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revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC revision 172 by ph10, Tue Jun 5 10:40:13 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
# Line 253  there is no character to match. Line 277  there is no character to match.
277  .P  .P
278  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).
279  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
280  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
281    included in a Perl script, \es may match the VT character. In PCRE, it never
282    does.)
283  .P  .P
284  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
285  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 293  in the
293  .\" HREF  .\" HREF
294  \fBpcreapi\fP  \fBpcreapi\fP
295  .\"  .\"
296  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,
297  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
298    accented letters, and these are matched by \ew.
299  .P  .P
300  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
301  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
# Line 276  character property support is available. Line 303  character property support is available.
303  discouraged.  discouraged.
304  .  .
305  .  .
306    .SS "Newline sequences"
307    .rs
308    .sp
309    Outside a character class, the escape sequence \eR matches any Unicode newline
310    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
311    the following:
312    .sp
313      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
314    .sp
315    This is an example of an "atomic group", details of which are given
316    .\" HTML <a href="#atomicgroup">
317    .\" </a>
318    below.
319    .\"
320    This particular group matches either the two-character sequence CR followed by
321    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
322    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
323    line, U+0085). The two-character sequence is treated as a single unit that
324    cannot be split.
325    .P
326    In UTF-8 mode, two additional characters whose codepoints are greater than 255
327    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
328    Unicode character property support is not needed for these characters to be
329    recognized.
330    .P
331    Inside a character class, \eR matches the letter "R".
332    .
333    .
334  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
335  .SS Unicode character properties  .SS Unicode character properties
336  .rs  .rs
# Line 306  Those that are not part of an identified Line 361  Those that are not part of an identified
361  .P  .P
362  Arabic,  Arabic,
363  Armenian,  Armenian,
364    Balinese,
365  Bengali,  Bengali,
366  Bopomofo,  Bopomofo,
367  Braille,  Braille,
# Line 315  Canadian_Aboriginal, Line 371  Canadian_Aboriginal,
371  Cherokee,  Cherokee,
372  Common,  Common,
373  Coptic,  Coptic,
374    Cuneiform,
375  Cypriot,  Cypriot,
376  Cyrillic,  Cyrillic,
377  Deseret,  Deseret,
# Line 344  Malayalam, Line 401  Malayalam,
401  Mongolian,  Mongolian,
402  Myanmar,  Myanmar,
403  New_Tai_Lue,  New_Tai_Lue,
404    Nko,
405  Ogham,  Ogham,
406  Old_Italic,  Old_Italic,
407  Old_Persian,  Old_Persian,
408  Oriya,  Oriya,
409  Osmanya,  Osmanya,
410    Phags_Pa,
411    Phoenician,
412  Runic,  Runic,
413  Shavian,  Shavian,
414  Sinhala,  Sinhala,
# Line 430  the Lu, Ll, or Lt property, in other wor Line 490  the Lu, Ll, or Lt property, in other wor
490  a modifier or "other".  a modifier or "other".
491  .P  .P
492  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})
493  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
494  properties with "Is".  properties with "Is".
495  .P  .P
496  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 461  why the traditional escape sequences suc Line 521  why the traditional escape sequences suc
521  properties in PCRE.  properties in PCRE.
522  .  .
523  .  .
524    .\" HTML <a name="resetmatchstart"></a>
525    .SS "Resetting the match start"
526    .rs
527    .sp
528    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
529    matched characters not to be included in the final matched sequence. For
530    example, the pattern:
531    .sp
532      foo\eKbar
533    .sp
534    matches "foobar", but reports that it has matched "bar". This feature is
535    similar to a lookbehind assertion
536    .\" HTML <a href="#lookbehind">
537    .\" </a>
538    (described below).
539    .\"
540    However, in this case, the part of the subject before the real match does not
541    have to be of fixed length, as lookbehind assertions do. The use of \eK does
542    not interfere with the setting of
543    .\" HTML <a href="#subpattern">
544    .\" </a>
545    captured substrings.
546    .\"
547    For example, when the pattern
548    .sp
549      (foo)\eKbar
550    .sp
551    matches "foobar", the first substring is still set to "foo".
552    .
553    .
554  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
555  .SS "Simple assertions"  .SS "Simple assertions"
556  .rs  .rs
557  .sp  .sp
558  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
559  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,
560  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
561  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 473  subpatterns for more complicated asserti Line 563  subpatterns for more complicated asserti
563  .\" </a>  .\" </a>
564  below.  below.
