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revision 75 by nigel, Sat Feb 24 21:40:37 2007 UTC revision 167 by ph10, Wed May 9 15:53:54 2007 UTC
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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 26  in the main Line 26  in the main
26  .\"  .\"
27  page.  page.
28  .P  .P
29    The remainder of this document discusses the patterns that are supported by
30    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31    From release 6.0, PCRE offers a second matching function,
32    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33    Perl-compatible. The advantages and disadvantages of the alternative function,
34    and how it differs from the normal function, are discussed in the
35    .\" HREF
36    \fBpcrematching\fP
37    .\"
38    page.
39    .
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
47  .sp  .sp
48    The quick brown fox    The quick brown fox
49  .sp  .sp
50  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
51  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
52  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
53    case for characters whose values are less than 128, so caseless matching is
54    always possible. For characters with higher values, the concept of case is
55    supported if PCRE is compiled with Unicode property support, but not otherwise.
56    If you want to use caseless matching for characters 128 and above, you must
57    ensure that PCRE is compiled with Unicode property support as well as with
58    UTF-8 support.
59    .P
60    The power of regular expressions comes from the ability to include alternatives
61    and repetitions in the pattern. These are encoded in the pattern by the use of
62  \fImetacharacters\fP, which do not stand for themselves but instead are  \fImetacharacters\fP, which do not stand for themselves but instead are
63  interpreted in some special way.  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 72  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 88  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 128  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 136  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 171  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 198  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 210  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 233  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 247  in the Line 291  in the
291  .\" HREF  .\" HREF
292  \fBpcreapi\fP  \fBpcreapi\fP
293  .\"  .\"
294  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,
295  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
296    accented letters, and these are matched by \ew.
297  .P  .P
298  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
299  \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
300  character property support is available.  character property support is available. The use of locales with Unicode is
301    discouraged.
302    .
303    .
304    .SS "Newline sequences"
305    .rs
306    .sp
307    Outside a character class, the escape sequence \eR matches any Unicode newline
308    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
309    the following:
310    .sp
311      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
312    .sp
313    This is an example of an "atomic group", details of which are given
314    .\" HTML <a href="#atomicgroup">
315    .\" </a>
316    below.
317    .\"
318    This particular group matches either the two-character sequence CR followed by
319    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
320    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
321    line, U+0085). The two-character sequence is treated as a single unit that
322    cannot be split.
323    .P
324    In UTF-8 mode, two additional characters whose codepoints are greater than 255
325    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
326    Unicode character property support is not needed for these characters to be
327    recognized.
328    .P
329    Inside a character class, \eR matches the letter "R".
330  .  .
331  .  .
332  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 260  character property support is available. Line 334  character property support is available.
334  .rs  .rs
335  .sp  .sp
336  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
337  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
338  is selected. They are:  is selected. They are:
339  .sp  .sp
340   \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
341   \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
342   \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
343  .sp  .sp
344  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
345  Unicode general category properties. Each character has exactly one such  script names, the general category properties, and "Any", which matches any
346  property, specified by a two-letter abbreviation. For compatibility with Perl,  character (including newline). Other properties such as "InMusicalSymbols" are
347  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
348  and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.  characters, so always causes a match failure.
349  .P  .P
350  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
351  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
352  curly brackets in the escape sequence are optional; these two examples have  example:
353  the same effect:  .sp
354      \ep{Greek}
355      \eP{Han}
356    .sp
357    Those that are not part of an identified script are lumped together as
358    "Common". The current list of scripts is:
