
664    similar to a lookbehind assertion
665    .\" HTML <a href="#lookbehind">
666    .\" </a>
667    (described below).
668    .\"
669    However, in this case, the part of the subject before the real match does not
670    have to be of fixed length, as lookbehind assertions do. The use of \eK does
671    not interfere with the setting of
672    .\" HTML <a href="#subpattern">
673    .\" </a>
674    captured substrings.
675    .\"
676    For example, when the pattern
677    .sp
678      (foo)\eKbar
679    .sp
680    matches "foobar", the first substring is still set to "foo".
681    .
682    .
683  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
684  .SS "Simple assertions"  .SS "Simple assertions"
685  .rs  .rs
686  .sp  .sp
687  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
688  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,
689  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
690  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 386  subpatterns for more complicated asserti Line 692  subpatterns for more complicated asserti
692  .\" </a>  .\" </a>
693  below.  below.
694  .\"  .\"
695  The backslashed  The backslashed assertions are:
 assertions are:  
696  .sp  .sp
697    \eb     matches at a word boundary    \eb     matches at a word boundary
698    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
699    \eA     matches at start of subject    \eA     matches at the start of the subject
700    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
701    \ez     matches at end of subject            also matches before a newline at the end of the subject
702    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
703      \eG     matches at the first matching position in the subject
704  .sp  .sp
705  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
706  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 412  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 718  PCRE_NOTBOL or PCRE_NOTEOL options, whic
718  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
719  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
720  at a point other than the beginning of the subject, \eA can never match. The  at a point other than the beginning of the subject, \eA can never match. The
721  difference between \eZ and \ez is that \eZ matches before a newline that is the  difference between \eZ and \ez is that \eZ matches before a newline at the end
722  last character of the string as well as at the end of the string, whereas \ez  of the string as well as at the very end, whereas \ez matches only at the end.
 matches only at the end.  
723  .P  .P
724  The \eG assertion is true only when the current matching position is at the  The \eG assertion is true only when the current matching position is at the
725  start point of the match, as specified by the \fIstartoffset\fP argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
# Line 458  to be anchored.) Line 763  to be anchored.)
763  .P  .P
764  A dollar character is an assertion that is true only if the current matching  A dollar character is an assertion that is true only if the current matching
765  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
766  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
767  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
768  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
769  Dollar has no special meaning in a character class.  character class.
770  .P  .P
771  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
772  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
773  does not affect the \eZ assertion.  does not affect the \eZ assertion.
774  .P  .P
775  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
776  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
777  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
778  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
779  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
780  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
781  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
782  branches start with ^ are not anchored in multiline mode, and a match for  .P
783  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
784  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is  \en represents a newline) in multiline mode, but not otherwise. Consequently,
785  set.  patterns that are anchored in single line mode because all branches start with
786    ^ are not anchored in multiline mode, and a match for circumflex is possible
787    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
788    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
789  .P  .P
790  Note that the sequences \eA, \eZ, and \ez can be used to match the start and  Note that the sequences \eA, \eZ, and \ez can be used to match the start and
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  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
793  .  .
794  .  .
795  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
796  .rs  .rs
797  .sp  .sp
798  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
799  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
800  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
801  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
802  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
803  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
804  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
805    (including isolated CRs and LFs). When any Unicode line endings are being
806    recognized, dot does not match CR or LF or any of the other line ending
807    characters.
808    .P
809    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 the
811    two-character sequence CRLF is present in the subject string, it takes two dots
812    to match it.
813    .P
814    The handling of dot is entirely independent of the handling of circumflex and
815    dollar, the only relationship being that they both involve newlines. Dot has no
816    special meaning in a character class.
817  .  .
818  .  .
