/[pcre]/code/trunk/doc/pcrepattern.3
ViewVC logotype

Diff of /code/trunk/doc/pcrepattern.3

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC revision 454 by ph10, Tue Sep 22 09:42:11 2009 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this, you must
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  the PCRE_UTF8 option. There is also a special sequence that can be given at the
27  places below. There is also a summary of UTF-8 features in the  start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 30  The remainder of this document discusses Line 46  The remainder of this document discusses
46  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
48  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
50  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53  .\" HREF  .\" HREF
54  \fBpcrematching\fP  \fBpcrematching\fP
55  .\"  .\"
56  page.  page.
57    .
58    .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76  .P  .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP. For
87    example, on a Unix system where LF is the default newline sequence, the pattern
88    .sp
89      (*CR)a.b
90    .sp
91    changes the convention to CR. That pattern matches "a\enb" because LF is no
92    longer a newline. Note that these special settings, which are not
93    Perl-compatible, are recognized only at the very start of a pattern, and that
94    they must be in upper case. If more than one of them is present, the last one
95    is used.
96    .P
97    The newline convention does not affect what the \eR escape sequence matches. By
98    default, this is any Unicode newline sequence, for Perl compatibility. However,
99    this can be changed; see the description of \eR in the section entitled
100    .\" HTML <a href="#newlineseq">
101    .\" </a>
102    "Newline sequences"
103    .\"
104    below. A change of \eR setting can be combined with a change of newline
105    convention.
106    .
107    .
108    .SH "CHARACTERS AND METACHARACTERS"
109    .rs
110    .sp
111  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
112  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
113  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 131  interpreted in some special way.
131  .P  .P
132  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
133  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
134  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
135  as follows:  are as follows:
136  .sp  .sp
137    \e      general escape character with several uses    \e      general escape character with several uses
138    ^      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 163  a character class the only metacharacter
163  .sp  .sp
164  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
165  .  .
166    .
167  .SH BACKSLASH  .SH BACKSLASH
168  .rs  .rs
169  .sp  .sp
170  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
171  non-alphanumeric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
172  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
173  outside character classes.  outside character classes.
174  .P  .P
175  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 180  particular, if you want to match a backs
180  .P  .P
181  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
182  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
183  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
184  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.  
185  .P  .P
186  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
187  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 214  represents:
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
215    \ee        escape (hex 1B)    \ee        escape (hex 1B)
216    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
217    \en        newline (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 158  Thus \ecz becomes hex 1A, but \ec{ becom Line 229  Thus \ecz becomes hex 1A, but \ec{ becom
229  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
230  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
231  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
232  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
235  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
236  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240  .P  .P
241  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
242  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
243  example, \exdc is exactly the same as \ex{dc}.  example, \exdc is exactly the same as \ex{dc}.
244  .P  .P
245  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
246  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
247  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
248  (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
249  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
250  .P  .P
251  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.
252  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 265  parenthesized subpatterns.
265  .P  .P
266  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
267  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
268  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
269  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
270  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
271    to \e777 are permitted. For example:
272  .sp  .sp
273    \e040   is another way of writing a space    \e040   is another way of writing a space
274  .\" JOIN  .\" JOIN
# Line 218  For example: Line 293  For example:
293  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
294  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
295  .P  .P
296  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
297  (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
298  addition, inside a character class, the sequence \eb is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
299  backspace character (hex 08), and the sequence \eX is interpreted as the  sequences \eR and \eX are interpreted as the characters "R" and "X",
300  character "X". Outside a character class, these sequences have different  respectively. Outside a character class, these sequences have different
301  meanings  meanings
302  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
303  .\" </a>  .\" </a>
# Line 230  meanings Line 305  meanings
305  .\"  .\"
306  .  .
307  .  .
308    .SS "Absolute and relative back references"
309    .rs
310    .sp
311    The sequence \eg followed by an unsigned or a negative number, optionally
312    enclosed in braces, is an absolute or relative back reference. A named back
313    reference can be coded as \eg{name}. Back references are discussed
314    .\" HTML <a href="#backreferences">
315    .\" </a>
316    later,
317    .\"
318    following the discussion of
319    .\" HTML <a href="#subpattern">
320    .\" </a>
321    parenthesized subpatterns.
322    .\"
323    .
324    .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342    .
343    .
344  .SS "Generic character types"  .SS "Generic character types"
345  .rs  .rs
346  .sp  .sp
347  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
348  following are always recognized:  following are always recognized:
349  .sp  .sp
350    \ed     any decimal digit    \ed     any decimal digit
351    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
352      \eh     any horizontal whitespace character
353      \eH     any character that is not a horizontal whitespace character
354    \es     any whitespace character    \es     any whitespace character
355    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
356      \ev     any vertical whitespace character
357      \eV     any character that is not a vertical whitespace character
358    \ew     any "word" character    \ew     any "word" character
359    \eW     any "non-word" character    \eW     any "non-word" character
360  .sp  .sp
# Line 253  there is no character to match. Line 368  there is no character to match.
368  .P  .P
369  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
370  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
371  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372    included in a Perl script, \es may match the VT character. In PCRE, it never
373    does.
