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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"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions supported by PCRE are
8  described below. Regular expressions are also described in the Perl  described below. Regular expressions are also described in the Perl
9  documentation and in a number of other books, some of which have copious  documentation and in a number of books, some of which have copious examples.
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in great detail. This description of PCRE's regular
12  reference documentation.  expressions is intended as reference material.
13    .P
14  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
15  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
19  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
20  .\" </a>  .\" </a>
21  section on UTF-8 support  section on UTF-8 support
22  .\"  .\"
23  in the main  in the main
24  .\" HREF  .\" HREF
25  \fBpcre\fR  \fBpcre\fP
26  .\"  .\"
27  page.  page.
28    .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    .P
40  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
41  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
42  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
43    .sp
44    The quick brown fox    The quick brown fox
45    .sp
46  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
47  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
48  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
49  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
50    always possible. For characters with higher values, the concept of case is
51    supported if PCRE is compiled with Unicode property support, but not otherwise.
52    If you want to use caseless matching for characters 128 and above, you must
53    ensure that PCRE is compiled with Unicode property support as well as with
54    UTF-8 support.
55    .P
56    The power of regular expressions comes from the ability to include alternatives
57    and repetitions in the pattern. These are encoded in the pattern by the use of
58    \fImetacharacters\fP, which do not stand for themselves but instead are
59  interpreted in some special way.  interpreted in some special way.
60    .P
61  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
62  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
63  recognized in square brackets. Outside square brackets, the meta-characters are  recognized in square brackets. Outside square brackets, the metacharacters are
64  as follows:  as follows:
65    .sp
66    \\      general escape character with several uses    \e      general escape character with several uses
67    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
68    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
69    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 78  as follows:
78    +      1 or more quantifier    +      1 or more quantifier
79           also "possessive quantifier"           also "possessive quantifier"
80    {      start min/max quantifier    {      start min/max quantifier
81    .sp
82  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
83  a character class the only meta-characters are:  a character class the only metacharacters are:
84    .sp
85    \\      general escape character    \e      general escape character
86    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
87    -      indicates character range    -      indicates character range
88    .\" JOIN
89    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
90             syntax)             syntax)
91    ]      terminates the character class    ]      terminates the character class
92    .sp
93  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
94    .
95  .SH BACKSLASH  .SH BACKSLASH
96  .rs  .rs
97  .sp  .sp
98  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
99  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character may
100  have. This use of backslash as an escape character applies both inside and  have. This use of backslash as an escape character applies both inside and
101  outside character classes.  outside character classes.
102    .P
103  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
104  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
105  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
106  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
107  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
108    .P
109  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
110  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
111  a character class and the next newline character are ignored. An escaping  a character class and the next newline character are ignored. An escaping
112  backslash can be used to include a whitespace or # character as part of the  backslash can be used to include a whitespace or # character as part of the
113  pattern.  pattern.
114    .P
115  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
116  can do so by putting them between \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
117  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
118  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
119    .sp
120    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
121    .sp
122    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
123      \eQabc$xyz\eE        abc$xyz        abc followed by the
124                                        contents of $xyz                                        contents of $xyz
125    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
126    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
127    .sp
128  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
129    .
130    .
131    .\" HTML <a name="digitsafterbackslash"></a>
132    .SS "Non-printing characters"
133    .rs
134    .sp
135  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
136  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
137  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
138  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is usually easier to
139  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
140  represents:  represents:
141    .sp
142    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
143    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
144    \\e        escape (hex 1B)    \ee        escape (hex 1B)
145    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
146    \\n        newline (hex 0A)    \en        newline (hex 0A)
147    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
148    \\t        tab (hex 09)    \et        tab (hex 09)
149    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
150    \\xhh      character with hex code hh    \exhh      character with hex code hh
151    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
152    .sp
153  The precise effect of \\cx 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
154  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.
155  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
156  7B.  7B.
157    .P
158  After \\x, 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
159  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{
160  appear between \\x{ 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
161  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
162  other than hexadecimal digits appear between \\x{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
163  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.
164  \\x will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
165  digits, giving a byte whose value is zero.  with no following digits, giving a character whose value is zero.