565  .\"  .\"
566  The backslashed  The backslashed assertions are:
 assertions are:  
567  .sp  .sp
568    \eb     matches at a word boundary    \eb     matches at a word boundary
569    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
570    \eA     matches at start of subject    \eA     matches at the start of the subject
571    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
572    \ez     matches at end of subject            also matches before a newline at the end of the subject
573    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
574      \eG     matches at the first matching position in the subject
575  .sp  .sp
576  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
577  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 589  PCRE_NOTBOL or PCRE_NOTEOL options, whic
589  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
590  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
591  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
592  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
593  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.  
594  .P  .P
595  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
596  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 634  to be anchored.)
634  .P  .P
635  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
636  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
637  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
638  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
639  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
640  Dollar has no special meaning in a character class.  character class.
641  .P  .P
642  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
643  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
644  does not affect the \eZ assertion.  does not affect the \eZ assertion.
645  .P  .P
646  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
647  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
648  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
649  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
650  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
651  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
652  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
653  branches start with ^ are not anchored in multiline mode, and a match for  .P
654  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
655  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,
656  set.  patterns that are anchored in single line mode because all branches start with
657    ^ are not anchored in multiline mode, and a match for circumflex is possible
658    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
659    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
660  .P  .P
661  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
662  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
663  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
664  .  .
665  .  .
666  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
667  .rs  .rs
668  .sp  .sp
669  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
670  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
671  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.
672  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
673  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
674  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
675  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
676    (including isolated CRs and LFs). When any Unicode line endings are being
677    recognized, dot does not match CR or LF or any of the other line ending
678    characters.
679    .P
680    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
681    option is set, a dot matches any one character, without exception. If the
682    two-character sequence CRLF is present in the subject string, it takes two dots
683    to match it.
684    .P
685    The handling of dot is entirely independent of the handling of circumflex and
686    dollar, the only relationship being that they both involve newlines. Dot has no
687    special meaning in a character class.
688  .  .
689  .  .
690  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
691  .rs  .rs
692  .sp  .sp
693  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
694  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
695  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
696  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,
697  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,
698  best avoided.  the \eC escape sequence is best avoided.
699  .P  .P
700  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
701  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 642  If you want to use caseless matching for Line 746  If you want to use caseless matching for
746  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
747  UTF-8 support.  UTF-8 support.
748  .P  .P
749  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
750  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
751  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
752    such as [^a] always matches one of these characters.
753  .P  .P
754  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
755  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 773  example [\ex{100}-\ex{2ff}].
773  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
774  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
775  [][\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
776  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
777  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
778  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
779  property support.  property support.
# Line 743  the pattern Line 848  the pattern
848    gilbert|sullivan    gilbert|sullivan
849  .sp  .sp
850  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
851  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
852  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
853  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  
854  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
855  .\" </a>  .\" </a>
856  (defined below),  (defined below),
# Line 780  If the change is placed right at the sta Line 884  If the change is placed right at the sta
884  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
885  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
886  .P  .P
887  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
888  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
889  .sp  .sp
890    (a(?i)b)c    (a(?i)b)c
891  .sp  .sp
# Line 797  branch is abandoned before the option se Line 901  branch is abandoned before the option se
901  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
902  behaviour otherwise.  behaviour otherwise.
903  .P  .P
904  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
905  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
906  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.  
907  .  .
908  .  .
909  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 816  Turning part of a pattern into a subpatt Line 918  Turning part of a pattern into a subpatt
918    cat(aract|erpillar|)    cat(aract|erpillar|)
919  .sp  .sp
920  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
921  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
922  .sp  .sp
923  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
924  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 943  the string "the white queen" is matched
943    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
944  .sp  .sp
945  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
946  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.  
947  .P  .P
948  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
949  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 863  the above patterns match "SUNDAY" as wel Line 964  the above patterns match "SUNDAY" as wel
964  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
965  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
966  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
967  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
968  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
969  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
970  .P  the Perl and the Python syntax.
971  Named capturing parentheses are still allocated numbers as well as names. The  .P
972  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
973  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
974  extracting a captured substring by name. For further details see the  parentheses from other parts of the pattern, such as
975    .\" HTML <a href="#backreferences">
976    .\" </a>
977    backreferences,
978    .\"
979    .\" HTML <a href="#recursion">
980    .\" </a>
981    recursion,
982    .\"
983    and
984    .\" HTML <a href="#conditions">
985    .\" </a>
986    conditions,
987    .\"
988    can be made by name as well as by number.