374    .P
375    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377    character property support is available. These sequences retain their original
378    meanings from before UTF-8 support was available, mainly for efficiency
379    reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381    .P
382    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384    The horizontal space characters are:
385    .sp
386      U+0009     Horizontal tab
387      U+0020     Space
388      U+00A0     Non-break space
389      U+1680     Ogham space mark
390      U+180E     Mongolian vowel separator
391      U+2000     En quad
392      U+2001     Em quad
393      U+2002     En space
394      U+2003     Em space
395      U+2004     Three-per-em space
396      U+2005     Four-per-em space
397      U+2006     Six-per-em space
398      U+2007     Figure space
399      U+2008     Punctuation space
400      U+2009     Thin space
401      U+200A     Hair space
402      U+202F     Narrow no-break space
403      U+205F     Medium mathematical space
404      U+3000     Ideographic space
405    .sp
406    The vertical space characters are:
407    .sp
408      U+000A     Linefeed
409      U+000B     Vertical tab
410      U+000C     Formfeed
411      U+000D     Carriage return
412      U+0085     Next line
413      U+2028     Line separator
414      U+2029     Paragraph separator
415  .P  .P
416  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
417  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 267  in the Line 425  in the
425  .\" HREF  .\" HREF
426  \fBpcreapi\fP  \fBpcreapi\fP
427  .\"  .\"
428  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
430  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
431  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
432  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
433  character property support is available. The use of locales with Unicode is  .
434  discouraged.  .\" HTML <a name="newlineseq"></a>
435    .SS "Newline sequences"
436    .rs
437    .sp
438    Outside a character class, by default, the escape sequence \eR matches any
439    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440    equivalent to the following:
441    .sp
442      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443    .sp
444    This is an example of an "atomic group", details of which are given
445    .\" HTML <a href="#atomicgroup">
446    .\" </a>
447    below.
448    .\"
449    This particular group matches either the two-character sequence CR followed by
450    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
451    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
452    line, U+0085). The two-character sequence is treated as a single unit that
453    cannot be split.
454    .P
455    In UTF-8 mode, two additional characters whose codepoints are greater than 255
456    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
457    Unicode character property support is not needed for these characters to be
458    recognized.
459    .P
460    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462    either at compile time or when the pattern is matched. (BSR is an abbrevation
463    for "backslash R".) This can be made the default when PCRE is built; if this is
464    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465    It is also possible to specify these settings by starting a pattern string with
466    one of the following sequences:
467    .sp
468      (*BSR_ANYCRLF)   CR, LF, or CRLF only
469      (*BSR_UNICODE)   any Unicode newline sequence
470    .sp
471    These override the default and the options given to \fBpcre_compile()\fP, but
472    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
473    special settings, which are not Perl-compatible, are recognized only at the
474    very start of a pattern, and that they must be in upper case. If more than one
475    of them is present, the last one is used. They can be combined with a change of
476    newline convention, for example, a pattern can start with:
477    .sp
478      (*ANY)(*BSR_ANYCRLF)
479    .sp
480    Inside a character class, \eR matches the letter "R".
481  .  .
482  .  .
483  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 281  discouraged. Line 485  discouraged.
485  .rs  .rs
486  .sp  .sp
487  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
488  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
489  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
490    characters whose codepoints are less than 256, but they do work in this mode.
491    The extra escape sequences are:
492  .sp  .sp
493    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
494    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 306  Those that are not part of an identified Line 512  Those that are not part of an identified
512  .P  .P
513  Arabic,  Arabic,
514  Armenian,  Armenian,
515    Balinese,
516  Bengali,  Bengali,
517  Bopomofo,  Bopomofo,
518  Braille,  Braille,
# Line 315  Canadian_Aboriginal, Line 522  Canadian_Aboriginal,
522  Cherokee,  Cherokee,
523  Common,  Common,
524  Coptic,  Coptic,
525    Cuneiform,
526  Cypriot,  Cypriot,
527  Cyrillic,  Cyrillic,
528  Deseret,  Deseret,
# Line 344  Malayalam, Line 552  Malayalam,
552  Mongolian,  Mongolian,
553  Myanmar,  Myanmar,
554  New_Tai_Lue,  New_Tai_Lue,
555    Nko,
556  Ogham,  Ogham,
557  Old_Italic,  Old_Italic,
558  Old_Persian,  Old_Persian,
559  Oriya,  Oriya,
560  Osmanya,  Osmanya,
561    Phags_Pa,
562    Phoenician,
563  Runic,  Runic,
564  Shavian,  Shavian,
565  Sinhala,  Sinhala,
# Line 429  The special property L& is also supporte Line 640  The special property L& is also supporte
640  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
641  a modifier or "other".  a modifier or "other".
642  .P  .P
643  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
644  are not supported by PCRE. Nor is is permitted to prefix any of these  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
645    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
646    (see the discussion of PCRE_NO_UTF8_CHECK in the
647    .\" HREF
648    \fBpcreapi\fP
649    .\"
650    page). Perl does not support the Cs property.
651    .P
652    The long synonyms for property names that Perl supports (such as \ep{Letter})
653    are not supported by PCRE, nor is it permitted to prefix any of these
654  properties with "Is".  properties with "Is".
655  .P  .P
656  No character that is in the Unicode table has the Cn (unassigned) property.  No character that is in the Unicode table has the Cn (unassigned) property.
# Line 453  atomic group Line 673  atomic group
673  (see below).  (see below).
674  .\"  .\"
675  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
676  preceding character.  preceding character. None of them have codepoints less than 256, so in
677    non-UTF-8 mode \eX matches any one character.
678  .P  .P
679  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
680  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 461  why the traditional escape sequences suc Line 682  why the traditional escape sequences suc
682  properties in PCRE.  properties in PCRE.
683  .  .
684  .  .
685    .\" HTML <a name="resetmatchstart"></a>
686    .SS "Resetting the match start"
687    .rs
688    .sp
689    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
690    matched characters not to be included in the final matched sequence. For
691    example, the pattern:
692    .sp
693      foo\eKbar
694    .sp
695    matches "foobar", but reports that it has matched "bar". This feature is
696    similar to a lookbehind assertion
697    .\" HTML <a href="#lookbehind">
698    .\" </a>
699    (described below).
700    .\"
701    However, in this case, the part of the subject before the real match does not
702    have to be of fixed length, as lookbehind assertions do. The use of \eK does
703    not interfere with the setting of
704    .\" HTML <a href="#subpattern">
705    .\" </a>
706    captured substrings.
