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revision 79 by nigel, Sat Feb 24 21:40:52 2007 UTC revision 93 by nigel, Sat Feb 24 21:41:42 2007 UTC
# Line 36  and how it differs from the normal funct Line 36  and how it differs from the normal funct
36  \fBpcrematching\fP  \fBpcrematching\fP
37  .\"  .\"
38  page.  page.
39  .P  .
40    .
41    .SH "CHARACTERS AND METACHARACTERS"
42    .rs
43    .sp
44  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
45  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
46  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 64  interpreted in some special way.
64  .P  .P
65  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
66  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
67  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
68  as follows:  are as follows:
69  .sp  .sp
70    \e      general escape character with several uses    \e      general escape character with several uses
71    ^      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 96  a character class the only metacharacter
96  .sp  .sp
97  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
98  .  .
99    .
100  .SH BACKSLASH  .SH BACKSLASH
101  .rs  .rs
102  .sp  .sp
103  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
104  non-alphanumeric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
105  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
106  outside character classes.  outside character classes.
107  .P  .P
108  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 113  particular, if you want to match a backs
113  .P  .P
114  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
115  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
116  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
117  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.  
118  .P  .P
119  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
120  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 148  represents: Line 152  represents:
152    \et        tab (hex 09)    \et        tab (hex 09)
153    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
154    \exhh      character with hex code hh    \exhh      character with hex code hh
155    \ex{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
156  .sp  .sp
157  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
158  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
# Line 156  Thus \ecz becomes hex 1A, but \ec{ becom Line 160  Thus \ecz becomes hex 1A, but \ec{ becom
160  7B.  7B.
161  .P  .P
162  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
163  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
164  appear between \ex{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
165  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
166  other than hexadecimal digits appear between \ex{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
167  terminating }, this form of escape is not recognized. Instead, the initial  and }, or if there is no terminating }, this form of escape is not recognized.
168  \ex will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
169  digits, giving a character whose value is zero.  with no following digits, giving a character whose value is zero.
170  .P  .P
171  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
172  syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
173  way they are handled. For example, \exdc is exactly the same as \ex{dc}.  example, \exdc is exactly the same as \ex{dc}.
174  .P  .P
175  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
176  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
177  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
178  (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
179  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
180  .P  .P
181  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.
182  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 195  parenthesized subpatterns.
195  .P  .P
196  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
197  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
198  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
199  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
200  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
201    to \e777 are permitted. For example:
202  .sp  .sp
203    \e040   is another way of writing a space    \e040   is another way of writing a space
204  .\" JOIN  .\" JOIN
# Line 218  For example: Line 223  For example:
223  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
224  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
225  .P  .P
226  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
227  (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
228  addition, inside a character class, the sequence \eb is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
229  backspace character (hex 08), and the sequence \eX is interpreted as the  sequences \eR and \eX are interpreted as the characters "R" and "X",
230  character "X". Outside a character class, these sequences have different  respectively. Outside a character class, these sequences have different
231  meanings  meanings
232  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
233  .\" </a>  .\" </a>
# Line 230  meanings Line 235  meanings
235  .\"  .\"
236  .  .
237  .  .
238    .SS "Absolute and relative back references"
239    .rs
240    .sp
241    The sequence \eg followed by a positive or negative number, optionally enclosed
242    in braces, is an absolute or relative back reference. Back references are
243    discussed
244    .\" HTML <a href="#backreferences">
245    .\" </a>
246    later,
247    .\"
248    following the discussion of
249    .\" HTML <a href="#subpattern">
250    .\" </a>
251    parenthesized subpatterns.
252    .\"
253    .
254    .
255  .SS "Generic character types"  .SS "Generic character types"
256  .rs  .rs
257  .sp  .sp
258  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
259  following are always recognized:  following are always recognized:
260  .sp  .sp
261    \ed     any decimal digit    \ed     any decimal digit
# Line 253  there is no character to match. Line 275  there is no character to match.
275  .P  .P
276  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).