989    .P
990    Names consist of up to 32 alphanumeric characters and underscores. Named
991    capturing parentheses are still allocated numbers as well as names, exactly as
992    if the names were not present. The PCRE API provides function calls for
993    extracting the name-to-number translation table from a compiled pattern. There
994    is also a convenience function for extracting a captured substring by name.
995    .P
996    By default, a name must be unique within a pattern, but it is possible to relax
997    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
998    be useful for patterns where only one instance of the named parentheses can
999    match. Suppose you want to match the name of a weekday, either as a 3-letter
1000    abbreviation or as the full name, and in both cases you want to extract the
1001    abbreviation. This pattern (ignoring the line breaks) does the job:
1002    .sp
1003      (?<DN>Mon|Fri|Sun)(?:day)?|
1004      (?<DN>Tue)(?:sday)?|
1005      (?<DN>Wed)(?:nesday)?|
1006      (?<DN>Thu)(?:rsday)?|
1007      (?<DN>Sat)(?:urday)?
1008    .sp
1009    There are five capturing substrings, but only one is ever set after a match.
1010    The convenience function for extracting the data by name returns the substring
1011    for the first (and in this example, the only) subpattern of that name that
1012    matched. This saves searching to find which numbered subpattern it was. If you
1013    make a reference to a non-unique named subpattern from elsewhere in the
1014    pattern, the one that corresponds to the lowest number is used. For further
1015    details of the interfaces for handling named subpatterns, see the
1016  .\" HREF  .\" HREF
1017  \fBpcreapi\fP  \fBpcreapi\fP
1018  .\"  .\"
# Line 884  Repetition is specified by quantifiers, Line 1026  Repetition is specified by quantifiers,
1026  items:  items:
1027  .sp  .sp
1028    a literal data character    a literal data character
1029    the . metacharacter    the dot metacharacter
1030    the \eC escape sequence    the \eC escape sequence
1031    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1032      the \eR escape sequence
1033    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1034    a character class    a character class
1035    a back reference (see next section)    a back reference (see next section)
# Line 924  which may be several bytes long (and the Line 1067  which may be several bytes long (and the
1067  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
1068  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1069  .P  .P
1070  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1071  quantifiers have single-character abbreviations:  abbreviations:
1072  .sp  .sp
1073    *    is equivalent to {0,}    *    is equivalent to {0,}
1074    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 973  own right. Because it has two uses, it c Line 1116  own right. Because it has two uses, it c
1116  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
1117  way the rest of the pattern matches.  way the rest of the pattern matches.
1118  .P  .P
1119  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),
1120  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
1121  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
1122  default behaviour.  default behaviour.
# Line 983  is greater than 1 or with a limited maxi Line 1126  is greater than 1 or with a limited maxi
1126  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1127  .P  .P
1128  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
1129  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
1130  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1131  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
1132  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 1138  alternatively using ^ to indicate anchor
1138  .P  .P
1139  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1140  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1141  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
1142  succeed. Consider, for example:  succeeds. Consider, for example:
1143  .sp  .sp
1144    (.*)abc\e1    (.*)abc\e1
1145  .sp  .sp
# Line 1022  matches "aba" the value of the second ca Line 1165  matches "aba" the value of the second ca
1165  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1166  .rs  .rs
1167  .sp  .sp
1168  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1169  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
1170  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
1171  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
1172  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
1173  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1174  .P  .P
1175  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
1176  .sp  .sp
# Line 1039  item, and then with 4, and so on, before Line 1182  item, and then with 4, and so on, before
1182  (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
1183  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.
1184  .P  .P
1185  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
1186  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
1187  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1188  .sp  .sp
# Line 1071  previous example can be rewritten as Line 1214  previous example can be rewritten as
1214  .sp  .sp
1215  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1216  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
1217  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
1218  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1219  .P  difference; possessive quantifiers should be slightly faster.
1220  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1221  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1222    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1223    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1224    package, and PCRE copied it from there. It ultimately found its way into Perl
1225    at release 5.10.
1226    .P
1227    PCRE has an optimization that automatically "possessifies" certain simple
1228    pattern constructs. For example, the sequence A+B is treated as A++B because
1229    there is no point in backtracking into a sequence of A's when B must follow.