707    .\"
708    For example, when the pattern
709    .sp
710      (foo)\eKbar
711    .sp
712    matches "foobar", the first substring is still set to "foo".
713    .
714    .
715  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
716  .SS "Simple assertions"  .SS "Simple assertions"
717  .rs  .rs
718  .sp  .sp
719  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
720  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,
721  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
722  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 473  subpatterns for more complicated asserti Line 724  subpatterns for more complicated asserti
724  .\" </a>  .\" </a>
725  below.  below.
726  .\"  .\"
727  The backslashed  The backslashed assertions are:
 assertions are:  
728  .sp  .sp
729    \eb     matches at a word boundary    \eb     matches at a word boundary
730    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
731    \eA     matches at start of subject    \eA     matches at the start of the subject
732    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
733    \ez     matches at end of subject            also matches before a newline at the end of the subject
734    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
735      \eG     matches at the first matching position in the subject
736  .sp  .sp
737  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
738  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 499  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 750  PCRE_NOTBOL or PCRE_NOTEOL options, whic
750  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
751  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
752  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
753  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
754  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.  
755  .P  .P
756  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
757  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 545  to be anchored.) Line 795  to be anchored.)
795  .P  .P
796  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
797  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
798  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
799  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
800  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
801  Dollar has no special meaning in a character class.  character class.
802  .P  .P
803  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
804  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
805  does not affect the \eZ assertion.  does not affect the \eZ assertion.
806  .P  .P
807  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
808  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
809  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
810  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
811  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
812  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
813  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
814  branches start with ^ are not anchored in multiline mode, and a match for  .P
815  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
816  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,
817  set.  patterns that are anchored in single line mode because all branches start with
818    ^ are not anchored in multiline mode, and a match for circumflex is possible
819    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
820    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
821  .P  .P
822  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
823  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
824  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
825  .  .
826  .  .
827  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
828  .rs  .rs
829  .sp  .sp
830  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
831  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
832  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.
833  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
834  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
835  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
836  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
837    (including isolated CRs and LFs). When any Unicode line endings are being
838    recognized, dot does not match CR or LF or any of the other line ending
839    characters.
840    .P
841    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
842    option is set, a dot matches any one character, without exception. If the
843    two-character sequence CRLF is present in the subject string, it takes two dots
844    to match it.
845    .P
846    The handling of dot is entirely independent of the handling of circumflex and
847    dollar, the only relationship being that they both involve newlines. Dot has no
848    special meaning in a character class.
849  .  .
850  .  .
851  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
852  .rs  .rs
853  .sp  .sp
854  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
855  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
856  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
857  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,
858  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,
859  best avoided.  the \eC escape sequence is best avoided.
860  .P  .P
861  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
862  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 642  If you want to use caseless matching for Line 907  If you want to use caseless matching for
907  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
908  UTF-8 support.  UTF-8 support.
909  .P  .P
910  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
911  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
912  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
913    such as [^a] always matches one of these characters.
914  .P  .P
915  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
916  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 668  example [\ex{100}-\ex{2ff}]. Line 934  example [\ex{100}-\ex{2ff}].
934  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
935  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
936  [][\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
937  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
938  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
939  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
940  property support.  property support.
# Line 743  the pattern Line 1009  the pattern
1009    gilbert|sullivan    gilbert|sullivan
1010  .sp  .sp
1011  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1012  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1013  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
1014  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  
1015  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
1016  .\" </a>  .\" </a>
1017  (defined below),  (defined below),
# Line 759  alternative in the subpattern. Line 1024  alternative in the subpattern.
1024  .rs  .rs
1025  .sp  .sp
1026  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1027  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
1028  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1029    The option letters are
1030  .sp  .sp
1031    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1032    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 774  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1040  PCRE_MULTILINE while unsetting PCRE_DOTA
1040  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
1041  unset.  unset.
1042  .P  .P
1043  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1044  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1045  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1046  the global options (and it will therefore show up in data extracted by the  .P
1047  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1048    subpattern parentheses), the change applies to the remainder of the pattern
1049    that follows. If the change is placed right at the start of a pattern, PCRE
1050    extracts it into the global options (and it will therefore show up in data
1051    extracted by the \fBpcre_fullinfo()\fP function).
1052  .P  .P
1053  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
1054  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1055  .sp  .sp
1056    (a(?i)b)c    (a(?i)b)c
1057  .sp  .sp
# Line 797  branch is abandoned before the option se Line 1067  branch is abandoned before the option se
1067  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1068  behaviour otherwise.  behaviour otherwise.
1069  .P  .P
1070  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
1071  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
1072  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1073  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1074  when it is at top level. It is best to put it at the start.  section entitled
1075    .\" HTML <a href="#newlineseq">
1076    .\" </a>
1077    "Newline sequences"
1078    .\"
1079    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1080    mode; this is equivalent to setting the PCRE_UTF8 option.
1081  .  .
1082  .  .
1083  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 816  Turning part of a pattern into a subpatt Line 1092  Turning part of a pattern into a subpatt
1092    cat(aract|erpillar|)    cat(aract|erpillar|)
1093  .sp  .sp
1094  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1095  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1096  .sp  .sp
1097  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
1098  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 841  the string "the white queen" is matched Line 1117  the string "the white queen" is matched
1117    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1118  .sp  .sp
1119  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
1120  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.  
1121  .P  .P
1122  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
1123  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 857  is reached, an option setting in one bra Line 1132  is reached, an option setting in one bra
1132  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1133  .  .
1134  .  .