277  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
278  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
279    included in a Perl script, \es may match the VT character. In PCRE, it never
280    does.)
281  .P  .P
282  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
283  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 272  greater than 128 are used for accented l Line 296  greater than 128 are used for accented l
296  .P  .P
297  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
298  \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
299  character property support is available.  character property support is available. The use of locales with Unicode is
300    discouraged.
301    .
302    .
303    .SS "Newline sequences"
304    .rs
305    .sp
306    Outside a character class, the escape sequence \eR matches any Unicode newline
307    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
308    the following:
309    .sp
310      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
311    .sp
312    This is an example of an "atomic group", details of which are given
313    .\" HTML <a href="#atomicgroup">
314    .\" </a>
315    below.
316    .\"
317    This particular group matches either the two-character sequence CR followed by
318    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
319    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
320    line, U+0085). The two-character sequence is treated as a single unit that
321    cannot be split.
322    .P
323    In UTF-8 mode, two additional characters whose codepoints are greater than 255
324    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
325    Unicode character property support is not needed for these characters to be
326    recognized.
327    .P
328    Inside a character class, \eR matches the letter "R".
329  .  .
330  .  .
331  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 280  character property support is available. Line 333  character property support is available.
333  .rs  .rs
334  .sp  .sp
335  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
336  escape sequences to match generic character types are available when UTF-8 mode  escape sequences to match character properties are available when UTF-8 mode
337  is selected. They are:  is selected. They are:
338  .sp  .sp
339   \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
340   \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
341   \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
342  .sp  .sp
343  The property names represented by \fIxx\fP above are limited to the  The property names represented by \fIxx\fP above are limited to the Unicode
344  Unicode general category properties. Each character has exactly one such  script names, the general category properties, and "Any", which matches any
345  property, specified by a two-letter abbreviation. For compatibility with Perl,  character (including newline). Other properties such as "InMusicalSymbols" are
346  negation can be specified by including a circumflex between the opening brace  not currently supported by PCRE. Note that \eP{Any} does not match any
347  and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.  characters, so always causes a match failure.
348  .P  .P
349  If only one letter is specified with \ep or \eP, it includes all the properties  Sets of Unicode characters are defined as belonging to certain scripts. A
350  that start with that letter. In this case, in the absence of negation, the  character from one of these sets can be matched using a script name. For
351  curly brackets in the escape sequence are optional; these two examples have  example:
352  the same effect:  .sp
353      \ep{Greek}
354      \eP{Han}
355    .sp
356    Those that are not part of an identified script are lumped together as
357    "Common". The current list of scripts is:
358    .P
359    Arabic,
360    Armenian,
361    Balinese,
362    Bengali,
363    Bopomofo,
364    Braille,
365    Buginese,
366    Buhid,
367    Canadian_Aboriginal,
368    Cherokee,
369    Common,
370    Coptic,
371    Cuneiform,
372    Cypriot,
373    Cyrillic,
374    Deseret,
375    Devanagari,
376    Ethiopic,
377    Georgian,
378    Glagolitic,
379    Gothic,
380    Greek,
381    Gujarati,
382    Gurmukhi,
383    Han,
384    Hangul,
385    Hanunoo,
386    Hebrew,
387    Hiragana,
388    Inherited,
389    Kannada,
390    Katakana,
391    Kharoshthi,
392    Khmer,
393    Lao,
394    Latin,
395    Limbu,
396    Linear_B,
397    Malayalam,
398    Mongolian,
399    Myanmar,
400    New_Tai_Lue,
401    Nko,
402    Ogham,
403    Old_Italic,
404    Old_Persian,
405    Oriya,
406    Osmanya,
407    Phags_Pa,
408    Phoenician,
409    Runic,
410    Shavian,
411    Sinhala,
412    Syloti_Nagri,
413    Syriac,
414    Tagalog,
415    Tagbanwa,
416    Tai_Le,
417    Tamil,
418    Telugu,
419    Thaana,
420    Thai,
421    Tibetan,
422    Tifinagh,
423    Ugaritic,
424    Yi.
425    .P
426    Each character has exactly one general category property, specified by a
427    two-letter abbreviation. For compatibility with Perl, negation can be specified
428    by including a circumflex between the opening brace and the property name. For
429    example, \ep{^Lu} is the same as \eP{Lu}.
