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revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC revision 185 by ph10, Tue Jun 19 13:39:46 2007 UTC
# Line 30  The remainder of this document discusses Line 30  The remainder of this document discusses
30  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
32  \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
33  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
34  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
35    alternative function, and how it differs from the normal function, are
36    discussed in the
37  .\" HREF  .\" HREF
38  \fBpcrematching\fP  \fBpcrematching\fP
39  .\"  .\"
40  page.  page.
41  .P  .
42    .
43    .SH "CHARACTERS AND METACHARACTERS"
44    .rs
45    .sp
46  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
47  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
48  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 66  interpreted in some special way.
66  .P  .P
67  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
68  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
69  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
70  as follows:  are as follows:
71  .sp  .sp
72    \e      general escape character with several uses    \e      general escape character with several uses
73    ^      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 98  a character class the only metacharacter
98  .sp  .sp
99  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
100  .  .
101    .
102  .SH BACKSLASH  .SH BACKSLASH
103  .rs  .rs
104  .sp  .sp
105  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
106  non-alphanumeric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
107  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
108  outside character classes.  outside character classes.
109  .P  .P
110  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 115  particular, if you want to match a backs
115  .P  .P
116  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
117  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
118  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
119  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.  
120  .P  .P
121  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
122  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 168  Characters whose value is less than 256 Line 174  Characters whose value is less than 256
174  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
175  example, \exdc is exactly the same as \ex{dc}.  example, \exdc is exactly the same as \ex{dc}.
176  .P  .P
177  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
178  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
179  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
180  (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
181  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
182  .P  .P
183  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.
184  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 197  parenthesized subpatterns.
197  .P  .P
198  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
199  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
200  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
201  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
202  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
203    to \e777 are permitted. For example:
204  .sp  .sp
205    \e040   is another way of writing a space    \e040   is another way of writing a space
206  .\" JOIN  .\" JOIN
# Line 218  For example: Line 225  For example:
225  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
226  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
227  .P  .P
228  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
229  (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
230  addition, inside a character class, the sequence \eb is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
231  backspace character (hex 08), and the sequence \eX is interpreted as the  sequences \eR and \eX are interpreted as the characters "R" and "X",
232  character "X". Outside a character class, these sequences have different  respectively. Outside a character class, these sequences have different
233  meanings  meanings
234  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
235  .\" </a>  .\" </a>
# Line 230  meanings Line 237  meanings
237  .\"  .\"
238  .  .
239  .  .
240    .SS "Absolute and relative back references"
241    .rs
242    .sp
243    The sequence \eg followed by a positive or negative number, optionally enclosed
244    in braces, is an absolute or relative back reference. A named back reference
245    can be coded as \eg{name}. Back references are discussed
246    .\" HTML <a href="#backreferences">
247    .\" </a>
248    later,
249    .\"
250    following the discussion of
251    .\" HTML <a href="#subpattern">
252    .\" </a>
253    parenthesized subpatterns.
254    .\"
255    .
256    .
257  .SS "Generic character types"  .SS "Generic character types"
258  .rs  .rs
259  .sp  .sp
260  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
261  following are always recognized:  following are always recognized:
262  .sp  .sp
263    \ed     any decimal digit    \ed     any decimal digit
264    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
265      \eh     any horizontal whitespace character
266      \eH     any character that is not a horizontal whitespace character
267    \es     any whitespace character    \es     any whitespace character
268    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
269      \ev     any vertical whitespace character
270      \eV     any character that is not a vertical whitespace character
271    \ew     any "word" character    \ew     any "word" character
272    \eW     any "non-word" character    \eW     any "non-word" character
273  .sp  .sp
# Line 253  there is no character to match. Line 281  there is no character to match.
281  .P  .P
282  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).
283  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
284  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
285    included in a Perl script, \es may match the VT character. In PCRE, it never
286    does.
287    .P
288    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
289    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
290    character property support is available. These sequences retain their original
291    meanings from before UTF-8 support was available, mainly for efficiency
292    reasons.