1135    .SH "DUPLICATE SUBPATTERN NUMBERS"
1136    .rs
1137    .sp
1138    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1139    the same numbers for its capturing parentheses. Such a subpattern starts with
1140    (?| and is itself a non-capturing subpattern. For example, consider this
1141    pattern:
1142    .sp
1143      (?|(Sat)ur|(Sun))day
1144    .sp
1145    Because the two alternatives are inside a (?| group, both sets of capturing
1146    parentheses are numbered one. Thus, when the pattern matches, you can look
1147    at captured substring number one, whichever alternative matched. This construct
1148    is useful when you want to capture part, but not all, of one of a number of
1149    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1150    number is reset at the start of each branch. The numbers of any capturing
1151    buffers that follow the subpattern start after the highest number used in any
1152    branch. The following example is taken from the Perl documentation.
1153    The numbers underneath show in which buffer the captured content will be
1154    stored.
1155    .sp
1156      # before  ---------------branch-reset----------- after
1157      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1158      # 1            2         2  3        2     3     4
1159    .sp
1160    A backreference or a recursive call to a numbered subpattern always refers to
1161    the first one in the pattern with the given number.
1162    .P
1163    An alternative approach to using this "branch reset" feature is to use
1164    duplicate named subpatterns, as described in the next section.
1165    .
1166    .
1167  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1168  .rs  .rs
1169  .sp  .sp
1170  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
1171  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1172  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
1173  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1174  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
1175  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
1176  .P  the Perl and the Python syntax.
1177  Named capturing parentheses are still allocated numbers as well as names. The  .P
1178  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
1179  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
1180  extracting a captured substring by name. For further details see the  parentheses from other parts of the pattern, such as
1181    .\" HTML <a href="#backreferences">
1182    .\" </a>
1183    backreferences,
1184    .\"
1185    .\" HTML <a href="#recursion">
1186    .\" </a>
1187    recursion,
1188    .\"
1189    and
1190    .\" HTML <a href="#conditions">
1191    .\" </a>
1192    conditions,
1193    .\"
1194    can be made by name as well as by number.
1195    .P
1196    Names consist of up to 32 alphanumeric characters and underscores. Named
1197    capturing parentheses are still allocated numbers as well as names, exactly as
1198    if the names were not present. The PCRE API provides function calls for
1199    extracting the name-to-number translation table from a compiled pattern. There
1200    is also a convenience function for extracting a captured substring by name.
1201    .P
1202    By default, a name must be unique within a pattern, but it is possible to relax
1203    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1204    be useful for patterns where only one instance of the named parentheses can
1205    match. Suppose you want to match the name of a weekday, either as a 3-letter
1206    abbreviation or as the full name, and in both cases you want to extract the
1207    abbreviation. This pattern (ignoring the line breaks) does the job:
1208    .sp
1209      (?<DN>Mon|Fri|Sun)(?:day)?|
1210      (?<DN>Tue)(?:sday)?|
1211      (?<DN>Wed)(?:nesday)?|
1212      (?<DN>Thu)(?:rsday)?|
1213      (?<DN>Sat)(?:urday)?
1214    .sp
1215    There are five capturing substrings, but only one is ever set after a match.
1216    (An alternative way of solving this problem is to use a "branch reset"
1217    subpattern, as described in the previous section.)
1218    .P
1219    The convenience function for extracting the data by name returns the substring
1220    for the first (and in this example, the only) subpattern of that name that
1221    matched. This saves searching to find which numbered subpattern it was. If you
1222    make a reference to a non-unique named subpattern from elsewhere in the
1223    pattern, the one that corresponds to the lowest number is used. For further
1224    details of the interfaces for handling named subpatterns, see the
1225  .\" HREF  .\" HREF
1226  \fBpcreapi\fP  \fBpcreapi\fP
1227  .\"  .\"
1228  documentation.  documentation.
1229    .P
1230    \fBWarning:\fP You cannot use different names to distinguish between two
1231    subpatterns with the same number (see the previous section) because PCRE uses
1232    only the numbers when matching.
1233  .  .
1234  .  .
1235  .SH REPETITION  .SH REPETITION
# Line 884  Repetition is specified by quantifiers, Line 1239  Repetition is specified by quantifiers,
1239  items:  items:
1240  .sp  .sp
1241    a literal data character    a literal data character
1242    the . metacharacter    the dot metacharacter
1243    the \eC escape sequence    the \eC escape sequence
1244    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1245      the \eR escape sequence
1246    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1247    a character class    a character class
1248    a back reference (see next section)    a back reference (see next section)
# Line 922  support is available, \eX{3} matches thr Line 1278  support is available, \eX{3} matches thr
1278  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1279  .P  .P
1280  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
1281  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1282    subpatterns that are referenced as
1283    .\" HTML <a href="#subpatternsassubroutines">
1284    .\" </a>
1285    subroutines
1286    .\"
1287    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1288    quantifier are omitted from the compiled pattern.
1289  .P  .P
1290  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1291  quantifiers have single-character abbreviations:  abbreviations:
1292  .sp  .sp
1293    *    is equivalent to {0,}    *    is equivalent to {0,}
1294    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 973  own right. Because it has two uses, it c Line 1336  own right. Because it has two uses, it c
1336  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
1337  way the rest of the pattern matches.  way the rest of the pattern matches.
1338  .P  .P
1339  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),
1340  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
1341  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
1342  default behaviour.  default behaviour.
# Line 983  is greater than 1 or with a limited maxi Line 1346  is greater than 1 or with a limited maxi
1346  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1347  .P  .P
1348  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
1349  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
1350  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1351  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
1352  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 995  alternatively using ^ to indicate anchor Line 1358  alternatively using ^ to indicate anchor
1358  .P  .P
1359  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1360  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1361  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
1362  succeed. Consider, for example:  succeeds. Consider, for example:
1363  .sp  .sp
1364    (.*)abc\e1    (.*)abc\e1
1365  .sp  .sp
# Line 1022  matches "aba" the value of the second ca Line 1385  matches "aba" the value of the second ca
1385  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1386  .rs  .rs
1387  .sp  .sp
1388  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1389  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
1390  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
1391  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
1392  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
1393  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1394  .P  .P
1395  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
1396  .sp  .sp
# Line 1039  item, and then with 4, and so on, before Line 1402  item, and then with 4, and so on, before
1402  (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
1403  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.