430    .P
431    If only one letter is specified with \ep or \eP, it includes all the general
432    category properties that start with that letter. In this case, in the absence
433    of negation, the curly brackets in the escape sequence are optional; these two
434    examples have the same effect:
435  .sp  .sp
436    \ep{L}    \ep{L}
437    \epL    \epL
438  .sp  .sp
439  The following property codes are supported:  The following general category property codes are supported:
440  .sp  .sp
441    C     Other    C     Other
442    Cc    Control    Cc    Control
# Line 347  The following property codes are support Line 482  The following property codes are support
482    Zp    Paragraph separator    Zp    Paragraph separator
483    Zs    Space separator    Zs    Space separator
484  .sp  .sp
485  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
486  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
487    a modifier or "other".
488    .P
489    The long synonyms for these properties that Perl supports (such as \ep{Letter})
490    are not supported by PCRE, nor is it permitted to prefix any of these
491    properties with "Is".
492    .P
493    No character that is in the Unicode table has the Cn (unassigned) property.
494    Instead, this property is assumed for any code point that is not in the
495    Unicode table.
496  .P  .P
497  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
498  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
# Line 378  properties in PCRE. Line 522  properties in PCRE.
522  .SS "Simple assertions"  .SS "Simple assertions"
523  .rs  .rs
524  .sp  .sp
525  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
526  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,
527  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
528  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 386  subpatterns for more complicated asserti Line 530  subpatterns for more complicated asserti
530  .\" </a>  .\" </a>
531  below.  below.
532  .\"  .\"
533  The backslashed  The backslashed assertions are:
 assertions are:  
534  .sp  .sp
535    \eb     matches at a word boundary    \eb     matches at a word boundary
536    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
537    \eA     matches at start of subject    \eA     matches at the start of the subject
538    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
539    \ez     matches at end of subject            also matches before a newline at the end of the subject
540    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
541      \eG     matches at the first matching position in the subject
542  .sp  .sp
543  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
544  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 412  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 556  PCRE_NOTBOL or PCRE_NOTEOL options, whic
556  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
557  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
558  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
559  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
560  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.  
561  .P  .P
562  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
563  start point of the match, as specified by the \fIstartoffset\fP argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
# Line 458  to be anchored.) Line 601  to be anchored.)
601  .P  .P
602  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
603  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
604  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
605  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
606  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
607  Dollar has no special meaning in a character class.  character class.
608  .P  .P
609  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
610  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
611  does not affect the \eZ assertion.  does not affect the \eZ assertion.
612  .P  .P
613  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
614  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
615  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
616  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
617  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
618  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
619  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
620  branches start with ^ are not anchored in multiline mode, and a match for  .P
621  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
622  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,
623  set.  patterns that are anchored in single line mode because all branches start with
624    ^ are not anchored in multiline mode, and a match for circumflex is possible
625    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
626    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
627  .P  .P
628  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
629  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
630  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
631  .  .
632  .  .
633  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
634  .rs  .rs
635  .sp  .sp
636  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
637  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
638  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.
639  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
640  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
641  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
642  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
643    (including isolated CRs and LFs). When any Unicode line endings are being
644    recognized, dot does not match CR or LF or any of the other line ending
645    characters.
646    .P
647    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
648    option is set, a dot matches any one character, without exception. If the
649    two-character sequence CRLF is present in the subject string, it takes two dots
650    to match it.
651    .P
652    The handling of dot is entirely independent of the handling of circumflex and
653    dollar, the only relationship being that they both involve newlines. Dot has no
654    special meaning in a character class.