293    .P
294    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
295    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
296    The horizontal space characters are:
297    .sp
298      U+0009     Horizontal tab
299      U+0020     Space
300      U+00A0     Non-break space
301      U+1680     Ogham space mark
302      U+180E     Mongolian vowel separator
303      U+2000     En quad
304      U+2001     Em quad
305      U+2002     En space
306      U+2003     Em space
307      U+2004     Three-per-em space
308      U+2005     Four-per-em space
309      U+2006     Six-per-em space
310      U+2007     Figure space
311      U+2008     Punctuation space
312      U+2009     Thin space
313      U+200A     Hair space
314      U+202F     Narrow no-break space
315      U+205F     Medium mathematical space
316      U+3000     Ideographic space
317    .sp
318    The vertical space characters are:
319    .sp
320      U+000A     Linefeed
321      U+000B     Vertical tab
322      U+000C     Formfeed
323      U+000D     Carriage return
324      U+0085     Next line
325      U+2028     Line separator
326      U+2029     Paragraph separator
327  .P  .P
328  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
329  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 337  in the
337  .\" HREF  .\" HREF
338  \fBpcreapi\fP  \fBpcreapi\fP
339  .\"  .\"
340  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,
341  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
342    accented letters, and these are matched by \ew. The use of locales with Unicode
343    is discouraged.
344    .
345    .
346    .SS "Newline sequences"
347    .rs
348    .sp
349    Outside a character class, the escape sequence \eR matches any Unicode newline
350    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
351    the following:
352    .sp
353      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
354    .sp
355    This is an example of an "atomic group", details of which are given
356    .\" HTML <a href="#atomicgroup">
357    .\" </a>
358    below.
359    .\"
360    This particular group matches either the two-character sequence CR followed by
361    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
362    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
363    line, U+0085). The two-character sequence is treated as a single unit that
364    cannot be split.
365    .P
366    In UTF-8 mode, two additional characters whose codepoints are greater than 255
367    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
368    Unicode character property support is not needed for these characters to be
369    recognized.
370  .P  .P
371  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Inside a character class, \eR matches the letter "R".
 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  
 character property support is available. The use of locales with Unicode is  
 discouraged.  
372  .  .
373  .  .
374  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 281  discouraged. Line 376  discouraged.
376  .rs  .rs
377  .sp  .sp
378  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
379  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
380  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
381    characters whose codepoints are less than 256, but they do work in this mode.
382    The extra escape sequences are:
383  .sp  .sp
384    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
385    \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 403  Those that are not part of an identified
403  .P  .P
404  Arabic,  Arabic,
405  Armenian,  Armenian,
406    Balinese,
407  Bengali,  Bengali,
408  Bopomofo,  Bopomofo,
409  Braille,  Braille,
# Line 315  Canadian_Aboriginal, Line 413  Canadian_Aboriginal,
413  Cherokee,  Cherokee,
414  Common,  Common,
415  Coptic,  Coptic,
416    Cuneiform,
417  Cypriot,  Cypriot,
418  Cyrillic,  Cyrillic,
419  Deseret,  Deseret,
# Line 344  Malayalam, Line 443  Malayalam,
443  Mongolian,  Mongolian,
444  Myanmar,  Myanmar,
445  New_Tai_Lue,  New_Tai_Lue,
446    Nko,
447  Ogham,  Ogham,
448  Old_Italic,  Old_Italic,
449  Old_Persian,  Old_Persian,
450  Oriya,  Oriya,
451  Osmanya,  Osmanya,
452    Phags_Pa,
453    Phoenician,
454  Runic,  Runic,
455  Shavian,  Shavian,
456  Sinhala,  Sinhala,
# Line 430  the Lu, Ll, or Lt property, in other wor Line 532  the Lu, Ll, or Lt property, in other wor
532  a modifier or "other".  a modifier or "other".
533  .P  .P
534  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
535  are not supported by PCRE. Nor is is permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
536  properties with "Is".  properties with "Is".
537  .P  .P
538  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 555  atomic group
555  (see below).  (see below).
556  .\"  .\"
557  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
558  preceding character.  preceding character. None of them have codepoints less than 256, so in
559    non-UTF-8 mode \eX matches any one character.
560  .P  .P
561  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
562  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 564  why the traditional escape sequences suc
564  properties in PCRE.  properties in PCRE.