819  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
820  .rs  .rs
821  .sp  .sp
822  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
823  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
824  provided in Perl in order to match individual bytes in UTF-8 mode. Because it  characters. The feature is provided in Perl in order to match individual bytes
825  breaks up UTF-8 characters into individual bytes, what remains in the string  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
826  may be a malformed UTF-8 string. For this reason, the \eC escape sequence is  what remains in the string may be a malformed UTF-8 string. For this reason,
827  best avoided.  the \eC escape sequence is best avoided.
828  .P  .P
829  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
830  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 555  If you want to use caseless matching for Line 875  If you want to use caseless matching for
875  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
876  UTF-8 support.  UTF-8 support.
877  .P  .P
878  The newline character is never treated in any special way in character classes,  Characters that might indicate line breaks are never treated in any special way
879  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  when matching character classes, whatever line-ending sequence is in use, and
880  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
881    such as [^a] always matches one of these characters.
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
884  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 581  example [\ex{100}-\ex{2ff}]. Line 902  example [\ex{100}-\ex{2ff}].
902  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
903  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
904  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
905  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
906  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
907  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
908  property support.  property support.
# Line 656  the pattern Line 977  the pattern
977    gilbert|sullivan    gilbert|sullivan
978  .sp  .sp
979  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
980  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
981  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
982  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
 subpattern  
983  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
984  .\" </a>  .\" </a>
985  (defined below),  (defined below),
# Line 672  alternative in the subpattern. Line 992  alternative in the subpattern.
992  .rs  .rs
993  .sp  .sp
994  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
995  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
996  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
997    The option letters are
998  .sp  .sp
999    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1000    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 687  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1008  PCRE_MULTILINE while unsetting PCRE_DOTA
1008  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1009  unset.  unset.
1010  .P  .P
1011    The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1012    changed in the same way as the Perl-compatible options by using the characters
1013    J, U and X respectively.
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
1018  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
1019  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
1020  .P  .P
1021  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
1022  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1023  .sp  .sp
1024    (a(?i)b)c    (a(?i)b)c
1025  .sp  .sp
# Line 710  branch is abandoned before the option se Line 1035  branch is abandoned before the option se
1035  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1036  behaviour otherwise.  behaviour otherwise.
1037  .P  .P
1038  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1039  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1040  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences to override what the application
1041  earlier in the pattern than any of the additional features it turns on, even  has set or what has been defaulted. Details are given in the section entitled
1042  when it is at top level. It is best to put it at the start.  .\" HTML <a href="#newlineseq">
1043    .\" </a>
1044    "Newline sequences"
1045    .\"
1046    above.
1047  .  .
1048  .  .
1049  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 729  Turning part of a pattern into a subpatt Line 1058  Turning part of a pattern into a subpatt
1058    cat(aract|erpillar|)    cat(aract|erpillar|)
1059  .sp  .sp
1060  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1061  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1062  .sp  .sp
1063  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
1064  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
# Line 754  the string "the white queen" is matched Line 1083  the string "the white queen" is matched
1083    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1084  .sp  .sp
1085  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
1086  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.  
1087  .P  .P
1088  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
1089  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 770  is reached, an option setting in one bra Line 1098  is reached, an option setting in one bra
1098  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1099  .  .
1100  .  .
1101    .SH "DUPLICATE SUBPATTERN NUMBERS"
1102    .rs
1103    .sp
1104    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1105    the same numbers for its capturing parentheses. Such a subpattern starts with
1106    (?| and is itself a non-capturing subpattern. For example, consider this
1107    pattern:
1108    .sp
1109      (?|(Sat)ur|(Sun))day
1110    .sp
1111    Because the two alternatives are inside a (?| group, both sets of capturing
1112    parentheses are numbered one. Thus, when the pattern matches, you can look
1113    at captured substring number one, whichever alternative matched. This construct
1114    is useful when you want to capture part, but not all, of one of a number of
1115    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1116    number is reset at the start of each branch. The numbers of any capturing
1117    buffers that follow the subpattern start after the highest number used in any
1118    branch. The following example is taken from the Perl documentation.
1119    The numbers underneath show in which buffer the captured content will be
1120    stored.