655  .  .
656  .  .
657  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
658  .rs  .rs
659  .sp  .sp
660  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
661  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
662  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
663  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,
664  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,
665  best avoided.  the \eC escape sequence is best avoided.
666  .P  .P
667  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
668  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 555  If you want to use caseless matching for Line 713  If you want to use caseless matching for
713  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
714  UTF-8 support.  UTF-8 support.
715  .P  .P
716  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
717  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
718  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
719    such as [^a] always matches one of these characters.
720  .P  .P
721  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
722  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 656  the pattern Line 815  the pattern
815    gilbert|sullivan    gilbert|sullivan
816  .sp  .sp
817  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
818  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
819  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
820  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  
821  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
822  .\" </a>  .\" </a>
823  (defined below),  (defined below),
# Line 693  If the change is placed right at the sta Line 851  If the change is placed right at the sta
851  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
852  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
853  .P  .P
854  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
855  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
856  .sp  .sp
857    (a(?i)b)c    (a(?i)b)c
858  .sp  .sp
# Line 710  branch is abandoned before the option se Line 868  branch is abandoned before the option se
868  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
869  behaviour otherwise.  behaviour otherwise.
870  .P  .P
871  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
872  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
873  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.  
874  .  .
875  .  .
876  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 729  Turning part of a pattern into a subpatt Line 885  Turning part of a pattern into a subpatt
885    cat(aract|erpillar|)    cat(aract|erpillar|)
886  .sp  .sp
887  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
888  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
889  .sp  .sp
890  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
891  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
# Line 754  the string "the white queen" is matched Line 910  the string "the white queen" is matched
910    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
911  .sp  .sp
912  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
913  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.  
914  .P  .P
915  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
916  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 776  the above patterns match "SUNDAY" as wel Line 931  the above patterns match "SUNDAY" as wel
931  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
932  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
933  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
934  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
935  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
936  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
937  .P  the Perl and the Python syntax.
938  Named capturing parentheses are still allocated numbers as well as names. The  .P
939  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
940  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
941  extracting a captured substring by name. For further details see the  parentheses from other parts of the pattern, such as
942    .\" HTML <a href="#backreferences">
943    .\" </a>
944    backreferences,
945    .\"
946    .\" HTML <a href="#recursion">
947    .\" </a>
948    recursion,
949    .\"
950    and
951    .\" HTML <a href="#conditions">
952    .\" </a>
953    conditions,
954    .\"
955    can be made by name as well as by number.
956    .P
957    Names consist of up to 32 alphanumeric characters and underscores. Named
958    capturing parentheses are still allocated numbers as well as names, exactly as
959    if the names were not present. The PCRE API provides function calls for
960    extracting the name-to-number translation table from a compiled pattern. There
961    is also a convenience function for extracting a captured substring by name.
962    .P
963    By default, a name must be unique within a pattern, but it is possible to relax
964    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
965    be useful for patterns where only one instance of the named parentheses can
966    match. Suppose you want to match the name of a weekday, either as a 3-letter
967    abbreviation or as the full name, and in both cases you want to extract the
968    abbreviation. This pattern (ignoring the line breaks) does the job:
969    .sp
970      (?<DN>Mon|Fri|Sun)(?:day)?|
971      (?<DN>Tue)(?:sday)?|
972      (?<DN>Wed)(?:nesday)?|
973      (?<DN>Thu)(?:rsday)?|
974      (?<DN>Sat)(?:urday)?