565  .  .
566  .  .
567    .\" HTML <a name="resetmatchstart"></a>
568    .SS "Resetting the match start"
569    .rs
570    .sp
571    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
572    matched characters not to be included in the final matched sequence. For
573    example, the pattern:
574    .sp
575      foo\eKbar
576    .sp
577    matches "foobar", but reports that it has matched "bar". This feature is
578    similar to a lookbehind assertion
579    .\" HTML <a href="#lookbehind">
580    .\" </a>
581    (described below).
582    .\"
583    However, in this case, the part of the subject before the real match does not
584    have to be of fixed length, as lookbehind assertions do. The use of \eK does
585    not interfere with the setting of
586    .\" HTML <a href="#subpattern">
587    .\" </a>
588    captured substrings.
589    .\"
590    For example, when the pattern
591    .sp
592      (foo)\eKbar
593    .sp
594    matches "foobar", the first substring is still set to "foo".
595    .
596    .
597  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
598  .SS "Simple assertions"  .SS "Simple assertions"
599  .rs  .rs
600  .sp  .sp
601  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
602  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,
603  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
604  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 473  subpatterns for more complicated asserti Line 606  subpatterns for more complicated asserti
606  .\" </a>  .\" </a>
607  below.  below.
608  .\"  .\"
609  The backslashed  The backslashed assertions are:
 assertions are:  
610  .sp  .sp
611    \eb     matches at a word boundary    \eb     matches at a word boundary
612    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
613    \eA     matches at start of subject    \eA     matches at the start of the subject
614    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
615    \ez     matches at end of subject            also matches before a newline at the end of the subject
616    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
617      \eG     matches at the first matching position in the subject
618  .sp  .sp
619  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
620  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 632  PCRE_NOTBOL or PCRE_NOTEOL options, whic
632  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
633  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
634  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
635  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
636  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.  
637  .P  .P
638  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
639  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 677  to be anchored.)
677  .P  .P
678  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
679  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
680  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
681  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
682  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
683  Dollar has no special meaning in a character class.  character class.
684  .P  .P
685  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
686  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
687  does not affect the \eZ assertion.  does not affect the \eZ assertion.
688  .P  .P
689  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
690  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
691  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
692  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
693  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
694  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
695  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
696  branches start with ^ are not anchored in multiline mode, and a match for  .P
697  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
698  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,
699  set.  patterns that are anchored in single line mode because all branches start with
700    ^ are not anchored in multiline mode, and a match for circumflex is possible
701    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
702    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
703  .P  .P
704  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
705  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
706  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
707  .  .
708  .  .
709  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
710  .rs  .rs
711  .sp  .sp
712  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
713  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
714  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.
715  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
716  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
717  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
718  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
719    (including isolated CRs and LFs). When any Unicode line endings are being
720    recognized, dot does not match CR or LF or any of the other line ending
721    characters.
722    .P
723    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
724    option is set, a dot matches any one character, without exception. If the
725    two-character sequence CRLF is present in the subject string, it takes two dots
726    to match it.
727    .P
728    The handling of dot is entirely independent of the handling of circumflex and
729    dollar, the only relationship being that they both involve newlines. Dot has no
730    special meaning in a character class.
731  .  .
732  .  .
733  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
734  .rs  .rs
735  .sp  .sp
736  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
737  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
738  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
739  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,
740  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,
741  best avoided.  the \eC escape sequence is best avoided.
742  .P  .P
743  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
744  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 642  If you want to use caseless matching for Line 789  If you want to use caseless matching for
789  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
790  UTF-8 support.  UTF-8 support.
791  .P  .P
792  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
793  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
794  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
795    such as [^a] always matches one of these characters.
796  .P  .P
797  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
798  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 816  example [\ex{100}-\ex{2ff}].
816  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
817  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
818  [][\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
819  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
820  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
821  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
822  property support.  property support.