1121    .sp
1122      # before  ---------------branch-reset----------- after
1123      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1124      # 1            2         2  3        2     3     4
1125    .sp
1126    A backreference or a recursive call to a numbered subpattern always refers to
1127    the first one in the pattern with the given number.
1128    .P
1129    An alternative approach to using this "branch reset" feature is to use
1130    duplicate named subpatterns, as described in the next section.
1131    .
1132    .
1133  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1134  .rs  .rs
1135  .sp  .sp
1136  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
1137  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1138  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
1139  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1140  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1141  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1142  .P  the Perl and the Python syntax.
1143  Named capturing parentheses are still allocated numbers as well as names. The  .P
1144  PCRE API provides function calls for extracting the name-to-number translation  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1145  table from a compiled pattern. There is also a convenience function for  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1146  extracting a captured substring by name. For further details see the  parentheses from other parts of the pattern, such as
1147    .\" HTML <a href="#backreferences">
1148    .\" </a>
1149    backreferences,
1150    .\"
1151    .\" HTML <a href="#recursion">
1152    .\" </a>
1153    recursion,
1154    .\"
1155    and
1156    .\" HTML <a href="#conditions">
1157    .\" </a>
1158    conditions,
1159    .\"
1160    can be made by name as well as by number.
1161    .P
1162    Names consist of up to 32 alphanumeric characters and underscores. Named
1163    capturing parentheses are still allocated numbers as well as names, exactly as
1164    if the names were not present. The PCRE API provides function calls for
1165    extracting the name-to-number translation table from a compiled pattern. There
1166    is also a convenience function for extracting a captured substring by name.
1167    .P
1168    By default, a name must be unique within a pattern, but it is possible to relax
1169    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1170    be useful for patterns where only one instance of the named parentheses can
1171    match. Suppose you want to match the name of a weekday, either as a 3-letter
1172    abbreviation or as the full name, and in both cases you want to extract the
1173    abbreviation. This pattern (ignoring the line breaks) does the job:
1174    .sp
1175      (?<DN>Mon|Fri|Sun)(?:day)?|
1176      (?<DN>Tue)(?:sday)?|
1177      (?<DN>Wed)(?:nesday)?|
1178      (?<DN>Thu)(?:rsday)?|
1179      (?<DN>Sat)(?:urday)?
1180    .sp
1181    There are five capturing substrings, but only one is ever set after a match.
1182    (An alternative way of solving this problem is to use a "branch reset"
1183    subpattern, as described in the previous section.)
1184    .P
1185    The convenience function for extracting the data by name returns the substring
1186    for the first (and in this example, the only) subpattern of that name that
1187    matched. This saves searching to find which numbered subpattern it was. If you
1188    make a reference to a non-unique named subpattern from elsewhere in the
1189    pattern, the one that corresponds to the lowest number is used. For further
1190    details of the interfaces for handling named subpatterns, see the
1191  .\" HREF  .\" HREF
1192  \fBpcreapi\fP  \fBpcreapi\fP
1193  .\"  .\"
# Line 797  Repetition is specified by quantifiers, Line 1201  Repetition is specified by quantifiers,
1201  items:  items:
1202  .sp  .sp
1203    a literal data character    a literal data character
1204    the . metacharacter    the dot metacharacter
1205    the \eC escape sequence    the \eC escape sequence
1206    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1207      the \eR escape sequence
1208    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1209    a character class    a character class
1210    a back reference (see next section)    a back reference (see next section)
# Line 837  which may be several bytes long (and the Line 1242  which may be several bytes long (and the
1242  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1243  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1244  .P  .P
1245  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1246  quantifiers have single-character abbreviations:  abbreviations:
1247  .sp  .sp
1248    *    is equivalent to {0,}    *    is equivalent to {0,}
1249    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 886  own right. Because it has two uses, it c Line 1291  own right. Because it has two uses, it c
1291  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1292  way the rest of the pattern matches.  way the rest of the pattern matches.