975    .sp
976    There are five capturing substrings, but only one is ever set after a match.
977    The convenience function for extracting the data by name returns the substring
978    for the first (and in this example, the only) subpattern of that name that
979    matched. This saves searching to find which numbered subpattern it was. If you
980    make a reference to a non-unique named subpattern from elsewhere in the
981    pattern, the one that corresponds to the lowest number is used. For further
982    details of the interfaces for handling named subpatterns, see the
983  .\" HREF  .\" HREF
984  \fBpcreapi\fP  \fBpcreapi\fP
985  .\"  .\"
# Line 797  Repetition is specified by quantifiers, Line 993  Repetition is specified by quantifiers,
993  items:  items:
994  .sp  .sp
995    a literal data character    a literal data character
996    the . metacharacter    the dot metacharacter
997    the \eC escape sequence    the \eC escape sequence
998    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
999      the \eR escape sequence
1000    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1001    a character class    a character class
1002    a back reference (see next section)    a back reference (see next section)
# Line 837  which may be several bytes long (and the Line 1034  which may be several bytes long (and the
1034  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
1035  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1036  .P  .P
1037  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1038  quantifiers have single-character abbreviations:  abbreviations:
1039  .sp  .sp
1040    *    is equivalent to {0,}    *    is equivalent to {0,}
1041    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 886  own right. Because it has two uses, it c Line 1083  own right. Because it has two uses, it c
1083  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
1084  way the rest of the pattern matches.  way the rest of the pattern matches.
1085  .P  .P
1086  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),
1087  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
1088  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
1089  default behaviour.  default behaviour.
# Line 896  is greater than 1 or with a limited maxi Line 1093  is greater than 1 or with a limited maxi
1093  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1094  .P  .P
1095  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
1096  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
1097  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1098  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
1099  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 908  alternatively using ^ to indicate anchor Line 1105  alternatively using ^ to indicate anchor
1105  .P  .P
1106  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1107  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1108  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
1109  succeed. Consider, for example:  succeeds. Consider, for example:
1110  .sp  .sp
1111    (.*)abc\e1    (.*)abc\e1
1112  .sp  .sp
# Line 935  matches "aba" the value of the second ca Line 1132  matches "aba" the value of the second ca
1132  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1133  .rs  .rs
1134  .sp  .sp
1135  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1136  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
1137  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
1138  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
1139  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
1140  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1141  .P  .P
1142  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
1143  .sp  .sp
# Line 952  item, and then with 4, and so on, before Line 1149  item, and then with 4, and so on, before
1149  (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
1150  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.
1151  .P  .P
1152  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
1153  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
1154  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1155  .sp  .sp
# Line 984  previous example can be rewritten as Line 1181  previous example can be rewritten as
1181  .sp  .sp
1182  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1183  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
1184  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
1185  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1186  .P  difference; possessive quantifiers should be slightly faster.
1187  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1188  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1189    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1190    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1191    package, and PCRE copied it from there. It ultimately found its way into Perl
1192    at release 5.10.
1193    .P
1194    PCRE has an optimization that automatically "possessifies" certain simple
1195    pattern constructs. For example, the sequence A+B is treated as A++B because
1196    there is no point in backtracking into a sequence of A's when B must follow.
1197  .P  .P
1198  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
1199  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1030  However, if the decimal number following Line 1235  However, if the decimal number following
1235  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
1236  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
1237  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
1238  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
1239    when a repetition is involved and the subpattern to the right has participated
1240    in an earlier iteration.
1241    .P
1242    It is not possible to have a numerical "forward back reference" to a subpattern
1243    whose number is 10 or more using this syntax because a sequence such as \e50 is
1244    interpreted as a character defined in octal. See the subsection entitled
1245    "Non-printing characters"
1246  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1247  .\" </a>  .\" </a>
1248  above  above
1249  .\"  .\"
1250  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1251    no such problem when named parentheses are used. A back reference to any
1252    subpattern is possible using named parentheses (see below).
1253    .P
1254    Another way of avoiding the ambiguity inherent in the use of digits following a
1255    backslash is to use the \eg escape sequence, which is a feature introduced in
1256    Perl 5.10. This escape must be followed by a positive or a negative number,
1257    optionally enclosed in braces. These examples are all identical:
1258    .sp
1259      (ring), \e1
1260      (ring), \eg1
1261      (ring), \eg{1}
1262    .sp
1263    A positive number specifies an absolute reference without the ambiguity that is
1264    present in the older syntax. It is also useful when literal digits follow the
1265    reference. A negative number is a relative reference. Consider this example:
1266    .sp
1267      (abc(def)ghi)\eg{-1}
1268    .sp
1269    The sequence \eg{-1} is a reference to the most recently started capturing
1270    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1271    would be equivalent to \e1. The use of relative references can be helpful in
1272    long patterns, and also in patterns that are created by joining together
1273    fragments that contain references within themselves.