1404  .P  .P
1405  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
1406  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
1407  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1408  .sp  .sp
# Line 1069  previous example can be rewritten as Line 1432  previous example can be rewritten as
1432  .sp  .sp
1433    \ed++foo    \ed++foo
1434  .sp  .sp
1435    Note that a possessive quantifier can be used with an entire group, for
1436    example:
1437    .sp
1438      (abc|xyz){2,3}+
1439    .sp
1440  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1441  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
1442  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
1443  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1444  .P  difference; possessive quantifiers should be slightly faster.
1445  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1446  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1447    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1448    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1449    package, and PCRE copied it from there. It ultimately found its way into Perl
1450    at release 5.10.
1451    .P
1452    PCRE has an optimization that automatically "possessifies" certain simple
1453    pattern constructs. For example, the sequence A+B is treated as A++B because
1454    there is no point in backtracking into a sequence of A's when B must follow.
1455  .P  .P
1456  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
1457  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 1117  However, if the decimal number following Line 1493  However, if the decimal number following
1493  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
1494  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
1495  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
1496  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
1497    when a repetition is involved and the subpattern to the right has participated
1498    in an earlier iteration.
1499    .P
1500    It is not possible to have a numerical "forward back reference" to a subpattern
1501    whose number is 10 or more using this syntax because a sequence such as \e50 is
1502    interpreted as a character defined in octal. See the subsection entitled
1503    "Non-printing characters"
1504  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1505  .\" </a>  .\" </a>
1506  above  above
1507  .\"  .\"
1508  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1509    no such problem when named parentheses are used. A back reference to any
1510    subpattern is possible using named parentheses (see below).
1511    .P
1512    Another way of avoiding the ambiguity inherent in the use of digits following a
1513    backslash is to use the \eg escape sequence, which is a feature introduced in
1514    Perl 5.10. This escape must be followed by an unsigned number or a negative
1515    number, optionally enclosed in braces. These examples are all identical:
1516    .sp
1517      (ring), \e1
1518      (ring), \eg1
1519      (ring), \eg{1}
1520    .sp
1521    An unsigned number specifies an absolute reference without the ambiguity that
1522    is present in the older syntax. It is also useful when literal digits follow
1523    the reference. A negative number is a relative reference. Consider this
1524    example:
1525    .sp
1526      (abc(def)ghi)\eg{-1}
1527    .sp
1528    The sequence \eg{-1} is a reference to the most recently started capturing
1529    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1530    would be equivalent to \e1. The use of relative references can be helpful in
1531    long patterns, and also in patterns that are created by joining together
1532    fragments that contain references within themselves.
1533  .P  .P
1534  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1535  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1144  back reference, the case of letters is r Line 1551  back reference, the case of letters is r
1551  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
1552  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1553  .P  .P
1554  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1555  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1556  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1557    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1558    references, is also supported. We could rewrite the above example in any of
1559    the following ways:
1560    .sp
1561      (?<p1>(?i)rah)\es+\ek<p1>
1562      (?'p1'(?i)rah)\es+\ek{p1}
1563      (?P<p1>(?i)rah)\es+(?P=p1)
1564      (?<p1>(?i)rah)\es+\eg{p1}
1565  .sp  .sp
1566    A subpattern that is referenced by name may appear in the pattern before or
1567    after the reference.
1568    .P
1569  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
1570  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1571  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
# Line 1210  because it does not make sense for negat Line 1627  because it does not make sense for negat
1627  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1628  .rs  .rs
1629  .sp  .sp
1630  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1631  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1632  .sp  .sp
1633    \ew+(?=;)    \ew+(?=;)
1634  .sp  .sp
# Line 1246  negative assertions. For example, Line 1663  negative assertions. For example,
1663  .sp  .sp
1664  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
1665  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
1666  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
1667  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1668  .sp  .sp
1669    (?<=bullock|donkey)    (?<=bullock|donkey)
1670  .sp  .sp
# Line 1257  is permitted, but Line 1674  is permitted, but
1674  .sp  .sp
1675  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1676  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1677  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1678  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1679  .sp  .sp
1680    (?<=ab(c|de))    (?<=ab(c|de))
1681  .sp  .sp
1682  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1683  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1684    branches:
1685  .sp  .sp
1686    (?<=abc|abde)    (?<=abc|abde)
1687  .sp  .sp
1688    In some cases, the Perl 5.10 escape sequence \eK
1689    .\" HTML <a href="#resetmatchstart">
1690    .\" </a>
1691    (see above)
1692    .\"
1693    can be used instead of a lookbehind assertion to get round the fixed-length
1694    restriction.
1695    .P
1696  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1697  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
1698  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1699  match is deemed to fail.  assertion fails.
1700  .P  .P
1701  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)
1702  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1703  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
1704  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1705  .P  .P
1706  Atomic groups can be used in conjunction with lookbehind assertions to specify  .\" HTML <a href="#subpatternsassubroutines">
1707  efficient matching at the end of the subject string. Consider a simple pattern  .\" </a>
1708  such as  "Subroutine"
1709    .\"
1710    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1711    as the subpattern matches a fixed-length string.
1712    .\" HTML <a href="#recursion">
1713    .\" </a>
1714    Recursion,
1715    .\"
1716    however, is not supported.