1293  .P  .P
1294  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),
1295  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
1296  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
1297  default behaviour.  default behaviour.
# Line 896  is greater than 1 or with a limited maxi Line 1301  is greater than 1 or with a limited maxi
1301  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1302  .P  .P
1303  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
1304  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
1305  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1306  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
1307  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 908  alternatively using ^ to indicate anchor Line 1313  alternatively using ^ to indicate anchor
1313  .P  .P
1314  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1315  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1316  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
1317  succeed. Consider, for example:  succeeds. Consider, for example:
1318  .sp  .sp
1319    (.*)abc\e1    (.*)abc\e1
1320  .sp  .sp
# Line 935  matches "aba" the value of the second ca Line 1340  matches "aba" the value of the second ca
1340  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1341  .rs  .rs
1342  .sp  .sp
1343  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1344  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
1345  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
1346  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
1347  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
1348  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1349  .P  .P
1350  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1351  .sp  .sp
# Line 952  item, and then with 4, and so on, before Line 1357  item, and then with 4, and so on, before
1357  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1358  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.
1359  .P  .P
1360  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
1361  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
1362  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1363  .sp  .sp
# Line 982  previous example can be rewritten as Line 1387  previous example can be rewritten as
1387  .sp  .sp
1388    \ed++foo    \ed++foo
1389  .sp  .sp
1390    Note that a possessive quantifier can be used with an entire group, for
1391    example:
1392    .sp
1393      (abc|xyz){2,3}+
1394    .sp
1395  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1396  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
1397  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
1398  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1399  .P  difference; possessive quantifiers should be slightly faster.
1400  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1401  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1402    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1403    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1404    package, and PCRE copied it from there. It ultimately found its way into Perl
1405    at release 5.10.
1406    .P
1407    PCRE has an optimization that automatically "possessifies" certain simple
1408    pattern constructs. For example, the sequence A+B is treated as A++B because
1409    there is no point in backtracking into a sequence of A's when B must follow.
1410  .P  .P
1411  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
1412  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1030  However, if the decimal number following Line 1448  However, if the decimal number following
1448  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1449  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1450  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1451  numbers less than 10. See the subsection entitled "Non-printing characters"  numbers less than 10. A "forward back reference" of this type can make sense
1452    when a repetition is involved and the subpattern to the right has participated
1453    in an earlier iteration.
1454    .P
1455    It is not possible to have a numerical "forward back reference" to a subpattern
1456    whose number is 10 or more using this syntax because a sequence such as \e50 is
1457    interpreted as a character defined in octal. See the subsection entitled
1458    "Non-printing characters"
1459  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1460  .\" </a>  .\" </a>
1461  above  above
1462  .\"  .\"
1463  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1464    no such problem when named parentheses are used. A back reference to any
1465    subpattern is possible using named parentheses (see below).
1466    .P
1467    Another way of avoiding the ambiguity inherent in the use of digits following a
1468    backslash is to use the \eg escape sequence, which is a feature introduced in
1469    Perl 5.10. This escape must be followed by an unsigned number or a negative
1470    number, optionally enclosed in braces. These examples are all identical:
1471    .sp
1472      (ring), \e1
1473      (ring), \eg1
1474      (ring), \eg{1}
1475    .sp
1476    An unsigned number specifies an absolute reference without the ambiguity that
1477    is present in the older syntax. It is also useful when literal digits follow
1478    the reference. A negative number is a relative reference. Consider this
1479    example:
1480    .sp
1481      (abc(def)ghi)\eg{-1}
1482    .sp
1483    The sequence \eg{-1} is a reference to the most recently started capturing
1484    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1485    would be equivalent to \e1. The use of relative references can be helpful in
1486    long patterns, and also in patterns that are created by joining together
1487    fragments that contain references within themselves.