# Line 743  the pattern Line 891  the pattern
891    gilbert|sullivan    gilbert|sullivan
892  .sp  .sp
893  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
894  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
895  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
896  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  
897  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
898  .\" </a>  .\" </a>
899  (defined below),  (defined below),
# Line 780  If the change is placed right at the sta Line 927  If the change is placed right at the sta
927  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
928  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
929  .P  .P
930  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
931  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
932  .sp  .sp
933    (a(?i)b)c    (a(?i)b)c
934  .sp  .sp
# Line 797  branch is abandoned before the option se Line 944  branch is abandoned before the option se
944  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
945  behaviour otherwise.  behaviour otherwise.
946  .P  .P
947  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
948  same way as the Perl-compatible options by using the characters U and X  changed in the same way as the Perl-compatible options by using the characters
949  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
 earlier in the pattern than any of the additional features it turns on, even  
 when it is at top level. It is best to put it at the start.  
950  .  .
951  .  .
952  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 816  Turning part of a pattern into a subpatt Line 961  Turning part of a pattern into a subpatt
961    cat(aract|erpillar|)    cat(aract|erpillar|)
962  .sp  .sp
963  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
964  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
965  .sp  .sp
966  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
967  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 986  the string "the white queen" is matched
986    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
987  .sp  .sp
988  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
989  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.  
990  .P  .P
991  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
992  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 1001  is reached, an option setting in one bra
1001  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1002  .  .
1003  .  .
1004    .SH "DUPLICATE SUBPATTERN NUMBERS"
1005    .rs
1006    .sp
1007    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1008    the same numbers for its capturing parentheses. Such a subpattern starts with
1009    (?| and is itself a non-capturing subpattern. For example, consider this
1010    pattern:
1011    .sp
1012      (?|(Sat)ur|(Sun))day
1013    .sp
1014    Because the two alternatives are inside a (?| group, both sets of capturing
1015    parentheses are numbered one. Thus, when the pattern matches, you can look
1016    at captured substring number one, whichever alternative matched. This construct
1017    is useful when you want to capture part, but not all, of one of a number of
1018    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1019    number is reset at the start of each branch. The numbers of any capturing
1020    buffers that follow the subpattern start after the highest number used in any
1021    branch. The following example is taken from the Perl documentation.
1022    The numbers underneath show in which buffer the captured content will be
1023    stored.
1024    .sp
1025      # before  ---------------branch-reset----------- after
1026      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1027      # 1            2         2  3        2     3     4
1028    .sp
1029    A backreference or a recursive call to a numbered subpattern always refers to
1030    the first one in the pattern with the given number.
1031    .P
1032    An alternative approach to using this "branch reset" feature is to use
1033    duplicate named subpatterns, as described in the next section.
1034    .
1035    .
1036  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1037  .rs  .rs
1038  .sp  .sp
1039  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
1040  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1041  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
1042  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1043  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
1044  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
1045  .P  the Perl and the Python syntax.
1046  Named capturing parentheses are still allocated numbers as well as names. The  .P
1047  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
1048  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
1049  extracting a captured substring by name. For further details see the  parentheses from other parts of the pattern, such as
1050    .\" HTML <a href="#backreferences">
1051    .\" </a>
1052    backreferences,
1053    .\"
1054    .\" HTML <a href="#recursion">
1055    .\" </a>
1056    recursion,
1057    .\"
1058    and
1059    .\" HTML <a href="#conditions">
1060    .\" </a>
1061    conditions,
1062    .\"
1063    can be made by name as well as by number.
1064    .P
1065    Names consist of up to 32 alphanumeric characters and underscores. Named
1066    capturing parentheses are still allocated numbers as well as names, exactly as
1067    if the names were not present. The PCRE API provides function calls for
1068    extracting the name-to-number translation table from a compiled pattern. There
1069    is also a convenience function for extracting a captured substring by name.
1070    .P
1071    By default, a name must be unique within a pattern, but it is possible to relax
1072    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1073    be useful for patterns where only one instance of the named parentheses can
1074    match. Suppose you want to match the name of a weekday, either as a 3-letter
1075    abbreviation or as the full name, and in both cases you want to extract the
1076    abbreviation. This pattern (ignoring the line breaks) does the job:
1077    .sp
1078      (?<DN>Mon|Fri|Sun)(?:day)?|
1079      (?<DN>Tue)(?:sday)?|
1080      (?<DN>Wed)(?:nesday)?|
1081      (?<DN>Thu)(?:rsday)?|
1082      (?<DN>Sat)(?:urday)?