359    .P
360    Arabic,
361    Armenian,
362    Balinese,
363    Bengali,
364    Bopomofo,
365    Braille,
366    Buginese,
367    Buhid,
368    Canadian_Aboriginal,
369    Cherokee,
370    Common,
371    Coptic,
372    Cuneiform,
373    Cypriot,
374    Cyrillic,
375    Deseret,
376    Devanagari,
377    Ethiopic,
378    Georgian,
379    Glagolitic,
380    Gothic,
381    Greek,
382    Gujarati,
383    Gurmukhi,
384    Han,
385    Hangul,
386    Hanunoo,
387    Hebrew,
388    Hiragana,
389    Inherited,
390    Kannada,
391    Katakana,
392    Kharoshthi,
393    Khmer,
394    Lao,
395    Latin,
396    Limbu,
397    Linear_B,
398    Malayalam,
399    Mongolian,
400    Myanmar,
401    New_Tai_Lue,
402    Nko,
403    Ogham,
404    Old_Italic,
405    Old_Persian,
406    Oriya,
407    Osmanya,
408    Phags_Pa,
409    Phoenician,
410    Runic,
411    Shavian,
412    Sinhala,
413    Syloti_Nagri,
414    Syriac,
415    Tagalog,
416    Tagbanwa,
417    Tai_Le,
418    Tamil,
419    Telugu,
420    Thaana,
421    Thai,
422    Tibetan,
423    Tifinagh,
424    Ugaritic,
425    Yi.
426    .P
427    Each character has exactly one general category property, specified by a
428    two-letter abbreviation. For compatibility with Perl, negation can be specified
429    by including a circumflex between the opening brace and the property name. For
430    example, \ep{^Lu} is the same as \eP{Lu}.
431    .P
432    If only one letter is specified with \ep or \eP, it includes all the general
433    category properties that start with that letter. In this case, in the absence
434    of negation, the curly brackets in the escape sequence are optional; these two
435    examples have the same effect:
436  .sp  .sp
437    \ep{L}    \ep{L}
438    \epL    \epL
439  .sp  .sp
440  The following property codes are supported:  The following general category property codes are supported:
441  .sp  .sp
442    C     Other    C     Other
443    Cc    Control    Cc    Control
# Line 327  The following property codes are support Line 483  The following property codes are support
483    Zp    Paragraph separator    Zp    Paragraph separator
484    Zs    Space separator    Zs    Space separator
485  .sp  .sp
486  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
487  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
488    a modifier or "other".
489    .P
490    The long synonyms for these properties that Perl supports (such as \ep{Letter})
491    are not supported by PCRE, nor is it permitted to prefix any of these
492    properties with "Is".
493    .P
494    No character that is in the Unicode table has the Cn (unassigned) property.
495    Instead, this property is assumed for any code point that is not in the
496    Unicode table.
497  .P  .P
498  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
499  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
# Line 358  properties in PCRE. Line 523  properties in PCRE.
523  .SS "Simple assertions"  .SS "Simple assertions"
524  .rs  .rs
525  .sp  .sp
526  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
527  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,
528  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
529  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 366  subpatterns for more complicated asserti Line 531  subpatterns for more complicated asserti
531  .\" </a>  .\" </a>
532  below.  below.
533  .\"  .\"
534  The backslashed  The backslashed assertions are:
 assertions are:  
535  .sp  .sp
536    \eb     matches at a word boundary    \eb     matches at a word boundary
537    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
538    \eA     matches at start of subject    \eA     matches at the start of the subject
539    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
540    \ez     matches at end of subject            also matches before a newline at the end of the subject
541    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
542      \eG     matches at the first matching position in the subject
543  .sp  .sp
544  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
545  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 392  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 557  PCRE_NOTBOL or PCRE_NOTEOL options, whic
557  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
558  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
559  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
560  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
561  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.  
562  .P  .P
563  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
564  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 438  to be anchored.) Line 602  to be anchored.)
602  .P  .P
603  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
604  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
605  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
606  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
607  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
608  Dollar has no special meaning in a character class.  character class.
609  .P  .P
610  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
611  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
612  does not affect the \eZ assertion.  does not affect the \eZ assertion.
613  .P  .P
614  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
615  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
616  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
617  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
618  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
619  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
620  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
621  branches start with ^ are not anchored in multiline mode, and a match for  .P
622  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
623  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,
624  set.  patterns that are anchored in single line mode because all branches start with
625    ^ are not anchored in multiline mode, and a match for circumflex is possible
626    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
627    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
628  .P  .P
629  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
630  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
631  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
632  .  .
633  .  .
634  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
635  .rs  .rs
636  .sp  .sp
637  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
638  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
639  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.
640  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
641  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
642  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
643  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
644    (including isolated CRs and LFs). When any Unicode line endings are being
645    recognized, dot does not match CR or LF or any of the other line ending
646    characters.