1230  .P  .P
1231  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
1232  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 1268  However, if the decimal number following
1268  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
1269  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
1270  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
1271  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
1272    when a repetition is involved and the subpattern to the right has participated
1273    in an earlier iteration.
1274    .P
1275    It is not possible to have a numerical "forward back reference" to a subpattern
1276    whose number is 10 or more using this syntax because a sequence such as \e50 is
1277    interpreted as a character defined in octal. See the subsection entitled
1278    "Non-printing characters"
1279  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1280  .\" </a>  .\" </a>
1281  above  above
1282  .\"  .\"
1283  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1284    no such problem when named parentheses are used. A back reference to any
1285    subpattern is possible using named parentheses (see below).
1286    .P
1287    Another way of avoiding the ambiguity inherent in the use of digits following a
1288    backslash is to use the \eg escape sequence, which is a feature introduced in
1289    Perl 5.10. This escape must be followed by a positive or a negative number,
1290    optionally enclosed in braces. These examples are all identical:
1291    .sp
1292      (ring), \e1
1293      (ring), \eg1
1294      (ring), \eg{1}
1295    .sp
1296    A positive number specifies an absolute reference without the ambiguity that is
1297    present in the older syntax. It is also useful when literal digits follow the
1298    reference. A negative number is a relative reference. Consider this example:
1299    .sp
1300      (abc(def)ghi)\eg{-1}
1301    .sp
1302    The sequence \eg{-1} is a reference to the most recently started capturing
1303    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1304    would be equivalent to \e1. The use of relative references can be helpful in
1305    long patterns, and also in patterns that are created by joining together
1306    fragments that contain references within themselves.
1307  .P  .P
1308  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1309  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 1325  back reference, the case of letters is r
1325  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
1326  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1327  .P  .P
1328  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1329  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1330  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1331    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1332    references, is also supported. We could rewrite the above example in any of
1333    the following ways:
1334    .sp
1335      (?<p1>(?i)rah)\es+\ek<p1>
1336      (?'p1'(?i)rah)\es+\ek{p1}
1337      (?P<p1>(?i)rah)\es+(?P=p1)
1338      (?<p1>(?i)rah)\es+\eg{p1}
1339  .sp  .sp
1340    A subpattern that is referenced by name may appear in the pattern before or
1341    after the reference.
1342    .P
1343  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
1344  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1345  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 1401  because it does not make sense for negat
1401  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1402  .rs  .rs
1403  .sp  .sp
1404  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1405  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1406  .sp  .sp
1407    \ew+(?=;)    \ew+(?=;)
1408  .sp  .sp
# Line 1246  negative assertions. For example, Line 1437  negative assertions. For example,
1437  .sp  .sp
1438  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
1439  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
1440  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
1441  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1442  .sp  .sp
1443    (?<=bullock|donkey)    (?<=bullock|donkey)
1444  .sp  .sp
# Line 1267  lengths, but it is acceptable if rewritt Line 1458  lengths, but it is acceptable if rewritt
1458  .sp  .sp
1459    (?<=abc|abde)    (?<=abc|abde)
1460  .sp  .sp
1461    In some cases, the Perl 5.10 escape sequence \eK
1462    .\" HTML <a href="#resetmatchstart">
1463    .\" </a>
1464    (see above)
1465    .\"
1466    can be used instead of a lookbehind assertion; this is not restricted to a
1467    fixed-length.
1468    .P
1469  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1470  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
1471  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1472  match is deemed to fail.  assertion fails.
1473  .P  .P
1474  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)
1475  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1476  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
1477  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1478  .P  .P
1479  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1480  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
1481  such as  pattern such as
1482  .sp  .sp
1483    abcd$    abcd$
1484  .sp  .sp
# Line 1295  then all but the last two characters, an Line 1494  then all but the last two characters, an
1494  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,
1495  if the pattern is written as  if the pattern is written as
1496  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1497    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1498  .sp  .sp
1499  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
1500  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
1501  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1502  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 1536  is another pattern that matches "foo" pr
1536  characters that are not "999".  characters that are not "999".
1537  .  .
1538  .  .