1083    .sp
1084    There are five capturing substrings, but only one is ever set after a match.
1085    (An alternative way of solving this problem is to use a "branch reset"
1086    subpattern, as described in the previous section.)
1087    .P
1088    The convenience function for extracting the data by name returns the substring
1089    for the first (and in this example, the only) subpattern of that name that
1090    matched. This saves searching to find which numbered subpattern it was. If you
1091    make a reference to a non-unique named subpattern from elsewhere in the
1092    pattern, the one that corresponds to the lowest number is used. For further
1093    details of the interfaces for handling named subpatterns, see the
1094  .\" HREF  .\" HREF
1095  \fBpcreapi\fP  \fBpcreapi\fP
1096  .\"  .\"
# Line 884  Repetition is specified by quantifiers, Line 1104  Repetition is specified by quantifiers,
1104  items:  items:
1105  .sp  .sp
1106    a literal data character    a literal data character
1107    the . metacharacter    the dot metacharacter
1108    the \eC escape sequence    the \eC escape sequence
1109    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1110      the \eR escape sequence
1111    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1112    a character class    a character class
1113    a back reference (see next section)    a back reference (see next section)
# Line 924  which may be several bytes long (and the Line 1145  which may be several bytes long (and the
1145  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
1146  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1147  .P  .P
1148  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1149  quantifiers have single-character abbreviations:  abbreviations:
1150  .sp  .sp
1151    *    is equivalent to {0,}    *    is equivalent to {0,}
1152    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 973  own right. Because it has two uses, it c Line 1194  own right. Because it has two uses, it c
1194  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
1195  way the rest of the pattern matches.  way the rest of the pattern matches.
1196  .P  .P
1197  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),
1198  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
1199  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
1200  default behaviour.  default behaviour.
# Line 983  is greater than 1 or with a limited maxi Line 1204  is greater than 1 or with a limited maxi
1204  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1205  .P  .P
1206  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
1207  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
1208  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1209  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
1210  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 1216  alternatively using ^ to indicate anchor
1216  .P  .P
1217  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1218  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1219  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
1220  succeed. Consider, for example:  succeeds. Consider, for example:
1221  .sp  .sp
1222    (.*)abc\e1    (.*)abc\e1
1223  .sp  .sp
# Line 1022  matches "aba" the value of the second ca Line 1243  matches "aba" the value of the second ca
1243  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1244  .rs  .rs
1245  .sp  .sp
1246  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1247  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
1248  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
1249  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
1250  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
1251  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1252  .P  .P
1253  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
1254  .sp  .sp
# Line 1039  item, and then with 4, and so on, before Line 1260  item, and then with 4, and so on, before
1260  (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
1261  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.
1262  .P  .P
1263  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
1264  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
1265  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1266  .sp  .sp
# Line 1071  previous example can be rewritten as Line 1292  previous example can be rewritten as
1292  .sp  .sp
1293  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1294  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
1295  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
1296  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1297  .P  difference; possessive quantifiers should be slightly faster.
1298  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1299  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1300    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1301    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1302    package, and PCRE copied it from there. It ultimately found its way into Perl
1303    at release 5.10.
1304    .P
1305    PCRE has an optimization that automatically "possessifies" certain simple
1306    pattern constructs. For example, the sequence A+B is treated as A++B because
1307    there is no point in backtracking into a sequence of A's when B must follow.
1308  .P  .P
1309  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
1310  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 1346  However, if the decimal number following
1346  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
1347  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
1348  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
1349  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
1350    when a repetition is involved and the subpattern to the right has participated
1351    in an earlier iteration.
1352    .P
1353    It is not possible to have a numerical "forward back reference" to a subpattern
1354    whose number is 10 or more using this syntax because a sequence such as \e50 is
1355    interpreted as a character defined in octal. See the subsection entitled
1356    "Non-printing characters"
1357  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1358  .\" </a>  .\" </a>
1359  above  above
1360  .\"  .\"
1361  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1362    no such problem when named parentheses are used. A back reference to any
1363    subpattern is possible using named parentheses (see below).