647    .P
648    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
649    option is set, a dot matches any one character, without exception. If the
650    two-character sequence CRLF is present in the subject string, it takes two dots
651    to match it.
652    .P
653    The handling of dot is entirely independent of the handling of circumflex and
654    dollar, the only relationship being that they both involve newlines. Dot has no
655    special meaning in a character class.
656  .  .
657  .  .
658  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
659  .rs  .rs
660  .sp  .sp
661  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
662  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
663  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
664  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,
665  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,
666  best avoided.  the \eC escape sequence is best avoided.
667  .P  .P
668  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
669  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 527  class as a literal string of bytes, or b Line 706  class as a literal string of bytes, or b
706  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
707  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
708  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
709  caseful version would. When running in UTF-8 mode, PCRE supports the concept of  caseful version would. In UTF-8 mode, PCRE always understands the concept of
710  case for characters with values greater than 128 only when it is compiled with  case for characters whose values are less than 128, so caseless matching is
711  Unicode property support.  always possible. For characters with higher values, the concept of case is
712  .P  supported if PCRE is compiled with Unicode property support, but not otherwise.
713  The newline character is never treated in any special way in character classes,  If you want to use caseless matching for characters 128 and above, you must
714  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  ensure that PCRE is compiled with Unicode property support as well as with
715  such as [^a] will always match a newline.  UTF-8 support.
716    .P
717    Characters that might indicate line breaks are never treated in any special way
718    when matching character classes, whatever line-ending sequence is in use, and
719    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
720    such as [^a] always matches one of these characters.
721  .P  .P
722  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
723  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 557  example [\ex{100}-\ex{2ff}]. Line 741  example [\ex{100}-\ex{2ff}].
741  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
742  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
743  [][\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
744  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
745  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
746  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
747  property support.  property support.
# Line 632  the pattern Line 816  the pattern
816    gilbert|sullivan    gilbert|sullivan
817  .sp  .sp
818  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
819  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
820  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
821  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  
822  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
823  .\" </a>  .\" </a>
824  (defined below),  (defined below),
# Line 669  If the change is placed right at the sta Line 852  If the change is placed right at the sta
852  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
853  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
854  .P  .P
855  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
856  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
857  .sp  .sp
858    (a(?i)b)c    (a(?i)b)c
859  .sp  .sp
# Line 686  branch is abandoned before the option se Line 869  branch is abandoned before the option se
869  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
870  behaviour otherwise.  behaviour otherwise.
871  .P  .P
872  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
873  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
874  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.  
875  .  .
876  .  .
877  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 705  Turning part of a pattern into a subpatt Line 886  Turning part of a pattern into a subpatt
886    cat(aract|erpillar|)    cat(aract|erpillar|)
887  .sp  .sp
888  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
889  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
890  .sp  .sp
891  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
892  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 730  the string "the white queen" is matched Line 911  the string "the white queen" is matched
911    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
912  .sp  .sp
913  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
914  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.  
915  .P  .P
916  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
917  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 752  the above patterns match "SUNDAY" as wel Line 932  the above patterns match "SUNDAY" as wel
932  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
933  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
934  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
935  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
936  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
937  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
938  .P  the Perl and the Python syntax.
939  Named capturing parentheses are still allocated numbers as well as names. The  .P
940  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
941  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
942  extracting a captured substring by name. For further details see the  parentheses from other parts of the pattern, such as
943    .\" HTML <a href="#backreferences">
944    .\" </a>
945    backreferences,
946    .\"
947    .\" HTML <a href="#recursion">
948    .\" </a>
949    recursion,
950    .\"
951    and
952    .\" HTML <a href="#conditions">
953    .\" </a>
954    conditions,
955    .\"
956    can be made by name as well as by number.
957    .P
958    Names consist of up to 32 alphanumeric characters and underscores. Named
959    capturing parentheses are still allocated numbers as well as names, exactly as
960    if the names were not present. The PCRE API provides function calls for
961    extracting the name-to-number translation table from a compiled pattern. There
962    is also a convenience function for extracting a captured substring by name.