1274  .P  .P
1275  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1276  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1057  back reference, the case of letters is r Line 1292  back reference, the case of letters is r
1292  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
1293  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1294  .P  .P
1295  Back references to named subpatterns use the Python syntax (?P=name). We could  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'
1296  rewrite the above example as follows:  or the Python syntax (?P=name). We could rewrite the above example in either of
1297    the following ways:
1298  .sp  .sp
1299    (?<p1>(?i)rah)\es+(?P=p1)    (?<p1>(?i)rah)\es+\ek<p1>
1300      (?P<p1>(?i)rah)\es+(?P=p1)
1301  .sp  .sp
1302    A subpattern that is referenced by name may appear in the pattern before or
1303    after the reference.
1304    .P
1305  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
1306  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1307  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
# Line 1123  because it does not make sense for negat Line 1363  because it does not make sense for negat
1363  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1364  .rs  .rs
1365  .sp  .sp
1366  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1367  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1368  .sp  .sp
1369    \ew+(?=;)    \ew+(?=;)
1370  .sp  .sp
# Line 1159  negative assertions. For example, Line 1399  negative assertions. For example,
1399  .sp  .sp
1400  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
1401  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
1402  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
1403  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1404  .sp  .sp
1405    (?<=bullock|donkey)    (?<=bullock|donkey)
1406  .sp  .sp
# Line 1181  lengths, but it is acceptable if rewritt Line 1421  lengths, but it is acceptable if rewritt
1421    (?<=abc|abde)    (?<=abc|abde)
1422  .sp  .sp
1423  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1424  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
1425  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1426  match is deemed to fail.  assertion fails.
1427  .P  .P
1428  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)
1429  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1430  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
1431  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1432  .P  .P
1433  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1434  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
1435  such as  pattern such as
1436  .sp  .sp
1437    abcd$    abcd$
1438  .sp  .sp
# Line 1208  then all but the last two characters, an Line 1448  then all but the last two characters, an
1448  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,
1449  if the pattern is written as  if the pattern is written as
1450  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1451    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1452  .sp  .sp
1453  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
1454  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
1455  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1456  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1254  is another pattern that matches "foo" pr Line 1490  is another pattern that matches "foo" pr
1490  characters that are not "999".  characters that are not "999".
1491  .  .
1492  .  .
1493    .\" HTML <a name="conditions"></a>
1494  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1495  .rs  .rs
1496  .sp  .sp
# Line 1269  If the condition is satisfied, the yes-p Line 1506  If the condition is satisfied, the yes-p
1506  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
1507  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1508  .P  .P
1509  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1510  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1511  subpattern of that number has previously matched. The number must be greater  .
1512  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1513  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1514  it into three parts for ease of discussion:  .sp
1515    If the text between the parentheses consists of a sequence of digits, the
1516    condition is true if the capturing subpattern of that number has previously
1517    matched.
1518    .P
1519    Consider the following pattern, which contains non-significant white space to
1520    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1521    three parts for ease of discussion:
1522  .sp  .sp
1523    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1524  .sp  .sp
# Line 1287  the condition is true, and so the yes-pa Line 1531  the condition is true, and so the yes-pa
1531  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1532  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1533  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1534    .
1535    .SS "Checking for a used subpattern by name"
1536    .rs
1537    .sp
1538    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1539    subpattern by name. For compatibility with earlier versions of PCRE, which had
1540    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1541    there is a possible ambiguity with this syntax, because subpattern names may
1542    consist entirely of digits. PCRE looks first for a named subpattern; if it
1543    cannot find one and the name consists entirely of digits, PCRE looks for a
1544    subpattern of that number, which must be greater than zero. Using subpattern
1545    names that consist entirely of digits is not recommended.