1717    .P
1718    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1719    specify efficient matching at the end of the subject string. Consider a simple
1720    pattern such as
1721  .sp  .sp
1722    abcd$    abcd$
1723  .sp  .sp
# Line 1295  then all but the last two characters, an Line 1733  then all but the last two characters, an
1733  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,
1734  if the pattern is written as  if the pattern is written as
1735  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1736    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1737  .sp  .sp
1738  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
1739  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
1740  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1741  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1341  is another pattern that matches "foo" pr Line 1775  is another pattern that matches "foo" pr
1775  characters that are not "999".  characters that are not "999".
1776  .  .
1777  .  .
1778    .\" HTML <a name="conditions"></a>
1779  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1780  .rs  .rs
1781  .sp  .sp
# Line 1356  If the condition is satisfied, the yes-p Line 1791  If the condition is satisfied, the yes-p
1791  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
1792  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1793  .P  .P
1794  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1795  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1796  subpattern of that number has previously matched. The number must be greater  .
1797  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1798  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1799  it into three parts for ease of discussion:  .sp
1800    If the text between the parentheses consists of a sequence of digits, the
1801    condition is true if the capturing subpattern of that number has previously
1802    matched. An alternative notation is to precede the digits with a plus or minus
1803    sign. In this case, the subpattern number is relative rather than absolute.
1804    The most recently opened parentheses can be referenced by (?(-1), the next most
1805    recent by (?(-2), and so on. In looping constructs it can also make sense to
1806    refer to subsequent groups with constructs such as (?(+2).
1807    .P
1808    Consider the following pattern, which contains non-significant white space to
1809    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1810    three parts for ease of discussion:
1811  .sp  .sp
1812    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1813  .sp  .sp
# Line 1375  parenthesis is required. Otherwise, sinc Line 1821  parenthesis is required. Otherwise, sinc
1821  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1822  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1823  .P  .P
1824  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
1825  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1826  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1827      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1828    .sp
1829    This makes the fragment independent of the parentheses in the larger pattern.
1830    .
1831    .SS "Checking for a used subpattern by name"
1832    .rs
1833    .sp
1834    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1835    subpattern by name. For compatibility with earlier versions of PCRE, which had
1836    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1837    there is a possible ambiguity with this syntax, because subpattern names may
1838    consist entirely of digits. PCRE looks first for a named subpattern; if it
1839    cannot find one and the name consists entirely of digits, PCRE looks for a
1840    subpattern of that number, which must be greater than zero. Using subpattern
1841    names that consist entirely of digits is not recommended.
1842  .P  .P
1843  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:
1844    .sp
1845      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1846    .sp
1847    .
1848    .SS "Checking for pattern recursion"
1849    .rs
1850    .sp
1851    If the condition is the string (R), and there is no subpattern with the name R,
1852    the condition is true if a recursive call to the whole pattern or any
1853    subpattern has been made. If digits or a name preceded by ampersand follow the
1854    letter R, for example:
1855    .sp
1856      (?(R3)...) or (?(R&name)...)
1857    .sp
1858    the condition is true if the most recent recursion is into the subpattern whose
1859    number or name is given. This condition does not check the entire recursion
1860    stack.
1861    .P
1862    At "top level", all these recursion test conditions are false.
1863    .\" HTML <a href="#recursion">
1864    .\" </a>
1865    Recursive patterns
1866    .\"
1867    are described below.
1868    .
1869    .SS "Defining subpatterns for use by reference only"
1870    .rs
1871    .sp
1872    If the condition is the string (DEFINE), and there is no subpattern with the
1873    name DEFINE, the condition is always false. In this case, there may be only one
1874    alternative in the subpattern. It is always skipped if control reaches this
1875    point in the pattern; the idea of DEFINE is that it can be used to define
1876    "subroutines" that can be referenced from elsewhere. (The use of
1877    .\" HTML <a href="#subpatternsassubroutines">
1878    .\" </a>
1879    "subroutines"
1880    .\"
1881    is described below.) For example, a pattern to match an IPv4 address could be
1882    written like this (ignore whitespace and line breaks):
1883    .sp
1884      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1885      \eb (?&byte) (\e.(?&byte)){3} \eb
1886    .sp
1887    The first part of the pattern is a DEFINE group inside which a another group
1888    named "byte" is defined. This matches an individual component of an IPv4
1889    address (a number less than 256). When matching takes place, this part of the
1890    pattern is skipped because DEFINE acts like a false condition.
1891    .P
1892    The rest of the pattern uses references to the named group to match the four
1893    dot-separated components of an IPv4 address, insisting on a word boundary at
1894    each end.
1895    .
1896    .SS "Assertion conditions"
1897    .rs
1898    .sp
1899    If the condition is not in any of the above formats, it must be an assertion.
1900  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1901  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1902  alternatives on the second line:  alternatives on the second line:
# Line 1404  closing parenthesis. Nested parentheses Line 1921  closing parenthesis. Nested parentheses
1921  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.
1922  .P  .P
1923  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1924  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1925  character in the pattern.  next newline in the pattern.
1926  .  .
1927  .  .
1928    .\" HTML <a name="recursion"></a>
1929  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
1930  .rs  .rs
1931  .sp  .sp
1932  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1933  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1934  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
1935  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1936  that allows regular expressions to recurse (amongst other things). It does this  .P
1937  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
1938  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
1939  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1940    pattern using code interpolation to solve the parentheses problem can be
1941    created like this:
1942  .sp  .sp
1943    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1944  .sp  .sp
1945  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
1946  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1947  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1948  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1949  .P  supports special syntax for recursion of the entire pattern, and also for
1950  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1951  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
1952  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1953  "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
1954  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1955  .P  provided that it occurs inside that subpattern. (If not, it is a
1956  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
1957  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
1958  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
1959    .\"
1960    call, which is described in the next section.) The special item (?R) or (?0) is
1961    a recursive call of the entire regular expression.