963    .P
964    By default, a name must be unique within a pattern, but it is possible to relax
965    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
966    be useful for patterns where only one instance of the named parentheses can
967    match. Suppose you want to match the name of a weekday, either as a 3-letter
968    abbreviation or as the full name, and in both cases you want to extract the
969    abbreviation. This pattern (ignoring the line breaks) does the job:
970    .sp
971      (?<DN>Mon|Fri|Sun)(?:day)?|
972      (?<DN>Tue)(?:sday)?|
973      (?<DN>Wed)(?:nesday)?|
974      (?<DN>Thu)(?:rsday)?|
975      (?<DN>Sat)(?:urday)?
976    .sp
977    There are five capturing substrings, but only one is ever set after a match.
978    The convenience function for extracting the data by name returns the substring
979    for the first (and in this example, the only) subpattern of that name that
980    matched. This saves searching to find which numbered subpattern it was. If you
981    make a reference to a non-unique named subpattern from elsewhere in the
982    pattern, the one that corresponds to the lowest number is used. For further
983    details of the interfaces for handling named subpatterns, see the
984  .\" HREF  .\" HREF
985  \fBpcreapi\fP  \fBpcreapi\fP
986  .\"  .\"
# Line 773  Repetition is specified by quantifiers, Line 994  Repetition is specified by quantifiers,
994  items:  items:
995  .sp  .sp
996    a literal data character    a literal data character
997    the . metacharacter    the dot metacharacter
998    the \eC escape sequence    the \eC escape sequence
999    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1000      the \eR escape sequence
1001    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1002    a character class    a character class
1003    a back reference (see next section)    a back reference (see next section)
# Line 813  which may be several bytes long (and the Line 1035  which may be several bytes long (and the
1035  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
1036  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1037  .P  .P
1038  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1039  quantifiers have single-character abbreviations:  abbreviations:
1040  .sp  .sp
1041    *    is equivalent to {0,}    *    is equivalent to {0,}
1042    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 862  own right. Because it has two uses, it c Line 1084  own right. Because it has two uses, it c
1084  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
1085  way the rest of the pattern matches.  way the rest of the pattern matches.
1086  .P  .P
1087  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),
1088  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
1089  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
1090  default behaviour.  default behaviour.
# Line 872  is greater than 1 or with a limited maxi Line 1094  is greater than 1 or with a limited maxi
1094  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1095  .P  .P
1096  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
1097  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
1098  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1099  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
1100  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 884  alternatively using ^ to indicate anchor Line 1106  alternatively using ^ to indicate anchor
1106  .P  .P
1107  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1108  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1109  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
1110  succeed. Consider, for example:  succeeds. Consider, for example:
1111  .sp  .sp
1112    (.*)abc\e1    (.*)abc\e1
1113  .sp  .sp
# Line 911  matches "aba" the value of the second ca Line 1133  matches "aba" the value of the second ca
1133  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1134  .rs  .rs
1135  .sp  .sp
1136  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1137  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
1138  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
1139  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
1140  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
1141  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1142  .P  .P
1143  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
1144  .sp  .sp
# Line 928  item, and then with 4, and so on, before Line 1150  item, and then with 4, and so on, before
1150  (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
1151  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.
1152  .P  .P
1153  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
1154  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
1155  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1156  .sp  .sp
# Line 960  previous example can be rewritten as Line 1182  previous example can be rewritten as
1182  .sp  .sp
1183  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1184  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
1185  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
1186  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1187  .P  difference; possessive quantifiers should be slightly faster.
1188  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1189  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1190    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1191    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1192    package, and PCRE copied it from there. It ultimately found its way into Perl
1193    at release 5.10.
1194    .P
1195    PCRE has an optimization that automatically "possessifies" certain simple
1196    pattern constructs. For example, the sequence A+B is treated as A++B because
1197    there is no point in backtracking into a sequence of A's when B must follow.
1198  .P  .P
1199  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
1200  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 1006  However, if the decimal number following Line 1236  However, if the decimal number following
1236  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
1237  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
1238  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
1239  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
1240    when a repetition is involved and the subpattern to the right has participated
1241    in an earlier iteration.
1242    .P
1243    It is not possible to have a numerical "forward back reference" to a subpattern
1244    whose number is 10 or more using this syntax because a sequence such as \e50 is
1245    interpreted as a character defined in octal. See the subsection entitled
1246    "Non-printing characters"
1247  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1248  .\" </a>  .\" </a>
1249  above  above
1250  .\"  .\"
1251  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1252    no such problem when named parentheses are used. A back reference to any
1253    subpattern is possible using named parentheses (see below).