1364    .P
1365    Another way of avoiding the ambiguity inherent in the use of digits following a
1366    backslash is to use the \eg escape sequence, which is a feature introduced in
1367    Perl 5.10. This escape must be followed by a positive or a negative number,
1368    optionally enclosed in braces. These examples are all identical:
1369    .sp
1370      (ring), \e1
1371      (ring), \eg1
1372      (ring), \eg{1}
1373    .sp
1374    A positive number specifies an absolute reference without the ambiguity that is
1375    present in the older syntax. It is also useful when literal digits follow the
1376    reference. A negative number is a relative reference. Consider this example:
1377    .sp
1378      (abc(def)ghi)\eg{-1}
1379    .sp
1380    The sequence \eg{-1} is a reference to the most recently started capturing
1381    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1382    would be equivalent to \e1. The use of relative references can be helpful in
1383    long patterns, and also in patterns that are created by joining together
1384    fragments that contain references within themselves.
1385  .P  .P
1386  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1387  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 1403  back reference, the case of letters is r
1403  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
1404  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1405  .P  .P
1406  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1407  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1408  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1409    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1410    references, is also supported. We could rewrite the above example in any of
1411    the following ways:
1412    .sp
1413      (?<p1>(?i)rah)\es+\ek<p1>
1414      (?'p1'(?i)rah)\es+\ek{p1}
1415      (?P<p1>(?i)rah)\es+(?P=p1)
1416      (?<p1>(?i)rah)\es+\eg{p1}
1417  .sp  .sp
1418    A subpattern that is referenced by name may appear in the pattern before or
1419    after the reference.
1420    .P
1421  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
1422  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1423  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 1479  because it does not make sense for negat
1479  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1480  .rs  .rs
1481  .sp  .sp
1482  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1483  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1484  .sp  .sp
1485    \ew+(?=;)    \ew+(?=;)
1486  .sp  .sp
# Line 1246  negative assertions. For example, Line 1515  negative assertions. For example,
1515  .sp  .sp
1516  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
1517  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
1518  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
1519  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1520  .sp  .sp
1521    (?<=bullock|donkey)    (?<=bullock|donkey)
1522  .sp  .sp
# Line 1267  lengths, but it is acceptable if rewritt Line 1536  lengths, but it is acceptable if rewritt
1536  .sp  .sp
1537    (?<=abc|abde)    (?<=abc|abde)
1538  .sp  .sp
1539    In some cases, the Perl 5.10 escape sequence \eK
1540    .\" HTML <a href="#resetmatchstart">
1541    .\" </a>
1542    (see above)
1543    .\"
1544    can be used instead of a lookbehind assertion; this is not restricted to a
1545    fixed-length.
1546    .P
1547  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1548  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
1549  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1550  match is deemed to fail.  assertion fails.
1551  .P  .P
1552  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)
1553  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1554  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
1555  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1556  .P  .P
1557  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1558  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching at the end of the subject string. Consider a simple
1559  such as  pattern such as
1560  .sp  .sp
1561    abcd$    abcd$
1562  .sp  .sp
# Line 1295  then all but the last two characters, an Line 1572  then all but the last two characters, an
1572  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,
1573  if the pattern is written as  if the pattern is written as
1574  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1575    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1576  .sp  .sp
1577  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
1578  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
1579  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1580  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 1614  is another pattern that matches "foo" pr
1614  characters that are not "999".  characters that are not "999".
1615  .  .
1616  .  .
1617    .\" HTML <a name="conditions"></a>
1618  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1619  .rs  .rs
1620  .sp  .sp
# Line 1356  If the condition is satisfied, the yes-p Line 1630  If the condition is satisfied, the yes-p
1630  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
1631  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1632  .P  .P
1633  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1634  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1635  subpattern of that number has previously matched. The number must be greater  .
1636  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1637  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1638  it into three parts for ease of discussion:  .sp
1639    If the text between the parentheses consists of a sequence of digits, the
1640    condition is true if the capturing subpattern of that number has previously
1641    matched. An alternative notation is to precede the digits with a plus or minus
1642    sign. In this case, the subpattern number is relative rather than absolute.