1488  .P  .P
1489  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1490  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1057  back reference, the case of letters is r Line 1506  back reference, the case of letters is r
1506  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1507  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1508  .P  .P
1509  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1510  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1511  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1512    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1513    references, is also supported. We could rewrite the above example in any of
1514    the following ways:
1515    .sp
1516      (?<p1>(?i)rah)\es+\ek<p1>
1517      (?'p1'(?i)rah)\es+\ek{p1}
1518      (?P<p1>(?i)rah)\es+(?P=p1)
1519      (?<p1>(?i)rah)\es+\eg{p1}
1520  .sp  .sp
1521    A subpattern that is referenced by name may appear in the pattern before or
1522    after the reference.
1523    .P
1524  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1525  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1526  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
# Line 1123  because it does not make sense for negat Line 1582  because it does not make sense for negat
1582  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1583  .rs  .rs
1584  .sp  .sp
1585  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1586  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1587  .sp  .sp
1588    \ew+(?=;)    \ew+(?=;)
1589  .sp  .sp
# Line 1159  negative assertions. For example, Line 1618  negative assertions. For example,
1618  .sp  .sp
1619  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1620  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1621  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1622  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1623  .sp  .sp
1624    (?<=bullock|donkey)    (?<=bullock|donkey)
1625  .sp  .sp
# Line 1180  lengths, but it is acceptable if rewritt Line 1639  lengths, but it is acceptable if rewritt
1639  .sp  .sp
1640    (?<=abc|abde)    (?<=abc|abde)
1641  .sp  .sp
1642    In some cases, the Perl 5.10 escape sequence \eK
1643    .\" HTML <a href="#resetmatchstart">
1644    .\" </a>
1645    (see above)
1646    .\"
1647    can be used instead of a lookbehind assertion; this is not restricted to a
1648    fixed-length.
1649    .P
1650  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1651  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
1652  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1653  match is deemed to fail.  assertion fails.
1654  .P  .P
1655  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1656  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1657  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1658  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1659  .P  .P
1660  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1661  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
1662  such as  pattern such as
1663  .sp  .sp
1664    abcd$    abcd$
1665  .sp  .sp
# Line 1208  then all but the last two characters, an Line 1675  then all but the last two characters, an
1675  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,
1676  if the pattern is written as  if the pattern is written as
1677  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1678    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1679  .sp  .sp
1680  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
1681  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
1682  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1683  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1254  is another pattern that matches "foo" pr Line 1717  is another pattern that matches "foo" pr
1717  characters that are not "999".  characters that are not "999".
1718  .  .
1719  .  .
1720    .\" HTML <a name="conditions"></a>
1721  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1722  .rs  .rs
1723  .sp  .sp
# Line 1269  If the condition is satisfied, the yes-p Line 1733  If the condition is satisfied, the yes-p
1733  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1734  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1735  .P  .P
1736  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1737  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1738  subpattern of that number has previously matched. The number must be greater  .
1739  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1740  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1741  it into three parts for ease of discussion:  .sp
1742    If the text between the parentheses consists of a sequence of digits, the
1743    condition is true if the capturing subpattern of that number has previously
1744    matched. An alternative notation is to precede the digits with a plus or minus
1745    sign. In this case, the subpattern number is relative rather than absolute.
1746    The most recently opened parentheses can be referenced by (?(-1), the next most
1747    recent by (?(-2), and so on. In looping constructs it can also make sense to
1748    refer to subsequent groups with constructs such as (?(+2).
1749    .P
1750    Consider the following pattern, which contains non-significant white space to
1751    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1752    three parts for ease of discussion:
1753  .sp  .sp
1754    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1755  .sp  .sp
# Line 1288  parenthesis is required. Otherwise, sinc Line 1763  parenthesis is required. Otherwise, sinc
1763  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1764  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1765  .P  .P
1766  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
1767  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1768  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1769      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1770    .sp
1771    This makes the fragment independent of the parentheses in the larger pattern.
1772    .