1254    .P
1255    Another way of avoiding the ambiguity inherent in the use of digits following a
1256    backslash is to use the \eg escape sequence, which is a feature introduced in
1257    Perl 5.10. This escape must be followed by a positive or a negative number,
1258    optionally enclosed in braces. These examples are all identical:
1259    .sp
1260      (ring), \e1
1261      (ring), \eg1
1262      (ring), \eg{1}
1263    .sp
1264    A positive number specifies an absolute reference without the ambiguity that is
1265    present in the older syntax. It is also useful when literal digits follow the
1266    reference. A negative number is a relative reference. Consider this example:
1267    .sp
1268      (abc(def)ghi)\eg{-1}
1269    .sp
1270    The sequence \eg{-1} is a reference to the most recently started capturing
1271    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1272    would be equivalent to \e1. The use of relative references can be helpful in
1273    long patterns, and also in patterns that are created by joining together
1274    fragments that contain references within themselves.
1275  .P  .P
1276  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1277  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1033  back reference, the case of letters is r Line 1293  back reference, the case of letters is r
1293  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
1294  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1295  .P  .P
1296  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'
1297  rewrite the above example as follows:  or the Python syntax (?P=name). We could rewrite the above example in either of
1298    the following ways:
1299  .sp  .sp
1300    (?<p1>(?i)rah)\es+(?P=p1)    (?<p1>(?i)rah)\es+\ek<p1>
1301      (?P<p1>(?i)rah)\es+(?P=p1)
1302  .sp  .sp
1303    A subpattern that is referenced by name may appear in the pattern before or
1304    after the reference.
1305    .P
1306  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
1307  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1308  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
# Line 1099  because it does not make sense for negat Line 1364  because it does not make sense for negat
1364  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1365  .rs  .rs
1366  .sp  .sp
1367  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1368  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1369  .sp  .sp
1370    \ew+(?=;)    \ew+(?=;)
1371  .sp  .sp
# Line 1135  negative assertions. For example, Line 1400  negative assertions. For example,
1400  .sp  .sp
1401  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
1402  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
1403  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
1404  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1405  .sp  .sp
1406    (?<=bullock|donkey)    (?<=bullock|donkey)
1407  .sp  .sp
# Line 1157  lengths, but it is acceptable if rewritt Line 1422  lengths, but it is acceptable if rewritt
1422    (?<=abc|abde)    (?<=abc|abde)
1423  .sp  .sp
1424  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1425  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
1426  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1427  match is deemed to fail.  assertion fails.
1428  .P  .P
1429  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)
1430  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1431  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
1432  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1433  .P  .P
1434  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1435  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
1436  such as  pattern such as
1437  .sp  .sp
1438    abcd$    abcd$
1439  .sp  .sp
# Line 1184  then all but the last two characters, an Line 1449  then all but the last two characters, an
1449  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,
1450  if the pattern is written as  if the pattern is written as
1451  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1452    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1453  .sp  .sp
1454  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
1455  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
1456  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1457  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1230  is another pattern that matches "foo" pr Line 1491  is another pattern that matches "foo" pr
1491  characters that are not "999".  characters that are not "999".
1492  .  .
1493  .  .
1494    .\" HTML <a name="conditions"></a>
1495  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1496  .rs  .rs
1497  .sp  .sp
# Line 1245  If the condition is satisfied, the yes-p Line 1507  If the condition is satisfied, the yes-p
1507  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
1508  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1509  .P  .P
1510  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1511  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1512  subpattern of that number has previously matched. The number must be greater  .
1513  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1514  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1515  it into three parts for ease of discussion:  .sp
1516    If the text between the parentheses consists of a sequence of digits, the
1517    condition is true if the capturing subpattern of that number has previously
1518    matched. An alternative notation is to precede the digits with a plus or minus
1519    sign. In this case, the subpattern number is relative rather than absolute.
1520    The most recently opened parentheses can be referenced by (?(-1), the next most
1521    recent by (?(-2), and so on. In looping constructs it can also make sense to
1522    refer to subsequent groups with constructs such as (?(+2).