1643    The most recently opened parentheses can be referenced by (?(-1), the next most
1644    recent by (?(-2), and so on. In looping constructs it can also make sense to
1645    refer to subsequent groups with constructs such as (?(+2).
1646    .P
1647    Consider the following pattern, which contains non-significant white space to
1648    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1649    three parts for ease of discussion:
1650  .sp  .sp
1651    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1652  .sp  .sp
# Line 1375  parenthesis is required. Otherwise, sinc Line 1660  parenthesis is required. Otherwise, sinc
1660  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1661  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1662  .P  .P
1663  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
1664  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1665  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1666      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1667    .sp
1668    This makes the fragment independent of the parentheses in the larger pattern.
1669    .
1670    .SS "Checking for a used subpattern by name"
1671    .rs
1672    .sp
1673    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1674    subpattern by name. For compatibility with earlier versions of PCRE, which had
1675    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1676    there is a possible ambiguity with this syntax, because subpattern names may
1677    consist entirely of digits. PCRE looks first for a named subpattern; if it
1678    cannot find one and the name consists entirely of digits, PCRE looks for a
1679    subpattern of that number, which must be greater than zero. Using subpattern
1680    names that consist entirely of digits is not recommended.
1681  .P  .P
1682  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:
1683    .sp
1684      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1685    .sp
1686    .
1687    .SS "Checking for pattern recursion"
1688    .rs
1689    .sp
1690    If the condition is the string (R), and there is no subpattern with the name R,
1691    the condition is true if a recursive call to the whole pattern or any
1692    subpattern has been made. If digits or a name preceded by ampersand follow the
1693    letter R, for example:
1694    .sp
1695      (?(R3)...) or (?(R&name)...)
1696    .sp
1697    the condition is true if the most recent recursion is into the subpattern whose
1698    number or name is given. This condition does not check the entire recursion
1699    stack.
1700    .P
1701    At "top level", all these recursion test conditions are false. Recursive
1702    patterns are described below.
1703    .
1704    .SS "Defining subpatterns for use by reference only"
1705    .rs
1706    .sp
1707    If the condition is the string (DEFINE), and there is no subpattern with the
1708    name DEFINE, the condition is always false. In this case, there may be only one
1709    alternative in the subpattern. It is always skipped if control reaches this
1710    point in the pattern; the idea of DEFINE is that it can be used to define
1711    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1712    is described below.) For example, a pattern to match an IPv4 address could be
1713    written like this (ignore whitespace and line breaks):
1714    .sp
1715      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1716      \eb (?&byte) (\e.(?&byte)){3} \eb
1717    .sp
1718    The first part of the pattern is a DEFINE group inside which a another group
1719    named "byte" is defined. This matches an individual component of an IPv4
1720    address (a number less than 256). When matching takes place, this part of the
1721    pattern is skipped because DEFINE acts like a false condition.
1722    .P
1723    The rest of the pattern uses references to the named group to match the four
1724    dot-separated components of an IPv4 address, insisting on a word boundary at
1725    each end.
1726    .
1727    .SS "Assertion conditions"
1728    .rs
1729    .sp
1730    If the condition is not in any of the above formats, it must be an assertion.
1731  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1732  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1733  alternatives on the second line:  alternatives on the second line:
# Line 1404  closing parenthesis. Nested parentheses Line 1752  closing parenthesis. Nested parentheses
1752  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.
1753  .P  .P
1754  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1755  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1756  character in the pattern.  next newline in the pattern.
1757  .  .
1758  .  .
1759    .\" HTML <a name="recursion"></a>
1760  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
1761  .rs  .rs
1762  .sp  .sp
1763  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1764  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1765  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
1766  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1767  that allows regular expressions to recurse (amongst other things). It does this  .P
1768  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
1769  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
1770  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1771    pattern using code interpolation to solve the parentheses problem can be
1772    created like this:
1773  .sp  .sp
1774    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1775  .sp  .sp
1776  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
1777  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1778  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1779  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1780  .P  supports special syntax for recursion of the entire pattern, and also for
1781  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1782  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1783  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1784  "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
1785  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1786  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1787  A recursive subpattern call is always treated as an atomic group. That is, once  call, which is described in the next section.) The special item (?R) or (?0) is
1788  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1789  it contains untried alternatives and there is a subsequent matching failure.  .P
1790    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1791    treated as an atomic group. That is, once it has matched some of the subject
1792    string, it is never re-entered, even if it contains untried alternatives and
1793    there is a subsequent matching failure.