1773    .SS "Checking for a used subpattern by name"
1774    .rs
1775    .sp
1776    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1777    subpattern by name. For compatibility with earlier versions of PCRE, which had
1778    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1779    there is a possible ambiguity with this syntax, because subpattern names may
1780    consist entirely of digits. PCRE looks first for a named subpattern; if it
1781    cannot find one and the name consists entirely of digits, PCRE looks for a
1782    subpattern of that number, which must be greater than zero. Using subpattern
1783    names that consist entirely of digits is not recommended.
1784  .P  .P
1785  If the condition is not a sequence of digits or (R), it must be an assertion.  Rewriting the above example to use a named subpattern gives this:
1786    .sp
1787      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1788    .sp
1789    .
1790    .SS "Checking for pattern recursion"
1791    .rs
1792    .sp
1793    If the condition is the string (R), and there is no subpattern with the name R,
1794    the condition is true if a recursive call to the whole pattern or any
1795    subpattern has been made. If digits or a name preceded by ampersand follow the
1796    letter R, for example:
1797    .sp
1798      (?(R3)...) or (?(R&name)...)
1799    .sp
1800    the condition is true if the most recent recursion is into the subpattern whose
1801    number or name is given. This condition does not check the entire recursion
1802    stack.
1803    .P
1804    At "top level", all these recursion test conditions are false. Recursive
1805    patterns are described below.
1806    .
1807    .SS "Defining subpatterns for use by reference only"
1808    .rs
1809    .sp
1810    If the condition is the string (DEFINE), and there is no subpattern with the
1811    name DEFINE, the condition is always false. In this case, there may be only one
1812    alternative in the subpattern. It is always skipped if control reaches this
1813    point in the pattern; the idea of DEFINE is that it can be used to define
1814    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1815    is described below.) For example, a pattern to match an IPv4 address could be
1816    written like this (ignore whitespace and line breaks):
1817    .sp
1818      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1819      \eb (?&byte) (\e.(?&byte)){3} \eb
1820    .sp
1821    The first part of the pattern is a DEFINE group inside which a another group
1822    named "byte" is defined. This matches an individual component of an IPv4
1823    address (a number less than 256). When matching takes place, this part of the
1824    pattern is skipped because DEFINE acts like a false condition.
1825    .P
1826    The rest of the pattern uses references to the named group to match the four
1827    dot-separated components of an IPv4 address, insisting on a word boundary at
1828    each end.
1829    .
1830    .SS "Assertion conditions"
1831    .rs
1832    .sp
1833    If the condition is not in any of the above formats, it must be an assertion.
1834  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1835  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1836  alternatives on the second line:  alternatives on the second line:
# Line 1317  closing parenthesis. Nested parentheses Line 1855  closing parenthesis. Nested parentheses
1855  that make up a comment play no part in the pattern matching at all.  that make up a comment play no part in the pattern matching at all.
1856  .P  .P
1857  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1858  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1859  character in the pattern.  next newline in the pattern.
1860  .  .
1861  .  .
1862    .\" HTML <a name="recursion"></a>
1863  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
1864  .rs  .rs
1865  .sp  .sp
1866  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1867  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1868  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
1869  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1870  that allows regular expressions to recurse (amongst other things). It does this  .P
1871  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
1872  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
1873  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1874    pattern using code interpolation to solve the parentheses problem can be
1875    created like this:
1876  .sp  .sp
1877    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1878  .sp  .sp
1879  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
1880  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1881  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1882  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1883  .P  supports special syntax for recursion of the entire pattern, and also for
1884  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1885  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1886  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1887  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
1888  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1889    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1890    call, which is described in the next section.) The special item (?R) or (?0) is
1891    a recursive call of the entire regular expression.
1892    .P
1893    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1894    treated as an atomic group. That is, once it has matched some of the subject
1895    string, it is never re-entered, even if it contains untried alternatives and
1896    there is a subsequent matching failure.