1523    .P
1524    Consider the following pattern, which contains non-significant white space to
1525    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1526    three parts for ease of discussion:
1527  .sp  .sp
1528    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1529  .sp  .sp
# Line 1264  parenthesis is required. Otherwise, sinc Line 1537  parenthesis is required. Otherwise, sinc
1537  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1538  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1539  .P  .P
1540  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
1541  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1542  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1543      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1544    .sp
1545    This makes the fragment independent of the parentheses in the larger pattern.
1546    .
1547    .SS "Checking for a used subpattern by name"
1548    .rs
1549    .sp
1550    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1551    subpattern by name. For compatibility with earlier versions of PCRE, which had
1552    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1553    there is a possible ambiguity with this syntax, because subpattern names may
1554    consist entirely of digits. PCRE looks first for a named subpattern; if it
1555    cannot find one and the name consists entirely of digits, PCRE looks for a
1556    subpattern of that number, which must be greater than zero. Using subpattern
1557    names that consist entirely of digits is not recommended.
1558    .P
1559    Rewriting the above example to use a named subpattern gives this:
1560    .sp
1561      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1562    .sp
1563    .
1564    .SS "Checking for pattern recursion"
1565    .rs
1566    .sp
1567    If the condition is the string (R), and there is no subpattern with the name R,
1568    the condition is true if a recursive call to the whole pattern or any
1569    subpattern has been made. If digits or a name preceded by ampersand follow the
1570    letter R, for example:
1571    .sp
1572      (?(R3)...) or (?(R&name)...)
1573    .sp
1574    the condition is true if the most recent recursion is into the subpattern whose
1575    number or name is given. This condition does not check the entire recursion
1576    stack.
1577    .P
1578    At "top level", all these recursion test conditions are false. Recursive
1579    patterns are described below.
1580    .
1581    .SS "Defining subpatterns for use by reference only"
1582    .rs
1583    .sp
1584    If the condition is the string (DEFINE), and there is no subpattern with the
1585    name DEFINE, the condition is always false. In this case, there may be only one
1586    alternative in the subpattern. It is always skipped if control reaches this
1587    point in the pattern; the idea of DEFINE is that it can be used to define
1588    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1589    is described below.) For example, a pattern to match an IPv4 address could be
1590    written like this (ignore whitespace and line breaks):
1591    .sp
1592      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1593      \eb (?&byte) (\e.(?&byte)){3} \eb
1594    .sp
1595    The first part of the pattern is a DEFINE group inside which a another group
1596    named "byte" is defined. This matches an individual component of an IPv4
1597    address (a number less than 256). When matching takes place, this part of the
1598    pattern is skipped because DEFINE acts like a false condition.
1599  .P  .P
1600  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
1601    dot-separated components of an IPv4 address, insisting on a word boundary at
1602    each end.
1603    .
1604    .SS "Assertion conditions"
1605    .rs
1606    .sp
1607    If the condition is not in any of the above formats, it must be an assertion.
1608  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1609  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1610  alternatives on the second line:  alternatives on the second line:
# Line 1293  closing parenthesis. Nested parentheses Line 1629  closing parenthesis. Nested parentheses
1629  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.
1630  .P  .P
1631  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1632  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1633  character in the pattern.  next newline in the pattern.
1634  .  .
1635  .  .
1636    .\" HTML <a name="recursion"></a>
1637  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
1638  .rs  .rs
1639  .sp  .sp
1640  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1641  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1642  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
1643  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1644  that allows regular expressions to recurse (amongst other things). It does this  .P
1645  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
1646  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
1647  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1648    pattern using code interpolation to solve the parentheses problem can be
1649    created like this:
1650  .sp  .sp
1651    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1652  .sp  .sp
1653  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
1654  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1655  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1656  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1657  .P  supports special syntax for recursion of the entire pattern, and also for
1658  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1659  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.
1660  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1661  "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
1662  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1663    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1664    call, which is described in the next section.) The special item (?R) or (?0) is
1665    a recursive call of the entire regular expression.
1666    .P
1667    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1668    treated as an atomic group. That is, once it has matched some of the subject
1669    string, it is never re-entered, even if it contains untried alternatives and
1670    there is a subsequent matching failure.