1962  .P  .P
1963  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1964  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
# Line 1453  pattern, so instead you could use this: Line 1976  pattern, so instead you could use this:
1976    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1977  .sp  .sp
1978  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
1979  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1980  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1981  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
1982  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.)
1983  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
1984  .sp  most recently opened parentheses preceding the recursion. In other words, a
1985    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1986  .sp  it is encountered.
1987  This particular example pattern contains nested unlimited repeats, and so the  .P
1988  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1989  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
1990  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1991    .\" HTML <a href="#subpatternsassubroutines">
1992    .\" </a>
1993    "subroutine"
1994    .\"
1995    calls, as described in the next section.
1996    .P
1997    An alternative approach is to use named parentheses instead. The Perl syntax
1998    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1999    could rewrite the above example as follows:
2000    .sp
2001      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
2002    .sp
2003    If there is more than one subpattern with the same name, the earliest one is
2004    used.
2005    .P
2006    This particular example pattern that we have been looking at contains nested
2007    unlimited repeats, and so the use of atomic grouping for matching strings of
2008    non-parentheses is important when applying the pattern to strings that do not
2009    match. For example, when this pattern is applied to
2010  .sp  .sp
2011    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2012  .sp  .sp
# Line 1476  before failure can be reported. Line 2018  before failure can be reported.
2018  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
2019  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.
2020  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
2021  the next section and the  below and the
2022  .\" HREF  .\" HREF
2023  \fBpcrecallout\fP  \fBpcrecallout\fP
2024  .\"  .\"
# Line 1509  different alternatives for the recursive Line 2051  different alternatives for the recursive
2051  is the actual recursive call.  is the actual recursive call.
2052  .  .
2053  .  .
2054    .\" HTML <a name="recursiondifference"></a>
2055    .SS "Recursion difference from Perl"
2056    .rs
2057    .sp
2058    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2059    treated as an atomic group. That is, once it has matched some of the subject
2060    string, it is never re-entered, even if it contains untried alternatives and
2061    there is a subsequent matching failure. This can be illustrated by the
2062    following pattern, which purports to match a palindromic string that contains
2063    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2064    .sp
2065      ^(.|(.)(?1)\e2)$
2066    .sp
2067    The idea is that it either matches a single character, or two identical
2068    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2069    it does not if the pattern is longer than three characters. Consider the
2070    subject string "abcba":
2071    .P
2072    At the top level, the first character is matched, but as it is not at the end
2073    of the string, the first alternative fails; the second alternative is taken
2074    and the recursion kicks in. The recursive call to subpattern 1 successfully
2075    matches the next character ("b"). (Note that the beginning and end of line
2076    tests are not part of the recursion).
2077    .P
2078    Back at the top level, the next character ("c") is compared with what
2079    subpattern 2 matched, which was "a". This fails. Because the recursion is
2080    treated as an atomic group, there are now no backtracking points, and so the
2081    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2082    try the second alternative.) However, if the pattern is written with the
2083    alternatives in the other order, things are different:
2084    .sp
2085      ^((.)(?1)\e2|.)$
2086    .sp
2087    This time, the recursing alternative is tried first, and continues to recurse
2088    until it runs out of characters, at which point the recursion fails. But this
2089    time we do have another alternative to try at the higher level. That is the big
2090    difference: in the previous case the remaining alternative is at a deeper
2091    recursion level, which PCRE cannot use.
2092    .P
2093    To change the pattern so that matches all palindromic strings, not just those
2094    with an odd number of characters, it is tempting to change the pattern to this:
2095    .sp
2096      ^((.)(?1)\e2|.?)$
2097    .sp
2098    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2099    deeper recursion has matched a single character, it cannot be entered again in
2100    order to match an empty string. The solution is to separate the two cases, and
2101    write out the odd and even cases as alternatives at the higher level:
2102    .sp
2103      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2104    .sp
2105    If you want to match typical palindromic phrases, the pattern has to ignore all
2106    non-word characters, which can be done like this:
2107    .sp
2108      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2109    .sp
2110    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2111    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2112    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2113    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2114    more) to match typical phrases, and Perl takes so long that you think it has
2115    gone into a loop.
2116    .
2117    .
2118  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2119  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2120  .rs  .rs
2121  .sp  .sp
2122  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
2123  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
2124  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2125  pattern  before or after the reference. A numbered reference can be absolute or
2126    relative, as in these examples:
2127    .sp
2128      (...(absolute)...)...(?2)...
2129      (...(relative)...)...(?-1)...
2130      (...(?+1)...(relative)...
2131    .sp
2132    An earlier example pointed out that the pattern
2133  .sp  .sp
2134    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2135  .sp  .sp
# Line 1526  matches "sense and sensibility" and "res Line 2139  matches "sense and sensibility" and "res
2139    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2140  .sp  .sp
2141  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
2142  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
 refer.  
2143  .P  .P
2144  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2145  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2146  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2147  matching failure.  matching failure.
2148    .P
2149    When a subpattern is used as a subroutine, processing options such as
2150    case-independence are fixed when the subpattern is defined. They cannot be
2151    changed for different calls. For example, consider this pattern:
2152    .sp
2153      (abc)(?i:(?-1))
2154    .sp
2155    It matches "abcabc". It does not match "abcABC" because the change of
2156    processing option does not affect the called subpattern.
2157    .
2158    .
2159    .\" HTML <a name="onigurumasubroutines"></a>
2160    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2161    .rs
2162    .sp
2163    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2164    a number enclosed either in angle brackets or single quotes, is an alternative
2165    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2166    are two of the examples used above, rewritten using this syntax:
2167    .sp
2168      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2169      (sens|respons)e and \eg'1'ibility
2170    .sp
2171    PCRE supports an extension to Oniguruma: if a number is preceded by a
2172    plus or a minus sign it is taken as a relative reference. For example:
2173    .sp
2174      (abc)(?i:\eg<-1>)
2175    .sp
2176    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2177    synonymous. The former is a back reference; the latter is a subroutine call.