1794  .P  .P
1795  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1796  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 1808  pattern, so instead you could use this:
1808    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1809  .sp  .sp
1810  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
1811  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1812  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1813  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
1814  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.)
1815  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
1816  .sp  most recently opened parentheses preceding the recursion. In other words, a
1817    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1818  .sp  it is encountered.
1819  This particular example pattern contains nested unlimited repeats, and so the  .P
1820  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1821  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
1822  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1823    "subroutine" calls, as described in the next section.
1824    .P
1825    An alternative approach is to use named parentheses instead. The Perl syntax
1826    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1827    could rewrite the above example as follows:
1828    .sp
1829      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1830    .sp
1831    If there is more than one subpattern with the same name, the earliest one is
1832    used.
1833    .P
1834    This particular example pattern that we have been looking at contains nested
1835    unlimited repeats, and so the use of atomic grouping for matching strings of
1836    non-parentheses is important when applying the pattern to strings that do not
1837    match. For example, when this pattern is applied to
1838  .sp  .sp
1839    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1840  .sp  .sp
# Line 1476  before failure can be reported. Line 1846  before failure can be reported.
1846  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
1847  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.
1848  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
1849  the next section and the  below and the
1850  .\" HREF  .\" HREF
1851  \fBpcrecallout\fP  \fBpcrecallout\fP
1852  .\"  .\"
# Line 1515  is the actual recursive call. Line 1885  is the actual recursive call.
1885  .sp  .sp
1886  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
1887  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
1888  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1889  pattern  before or after the reference. A numbered reference can be absolute or
1890    relative, as in these examples:
1891    .sp
1892      (...(absolute)...)...(?2)...
1893      (...(relative)...)...(?-1)...
1894      (...(?+1)...(relative)...
1895    .sp
1896    An earlier example pointed out that the pattern
1897  .sp  .sp
1898    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1899  .sp  .sp
# Line 1526  matches "sense and sensibility" and "res Line 1903  matches "sense and sensibility" and "res
1903    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1904  .sp  .sp
1905  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
1906  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
 refer.  
1907  .P  .P
1908  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1909  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
1910  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
1911  matching failure.  matching failure.
1912    .P
1913    When a subpattern is used as a subroutine, processing options such as
1914    case-independence are fixed when the subpattern is defined. They cannot be
1915    changed for different calls. For example, consider this pattern:
1916    .sp
1917      (abc)(?i:(?-1))
1918    .sp
1919    It matches "abcabc". It does not match "abcABC" because the change of
1920    processing option does not affect the called subpattern.
1921  .  .
1922  .  .
1923  .SH CALLOUTS  .SH CALLOUTS
# Line 1553  function is to be called. If you want to Line 1938  function is to be called. If you want to
1938  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.
1939  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1940  .sp  .sp
1941    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1942  .sp  .sp
1943  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
1944  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 1954  description of the interface to the call
1954  \fBpcrecallout\fP  \fBpcrecallout\fP
1955  .\"  .\"
1956  documentation.  documentation.
1957  .P  .
1958  .in 0  .
1959  Last updated: 24 January 2006  .SH "SEE ALSO"
1960  .br  .rs
1961  Copyright (c) 1997-2006 University of Cambridge.  .sp
1962    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1963    .
1964    .
1965    .SH AUTHOR
1966    .rs
1967    .sp
1968    .nf
1969    Philip Hazel
1970    University Computing Service
1971    Cambridge CB2 3QH, England.
1972    .fi
1973    .
1974    .
1975    .SH REVISION
1976    .rs
1977    .sp
1978    .nf
1979    Last updated: 19 June 2007
1980    Copyright (c) 1997-2007 University of Cambridge.
1981    .fi

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