1897  .P  .P
1898  For example, this PCRE pattern solves the nested parentheses problem (assume  This PCRE pattern solves the nested parentheses problem (assume the
1899  the PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1900  .sp  .sp
1901    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( (?>[^()]+) | (?R) )* \e)
1902  .sp  .sp
1903  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1904  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1905  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1906  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1907  .P  .P
1908  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 1362  pattern, so instead you could use this: Line 1911  pattern, so instead you could use this:
1911    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1912  .sp  .sp
1913  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
1914  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1915  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1916  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1917  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1918  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1919  .sp  most recently opened parentheses preceding the recursion. In other words, a
1920    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1921  .sp  it is encountered.
1922  This particular example pattern contains nested unlimited repeats, and so the  .P
1923  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1924  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1925  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1926    "subroutine" calls, as described in the next section.
1927    .P
1928    An alternative approach is to use named parentheses instead. The Perl syntax
1929    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1930    could rewrite the above example as follows:
1931    .sp
1932      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1933    .sp
1934    If there is more than one subpattern with the same name, the earliest one is
1935    used.
1936    .P
1937    This particular example pattern that we have been looking at contains nested
1938    unlimited repeats, and so the use of atomic grouping for matching strings of
1939    non-parentheses is important when applying the pattern to strings that do not
1940    match. For example, when this pattern is applied to
1941  .sp  .sp
1942    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1943  .sp  .sp
# Line 1385  before failure can be reported. Line 1949  before failure can be reported.
1949  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
1950  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.
1951  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
1952  the next section and the  below and the
1953  .\" HREF  .\" HREF
1954  \fBpcrecallout\fP  \fBpcrecallout\fP
1955  .\"  .\"
# Line 1424  is the actual recursive call. Line 1988  is the actual recursive call.
1988  .sp  .sp
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    .sp
1995      (...(absolute)...)...(?2)...
1996      (...(relative)...)...(?-1)...
1997      (...(?+1)...(relative)...
1998    .sp
1999    An earlier example pointed out that the pattern
2000  .sp  .sp
2001    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2002  .sp  .sp
# Line 1435  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  .sp  .sp
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 must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2010  refer.  .P
2011    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2012    group. That is, once it has matched some of the subject string, it is never
2013    re-entered, even if it contains untried alternatives and there is a subsequent
2014    matching failure.
2015    .P
2016    When a subpattern is used as a subroutine, processing options such as
2017    case-independence are fixed when the subpattern is defined. They cannot be
2018    changed for different calls. For example, consider this pattern:
2019    .sp
2020      (abc)(?i:(?-1))
2021    .sp
2022    It matches "abcabc". It does not match "abcABC" because the change of
2023    processing option does not affect the called subpattern.
2024  .  .
2025  .  .
2026  .SH CALLOUTS  .SH CALLOUTS
# Line 1457  function is to be called. If you want to Line 2041  function is to be called. If you want to
2041  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.
2042  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2043  .sp  .sp
2044    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2045  .sp  .sp
2046  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2047  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 1473  description of the interface to the call Line 2057  description of the interface to the call
2057  \fBpcrecallout\fP  \fBpcrecallout\fP
2058  .\"  .\"
2059  documentation.  documentation.
2060    .
2061    .
2062    .SH "BACKTRACKING CONTROL"
2063    .rs
2064    .sp
2065    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2066    are described in the Perl documentation as "experimental and subject to change
2067    or removal in a future version of Perl". It goes on to say: "Their usage in
2068    production code should be noted to avoid problems during upgrades." The same
2069    remarks apply to the PCRE features described in this section.
2070  .P  .P
2071  .in 0  Since these verbs are specifically related to backtracking, they can be used
2072  Last updated: 28 February 2005  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2073  .br  backtracking algorithm. They cause an error if encountered by
2074  Copyright (c) 1997-2005 University of Cambridge.  \fBpcre_dfa_exec()\fP.
2075    .P
2076    The new verbs make use of what was previously invalid syntax: an opening
2077    parenthesis followed by an asterisk. In Perl, they are generally of the form
2078    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2079    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2080    are two kinds:
2081    .