1539    .\" HTML <a name="conditions"></a>
1540  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1541  .rs  .rs
1542  .sp  .sp
# Line 1356  If the condition is satisfied, the yes-p Line 1552  If the condition is satisfied, the yes-p
1552  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
1553  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1554  .P  .P
1555  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1556  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1557  subpattern of that number has previously matched. The number must be greater  .
1558  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1559  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1560  it into three parts for ease of discussion:  .sp
1561    If the text between the parentheses consists of a sequence of digits, the
1562    condition is true if the capturing subpattern of that number has previously
1563    matched. An alternative notation is to precede the digits with a plus or minus
1564    sign. In this case, the subpattern number is relative rather than absolute.
1565    The most recently opened parentheses can be referenced by (?(-1), the next most
1566    recent by (?(-2), and so on. In looping constructs it can also make sense to
1567    refer to subsequent groups with constructs such as (?(+2).
1568    .P
1569    Consider the following pattern, which contains non-significant white space to
1570    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1571    three parts for ease of discussion:
1572  .sp  .sp
1573    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1574  .sp  .sp
# Line 1375  parenthesis is required. Otherwise, sinc Line 1582  parenthesis is required. Otherwise, sinc
1582  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1583  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1584  .P  .P
1585  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
1586  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1587  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1588      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1589    .sp
1590    This makes the fragment independent of the parentheses in the larger pattern.
1591    .
1592    .SS "Checking for a used subpattern by name"
1593    .rs
1594    .sp
1595    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1596    subpattern by name. For compatibility with earlier versions of PCRE, which had
1597    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1598    there is a possible ambiguity with this syntax, because subpattern names may
1599    consist entirely of digits. PCRE looks first for a named subpattern; if it
1600    cannot find one and the name consists entirely of digits, PCRE looks for a
1601    subpattern of that number, which must be greater than zero. Using subpattern
1602    names that consist entirely of digits is not recommended.
1603    .P
1604    Rewriting the above example to use a named subpattern gives this:
1605    .sp
1606      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1607    .sp
1608    .
1609    .SS "Checking for pattern recursion"
1610    .rs
1611    .sp
1612    If the condition is the string (R), and there is no subpattern with the name R,
1613    the condition is true if a recursive call to the whole pattern or any
1614    subpattern has been made. If digits or a name preceded by ampersand follow the
1615    letter R, for example:
1616    .sp
1617      (?(R3)...) or (?(R&name)...)
1618    .sp
1619    the condition is true if the most recent recursion is into the subpattern whose
1620    number or name is given. This condition does not check the entire recursion
1621    stack.
1622    .P
1623    At "top level", all these recursion test conditions are false. Recursive
1624    patterns are described below.
1625    .
1626    .SS "Defining subpatterns for use by reference only"
1627    .rs
1628    .sp
1629    If the condition is the string (DEFINE), and there is no subpattern with the
1630    name DEFINE, the condition is always false. In this case, there may be only one
1631    alternative in the subpattern. It is always skipped if control reaches this
1632    point in the pattern; the idea of DEFINE is that it can be used to define
1633    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1634    is described below.) For example, a pattern to match an IPv4 address could be
1635    written like this (ignore whitespace and line breaks):
1636    .sp
1637      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1638      \eb (?&byte) (\e.(?&byte)){3} \eb
1639    .sp
1640    The first part of the pattern is a DEFINE group inside which a another group
1641    named "byte" is defined. This matches an individual component of an IPv4
1642    address (a number less than 256). When matching takes place, this part of the
1643    pattern is skipped because DEFINE acts like a false condition.
1644  .P  .P
1645  If the condition is not a sequence of digits or (R), it must be an assertion.  The rest of the pattern uses references to the named group to match the four
1646    dot-separated components of an IPv4 address, insisting on a word boundary at
1647    each end.
1648    .
1649    .SS "Assertion conditions"
1650    .rs
1651    .sp
1652    If the condition is not in any of the above formats, it must be an assertion.
1653  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1654  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1655  alternatives on the second line:  alternatives on the second line:
# Line 1404  closing parenthesis. Nested parentheses Line 1674  closing parenthesis. Nested parentheses
1674  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.
1675  .P  .P
1676  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1677  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1678  character in the pattern.  next newline in the pattern.
1679  .  .
1680  .  .