1546    .P
1547    Rewriting the above example to use a named subpattern gives this:
1548    .sp
1549      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1550    .sp
1551    .
1552    .SS "Checking for pattern recursion"
1553    .rs
1554    .sp
1555    If the condition is the string (R), and there is no subpattern with the name R,
1556    the condition is true if a recursive call to the whole pattern or any
1557    subpattern has been made. If digits or a name preceded by ampersand follow the
1558    letter R, for example:
1559    .sp
1560      (?(R3)...) or (?(R&name)...)
1561    .sp
1562    the condition is true if the most recent recursion is into the subpattern whose
1563    number or name is given. This condition does not check the entire recursion
1564    stack.
1565  .P  .P
1566  If the condition is the string (R), it is satisfied if a recursive call to the  At "top level", all these recursion test conditions are false. Recursive
1567  pattern or subpattern has been made. At "top level", the condition is false.  patterns are described below.
1568  This is a PCRE extension. Recursive patterns are described in the next section.  .
1569    .SS "Defining subpatterns for use by reference only"
1570    .rs
1571    .sp
1572    If the condition is the string (DEFINE), and there is no subpattern with the
1573    name DEFINE, the condition is always false. In this case, there may be only one
1574    alternative in the subpattern. It is always skipped if control reaches this
1575    point in the pattern; the idea of DEFINE is that it can be used to define
1576    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1577    is described below.) For example, a pattern to match an IPv4 address could be
1578    written like this (ignore whitespace and line breaks):
1579    .sp
1580      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1581      \eb (?&byte) (\e.(?&byte)){3} \eb
1582    .sp
1583    The first part of the pattern is a DEFINE group inside which a another group
1584    named "byte" is defined. This matches an individual component of an IPv4
1585    address (a number less than 256). When matching takes place, this part of the
1586    pattern is skipped because DEFINE acts like a false condition.
1587  .P  .P
1588  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
1589    dot-separated components of an IPv4 address, insisting on a word boundary at
1590    each end.
1591    .
1592    .SS "Assertion conditions"
1593    .rs
1594    .sp
1595    If the condition is not in any of the above formats, it must be an assertion.
1596  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1597  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1598  alternatives on the second line:  alternatives on the second line:
# Line 1317  closing parenthesis. Nested parentheses Line 1617  closing parenthesis. Nested parentheses
1617  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.
1618  .P  .P
1619  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1620  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1621  character in the pattern.  next newline in the pattern.
1622  .  .
1623  .  .
1624    .\" HTML <a name="recursion"></a>
1625  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
1626  .rs  .rs
1627  .sp  .sp
1628  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1629  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1630  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
1631  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1632  that allows regular expressions to recurse (amongst other things). It does this  .P
1633  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
1634  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
1635  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1636    pattern using code interpolation to solve the parentheses problem can be
1637    created like this:
1638  .sp  .sp
1639    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1640  .sp  .sp
1641  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
1642  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
 the interpolation of Perl code. Instead, it supports some special syntax for  
 recursion of the entire pattern, and also for individual subpattern recursion.  
 .P  
 The special item that consists of (? followed by a number greater than zero and  
 a closing parenthesis is a recursive call of the subpattern of the given  
 number, provided that it occurs inside that subpattern. (If not, it is a  
 "subroutine" call, which is described in the next section.) The special item  
 (?R) is a recursive call of the entire regular expression.  
1643  .P  .P
1644  For example, this PCRE pattern solves the nested parentheses problem (assume  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1645  the PCRE_EXTENDED option is set so that white space is ignored):  supports special syntax for recursion of the entire pattern, and also for
1646    individual subpattern recursion. After its introduction in PCRE and Python,
1647    this kind of recursion was introduced into Perl at release 5.10.