2082    .SS "Verbs that act immediately"
2083    .rs
2084    .sp
2085    The following verbs act as soon as they are encountered:
2086    .sp
2087       (*ACCEPT)
2088    .sp
2089    This verb causes the match to end successfully, skipping the remainder of the
2090    pattern. When inside a recursion, only the innermost pattern is ended
2091    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2092    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2093    captured. For example:
2094    .sp
2095      A(A|B(*ACCEPT)|C)D
2096    .sp
2097    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2098    captured.
2099    .sp
2100      (*FAIL) or (*F)
2101    .sp
2102    This verb causes the match to fail, forcing backtracking to occur. It is
2103    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2104    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2105    Perl features that are not present in PCRE. The nearest equivalent is the
2106    callout feature, as for example in this pattern:
2107    .sp
2108      a+(?C)(*FAIL)
2109    .sp
2110    A match with the string "aaaa" always fails, but the callout is taken before
2111    each backtrack happens (in this example, 10 times).
2112    .
2113    .SS "Verbs that act after backtracking"
2114    .rs
2115    .sp
2116    The following verbs do nothing when they are encountered. Matching continues
2117    with what follows, but if there is no subsequent match, a failure is forced.
2118    The verbs differ in exactly what kind of failure occurs.
2119    .sp
2120      (*COMMIT)
2121    .sp
2122    This verb causes the whole match to fail outright if the rest of the pattern
2123    does not match. Even if the pattern is unanchored, no further attempts to find
2124    a match by advancing the start point take place. Once (*COMMIT) has been
2125    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2126    starting point, or not at all. For example:
2127    .sp
2128      a+(*COMMIT)b
2129    .sp
2130    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2131    dynamic anchor, or "I've started, so I must finish."
2132    .sp
2133      (*PRUNE)
2134    .sp
2135    This verb causes the match to fail at the current position if the rest of the
2136    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2137    advance to the next starting character then happens. Backtracking can occur as
2138    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2139    if there is no match to the right, backtracking cannot cross (*PRUNE).
2140    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2141    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2142    be expressed in any other way.
2143    .sp
2144      (*SKIP)
2145    .sp
2146    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2147    "bumpalong" advance is not to the next character, but to the position in the
2148    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2149    was matched leading up to it cannot be part of a successful match. Consider:
2150    .sp
2151      a+(*SKIP)b
2152    .sp
2153    If the subject is "aaaac...", after the first match attempt fails (starting at
2154    the first character in the string), the starting point skips on to start the
2155    next attempt at "c". Note that a possessive quantifer does not have the same
2156    effect in this example; although it would suppress backtracking during the
2157    first match attempt, the second attempt would start at the second character
2158    instead of skipping on to "c".
2159    .sp
2160      (*THEN)
2161    .sp
2162    This verb causes a skip to the next alternation if the rest of the pattern does
2163    not match. That is, it cancels pending backtracking, but only within the
2164    current alternation. Its name comes from the observation that it can be used
2165    for a pattern-based if-then-else block:
2166    .sp
2167      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2168    .sp
2169    If the COND1 pattern matches, FOO is tried (and possibly further items after
2170    the end of the group if FOO succeeds); on failure the matcher skips to the
2171    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2172    is used outside of any alternation, it acts exactly like (*PRUNE).
2173    .
2174    .
2175    .SH "SEE ALSO"
2176    .rs
2177    .sp
2178    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2179    .
2180    .
2181    .SH AUTHOR
2182    .rs
2183    .sp
2184    .nf
2185    Philip Hazel
2186    University Computing Service
2187    Cambridge CB2 3QH, England.
2188    .fi
2189    .
2190    .
2191    .SH REVISION
2192    .rs
2193    .sp
2194    .nf
2195    Last updated: 17 September 2007
2196    Copyright (c) 1997-2007 University of Cambridge.
2197    .fi

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