2178  .  .
2179  .  .
2180  .SH CALLOUTS  .SH CALLOUTS
# Line 1553  function is to be called. If you want to Line 2195  function is to be called. If you want to
2195  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.
2196  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2197  .sp  .sp
2198    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2199  .sp  .sp
2200  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
2201  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 1569  description of the interface to the call Line 2211  description of the interface to the call
2211  \fBpcrecallout\fP  \fBpcrecallout\fP
2212  .\"  .\"
2213  documentation.  documentation.
2214    .
2215    .
2216    .SH "BACKTRACKING CONTROL"
2217    .rs
2218    .sp
2219    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2220    are described in the Perl documentation as "experimental and subject to change
2221    or removal in a future version of Perl". It goes on to say: "Their usage in
2222    production code should be noted to avoid problems during upgrades." The same
2223    remarks apply to the PCRE features described in this section.
2224  .P  .P
2225  .in 0  Since these verbs are specifically related to backtracking, most of them can be
2226  Last updated: 24 January 2006  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2227  .br  a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2228  Copyright (c) 1997-2006 University of Cambridge.  failing negative assertion, they cause an error if encountered by
2229    \fBpcre_dfa_exec()\fP.
2230    .P
2231    If any of these verbs are used in an assertion subpattern, their effect is
2232    confined to that subpattern; it does not extend to the surrounding pattern.
2233    Note that assertion subpatterns are processed as anchored at the point where
2234    they are tested.
2235    .P
2236    The new verbs make use of what was previously invalid syntax: an opening
2237    parenthesis followed by an asterisk. In Perl, they are generally of the form
2238    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2239    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2240    are two kinds:
2241    .
2242    .SS "Verbs that act immediately"
2243    .rs
2244    .sp
2245    The following verbs act as soon as they are encountered:
2246    .sp
2247       (*ACCEPT)
2248    .sp
2249    This verb causes the match to end successfully, skipping the remainder of the
2250    pattern. When inside a recursion, only the innermost pattern is ended
2251    immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
2252    is captured. (This feature was added to PCRE at release 8.00.) For example:
2253    .sp
2254      A((?:A|B(*ACCEPT)|C)D)
2255    .sp
2256    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2257    the outer parentheses.
2258    .sp
2259      (*FAIL) or (*F)
2260    .sp
2261    This verb causes the match to fail, forcing backtracking to occur. It is
2262    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2263    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2264    Perl features that are not present in PCRE. The nearest equivalent is the
2265    callout feature, as for example in this pattern:
2266    .sp
2267      a+(?C)(*FAIL)
2268    .sp
2269    A match with the string "aaaa" always fails, but the callout is taken before
2270    each backtrack happens (in this example, 10 times).
2271    .
2272    .SS "Verbs that act after backtracking"
2273    .rs
2274    .sp
2275    The following verbs do nothing when they are encountered. Matching continues
2276    with what follows, but if there is no subsequent match, a failure is forced.
2277    The verbs differ in exactly what kind of failure occurs.
2278    .sp
2279      (*COMMIT)
2280    .sp
2281    This verb causes the whole match to fail outright if the rest of the pattern
2282    does not match. Even if the pattern is unanchored, no further attempts to find
2283    a match by advancing the start point take place. Once (*COMMIT) has been
2284    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2285    starting point, or not at all. For example:
2286    .sp
2287      a+(*COMMIT)b
2288    .sp
2289    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2290    dynamic anchor, or "I've started, so I must finish."
2291    .sp
2292      (*PRUNE)
2293    .sp
2294    This verb causes the match to fail at the current position if the rest of the
2295    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2296    advance to the next starting character then happens. Backtracking can occur as
2297    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2298    if there is no match to the right, backtracking cannot cross (*PRUNE).
2299    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2300    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2301    be expressed in any other way.
2302    .sp
2303      (*SKIP)
2304    .sp
2305    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2306    "bumpalong" advance is not to the next character, but to the position in the
2307    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2308    was matched leading up to it cannot be part of a successful match. Consider:
2309    .sp
2310      a+(*SKIP)b
2311    .sp
2312    If the subject is "aaaac...", after the first match attempt fails (starting at
2313    the first character in the string), the starting point skips on to start the
2314    next attempt at "c". Note that a possessive quantifer does not have the same
2315    effect in this example; although it would suppress backtracking during the
2316    first match attempt, the second attempt would start at the second character
2317    instead of skipping on to "c".
2318    .sp
2319      (*THEN)
2320    .sp
2321    This verb causes a skip to the next alternation if the rest of the pattern does
2322    not match. That is, it cancels pending backtracking, but only within the
2323    current alternation. Its name comes from the observation that it can be used
2324    for a pattern-based if-then-else block:
2325    .sp
2326      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2327    .sp
2328    If the COND1 pattern matches, FOO is tried (and possibly further items after
2329    the end of the group if FOO succeeds); on failure the matcher skips to the
2330    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2331    is used outside of any alternation, it acts exactly like (*PRUNE).
2332    .
2333    .
2334    .SH "SEE ALSO"
2335    .rs
2336    .sp
2337    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2338    .
2339    .
2340    .SH AUTHOR
2341    .rs
2342    .sp
2343    .nf
2344    Philip Hazel
2345    University Computing Service
2346    Cambridge CB2 3QH, England.
2347    .fi
2348    .
2349    .
2350    .SH REVISION
2351    .rs
2352    .sp
2353    .nf
2354    Last updated: 22 September 2009
2355    Copyright (c) 1997-2009 University of Cambridge.
2356    .fi

Legend:
Removed from v.87  
changed lines
  Added in v.454

webmaster@exim.org
ViewVC Help
Powered by ViewVC 1.1.12