1681    .\" HTML <a name="recursion"></a>
1682  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
1683  .rs  .rs
1684  .sp  .sp
1685  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1686  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1687  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
1688  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1689  that allows regular expressions to recurse (amongst other things). It does this  .P
1690  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
1691  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
1692  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1693    pattern using code interpolation to solve the parentheses problem can be
1694    created like this:
1695  .sp  .sp
1696    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1697  .sp  .sp
1698  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
1699  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1700  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1701  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1702  .P  supports special syntax for recursion of the entire pattern, and also for
1703  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1704  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.
1705  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1706  "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
1707  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1708  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1709  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
1710  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1711  it contains untried alternatives and there is a subsequent matching failure.  .P
1712    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1713    treated as an atomic group. That is, once it has matched some of the subject
1714    string, it is never re-entered, even if it contains untried alternatives and
1715    there is a subsequent matching failure.
1716  .P  .P
1717  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1718  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 1730  pattern, so instead you could use this:
1730    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1731  .sp  .sp
1732  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
1733  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1734  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1735  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
1736  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.)
1737  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
1738  .sp  most recently opened parentheses preceding the recursion. In other words, a
1739    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1740  .sp  it is encountered.
1741  This particular example pattern contains nested unlimited repeats, and so the  .P
1742  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1743  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
1744  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1745    "subroutine" calls, as described in the next section.
1746    .P
1747    An alternative approach is to use named parentheses instead. The Perl syntax
1748    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1749    could rewrite the above example as follows:
1750    .sp
1751      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1752    .sp
1753    If there is more than one subpattern with the same name, the earliest one is
1754    used.
1755    .P
1756    This particular example pattern that we have been looking at contains nested
1757    unlimited repeats, and so the use of atomic grouping for matching strings of
1758    non-parentheses is important when applying the pattern to strings that do not
1759    match. For example, when this pattern is applied to
1760  .sp  .sp
1761    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1762  .sp  .sp
# Line 1476  before failure can be reported. Line 1768  before failure can be reported.
1768  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
1769  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.
1770  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
1771  the next section and the  below and the
1772  .\" HREF  .\" HREF
1773  \fBpcrecallout\fP  \fBpcrecallout\fP
1774  .\"  .\"
# Line 1515  is the actual recursive call. Line 1807  is the actual recursive call.
1807  .sp  .sp
1808  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
1809  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
1810  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1811  pattern  before or after the reference. A numbered reference can be absolute or
1812    relative, as in these examples:
1813    .sp
1814      (...(absolute)...)...(?2)...
1815      (...(relative)...)...(?-1)...
1816      (...(?+1)...(relative)...
1817    .sp
1818    An earlier example pointed out that the pattern
1819  .sp  .sp
1820    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1821  .sp  .sp
# Line 1526  matches "sense and sensibility" and "res Line 1825  matches "sense and sensibility" and "res
1825    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1826  .sp  .sp
1827  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
1828  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
 refer.  
1829  .P  .P
1830  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1831  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
1832  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
1833  matching failure.  matching failure.
1834    .P
1835    When a subpattern is used as a subroutine, processing options such as
1836    case-independence are fixed when the subpattern is defined. They cannot be
1837    changed for different calls. For example, consider this pattern:
1838    .sp
1839      (abc)(?i:(?-1))
1840    .sp
1841    It matches "abcabc". It does not match "abcABC" because the change of
1842    processing option does not affect the called subpattern.
1843  .  .
1844  .  .
1845  .SH CALLOUTS  .SH CALLOUTS
# Line 1553  function is to be called. If you want to Line 1860  function is to be called. If you want to
1860  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.
1861  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1862  .sp  .sp
1863    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1864  .sp  .sp
1865  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
1866  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 1876  description of the interface to the call
1876  \fBpcrecallout\fP  \fBpcrecallout\fP
1877  .\"  .\"
1878  documentation.  documentation.
1879  .P  .
1880  .in 0  .
1881  Last updated: 24 January 2006  .SH "SEE ALSO"
1882  .br  .rs
1883  Copyright (c) 1997-2006 University of Cambridge.  .sp
1884    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1885    .
1886    .
1887    .SH AUTHOR
1888    .rs
1889    .sp
1890    .nf
1891    Philip Hazel
1892    University Computing Service
1893    Cambridge CB2 3QH, England.
1894    .fi
1895    .
1896    .
1897    .SH REVISION
1898    .rs
1899    .sp
1900    .nf
1901    Last updated: 29 May 2007
1902    Copyright (c) 1997-2007 University of Cambridge.
1903    .fi

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