1671  .P  .P
1672  For example, this PCRE pattern solves the nested parentheses problem (assume  This PCRE pattern solves the nested parentheses problem (assume the
1673  the PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1674  .sp  .sp
1675    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( (?>[^()]+) | (?R) )* \e)
1676  .sp  .sp
1677  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1678  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
1679  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1680  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1681  .P  .P
1682  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 1338  pattern, so instead you could use this: Line 1685  pattern, so instead you could use this:
1685    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1686  .sp  .sp
1687  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
1688  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1689  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1690  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
1691  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.)
1692  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
1693  .sp  most recently opened parentheses preceding the recursion. In other words, a
1694    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1695  .sp  it is encountered.
1696  This particular example pattern contains nested unlimited repeats, and so the  .P
1697  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1698  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
1699  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1700    "subroutine" calls, as described in the next section.
1701    .P
1702    An alternative approach is to use named parentheses instead. The Perl syntax
1703    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1704    could rewrite the above example as follows:
1705    .sp
1706      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1707    .sp
1708    If there is more than one subpattern with the same name, the earliest one is
1709    used.
1710    .P
1711    This particular example pattern that we have been looking at contains nested
1712    unlimited repeats, and so the use of atomic grouping for matching strings of
1713    non-parentheses is important when applying the pattern to strings that do not
1714    match. For example, when this pattern is applied to
1715  .sp  .sp
1716    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1717  .sp  .sp
# Line 1361  before failure can be reported. Line 1723  before failure can be reported.
1723  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
1724  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.
1725  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
1726  the next section and the  below and the
1727  .\" HREF  .\" HREF
1728  \fBpcrecallout\fP  \fBpcrecallout\fP
1729  .\"  .\"
# Line 1400  is the actual recursive call. Line 1762  is the actual recursive call.
1762  .sp  .sp
1763  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
1764  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
1765  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1766  pattern  before or after the reference. A numbered reference can be absolute or
1767    relative, as in these examples:
1768    .sp
1769      (...(absolute)...)...(?2)...
1770      (...(relative)...)...(?-1)...
1771      (...(?+1)...(relative)...
1772    .sp
1773    An earlier example pointed out that the pattern
1774  .sp  .sp
1775    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1776  .sp  .sp
# Line 1411  matches "sense and sensibility" and "res Line 1780  matches "sense and sensibility" and "res
1780    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1781  .sp  .sp
1782  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
1783  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1784  refer.  .P
1785    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1786    group. That is, once it has matched some of the subject string, it is never
1787    re-entered, even if it contains untried alternatives and there is a subsequent
1788    matching failure.
1789    .P
1790    When a subpattern is used as a subroutine, processing options such as
1791    case-independence are fixed when the subpattern is defined. They cannot be
1792    changed for different calls. For example, consider this pattern:
1793    .sp
1794      (abc)(?i:(?-1))
1795    .sp
1796    It matches "abcabc". It does not match "abcABC" because the change of
1797    processing option does not affect the called subpattern.
1798  .  .
1799  .  .
1800  .SH CALLOUTS  .SH CALLOUTS
# Line 1433  function is to be called. If you want to Line 1815  function is to be called. If you want to
1815  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.
1816  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1817  .sp  .sp
1818    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1819  .sp  .sp
1820  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
1821  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 1449  description of the interface to the call Line 1831  description of the interface to the call
1831  \fBpcrecallout\fP  \fBpcrecallout\fP
1832  .\"  .\"
1833  documentation.  documentation.
1834  .P  .
1835  .in 0  .
1836  Last updated: 09 September 2004  .SH "SEE ALSO"
1837  .br  .rs
1838  Copyright (c) 1997-2004 University of Cambridge.  .sp
1839    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1840    .
1841    .
1842    .SH AUTHOR
1843    .rs
1844    .sp
1845    .nf
1846    Philip Hazel
1847    University Computing Service
1848    Cambridge CB2 3QH, England.
1849    .fi
1850    .
1851    .
1852    .SH REVISION
1853    .rs
1854    .sp
1855    .nf
1856    Last updated: 09 May 2007
1857    Copyright (c) 1997-2007 University of Cambridge.
1858    .fi

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