1648    .P
1649    A special item that consists of (? followed by a number greater than zero and a
1650    closing parenthesis is a recursive call of the subpattern of the given number,
1651    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1652    call, which is described in the next section.) The special item (?R) or (?0) is
1653    a recursive call of the entire regular expression.
1654    .P
1655    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1656    treated as an atomic group. That is, once it has matched some of the subject
1657    string, it is never re-entered, even if it contains untried alternatives and
1658    there is a subsequent matching failure.
1659    .P
1660    This PCRE pattern solves the nested parentheses problem (assume the
1661    PCRE_EXTENDED option is set so that white space is ignored):
1662  .sp  .sp
1663    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( (?>[^()]+) | (?R) )* \e)
1664  .sp  .sp
1665  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1666  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1667  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1668  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1669  .P  .P
1670  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 1364  pattern, so instead you could use this: Line 1675  pattern, so instead you could use this:
1675  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
1676  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern. In a larger pattern, keeping track of
1677  parenthesis numbers can be tricky. It may be more convenient to use named  parenthesis numbers can be tricky. It may be more convenient to use named
1678  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier
1679  the Python syntax that PCRE uses for named parentheses (Perl does not provide  syntax (?P>name) is also supported. We could rewrite the above example as
1680  named parentheses). We could rewrite the above example as follows:  follows:
1681  .sp  .sp
1682    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1683  .sp  .sp
1684  This particular example pattern contains nested unlimited repeats, and so the  If there is more than one subpattern with the same name, the earliest one is
1685  use of atomic grouping for matching strings of non-parentheses is important  used. This particular example pattern contains nested unlimited repeats, and so
1686    the use of atomic grouping for matching strings of non-parentheses is important
1687  when applying the pattern to strings that do not match. For example, when this  when applying the pattern to strings that do not match. For example, when this
1688  pattern is applied to  pattern is applied to
1689  .sp  .sp
# Line 1385  before failure can be reported. Line 1697  before failure can be reported.
1697  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
1698  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.
1699  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
1700  the next section and the  below and the
1701  .\" HREF  .\" HREF
1702  \fBpcrecallout\fP  \fBpcrecallout\fP
1703  .\"  .\"
# Line 1424  is the actual recursive call. Line 1736  is the actual recursive call.
1736  .sp  .sp
1737  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
1738  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
1739  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1740  pattern  before or after the reference. An earlier example pointed out that the pattern
1741  .sp  .sp
1742    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1743  .sp  .sp
# Line 1435  matches "sense and sensibility" and "res Line 1747  matches "sense and sensibility" and "res
1747    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1748  .sp  .sp
1749  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
1750  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1751  refer.  .P
1752    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1753    group. That is, once it has matched some of the subject string, it is never
1754    re-entered, even if it contains untried alternatives and there is a subsequent
1755    matching failure.
1756    .P
1757    When a subpattern is used as a subroutine, processing options such as
1758    case-independence are fixed when the subpattern is defined. They cannot be
1759    changed for different calls. For example, consider this pattern:
1760    .sp
1761      (abc)(?i:(?1))
1762    .sp
1763    It matches "abcabc". It does not match "abcABC" because the change of
1764    processing option does not affect the called subpattern.
1765  .  .
1766  .  .
1767  .SH CALLOUTS  .SH CALLOUTS
# Line 1473  description of the interface to the call Line 1798  description of the interface to the call
1798  \fBpcrecallout\fP  \fBpcrecallout\fP
1799  .\"  .\"
1800  documentation.  documentation.
1801    .
1802    .
1803    .SH "SEE ALSO"
1804    .rs
1805    .sp
1806    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1807  .P  .P
1808  .in 0  .in 0
1809  Last updated: 28 February 2005  Last updated: 06 December 2006
1810  .br  .br
1811  Copyright (c) 1997-2005 University of Cambridge.  Copyright (c) 1997-2006 University of Cambridge.

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