166    .P
167  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
168  syntaxes for \\x 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
169  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
170    .P
171  After \\0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. In both cases, if there
172  are fewer than two digits, just those that are present are used. Thus the  are fewer than two digits, just those that are present are used. Thus the
173  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character
174  (code value 7). Make sure you supply two digits after the initial zero if the  (code value 7). Make sure you supply two digits after the initial zero if the
175  character that follows is itself an octal digit.  pattern character that follows is itself an octal digit.
176    .P
177  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.
178  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
179  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
180  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
181  taken as a \fIback reference\fR. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
182  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
183    .\" </a>
184    later,
185    .\"
186    following the discussion of
187    .\" HTML <a href="#subpattern">
188    .\" </a>
189    parenthesized subpatterns.
190    .\"
191    .P
192  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
193  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
194  digits following the backslash, and generates a single byte from the least  digits following the backslash, and generates a single byte from the least
195  significant 8 bits of the value. Any subsequent digits stand for themselves.  significant 8 bits of the value. Any subsequent digits stand for themselves.
196  For example:  For example:
197    .sp
198    \\040   is another way of writing a space    \e040   is another way of writing a space
199    \\40    is the same, provided there are fewer than 40  .\" JOIN
200      \e40    is the same, provided there are fewer than 40
201              previous capturing subpatterns              previous capturing subpatterns
202    \\7     is always a back reference    \e7     is always a back reference
203    \\11    might be a back reference, or another way of  .\" JOIN
204      \e11    might be a back reference, or another way of
205              writing a tab              writing a tab
206    \\011   is always a tab    \e011   is always a tab
207    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
208    \\113   might be a back reference, otherwise the  .\" JOIN
209      \e113   might be a back reference, otherwise the
210              character with octal code 113              character with octal code 113
211    \\377   might be a back reference, otherwise  .\" JOIN
212      \e377   might be a back reference, otherwise
213              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
214    \\81    is either a back reference, or a binary zero  .\" JOIN
215      \e81    is either a back reference, or a binary zero
216              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
217    .sp
218  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
219  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
220    .P
221  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single byte value or a single UTF-8 character
222  (in UTF-8 mode) can be used both inside and outside character classes. In  (in UTF-8 mode) can be used both inside and outside character classes. In
223  addition, inside a character class, the sequence \\b is interpreted as the  addition, inside a character class, the sequence \eb is interpreted as the
224  backspace character (hex 08). Outside a character class it has a different  backspace character (hex 08), and the sequence \eX is interpreted as the
225  meaning (see below).  character "X". Outside a character class, these sequences have different
226    meanings
227  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
228    .\" </a>
229    \\d     any decimal digit  (see below).
230    \\D     any character that is not a decimal digit  .\"
231    \\s     any whitespace character  .
232    \\S     any character that is not a whitespace character  .
233    \\w     any "word" character  .SS "Generic character types"
234    \\W     any "non-word" character  .rs
235    .sp
236    The third use of backslash is for specifying generic character types. The
237    following are always recognized:
238    .sp
239      \ed     any decimal digit
240      \eD     any character that is not a decimal digit
241      \es     any whitespace character
242      \eS     any character that is not a whitespace character
243      \ew     any "word" character
244      \eW     any "non-word" character
245    .sp
246  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
247  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
248    .P
249  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  These character type sequences can appear both inside and outside character
250  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
251    matching point is at the end of the subject string, all of them fail, since
252  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
253  This makes it different from the the POSIX "space" class. The \\s characters  .P
254    For compatibility with Perl, \es does not match the VT character (code 11).
255    This makes it different from the the POSIX "space" class. The \es characters
256  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32).
257    .P
258  A "word" character is any letter or digit or the underscore character, that is,  A "word" character is an underscore or any character less than 256 that is a
259  any character which can be part of a Perl "word". The definition of letters and  letter or digit. The definition of letters and digits is controlled by PCRE's
260  digits is controlled by PCRE's character tables, and may vary if locale-  low-valued character tables, and may vary if locale-specific matching is taking
261  specific matching is taking place (see  place (see
262  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
263  .\" </a>  .\" </a>
264  "Locale support"  "Locale support"
265  .\"  .\"
266  in the  in the
267  .\" HREF  .\" HREF
268  \fBpcreapi\fR  \fBpcreapi\fP
269  .\"  .\"
270  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in the "fr_FR" (French) locale, some character codes
271  than 128 are used for accented letters, and these are matched by \\w.  greater than 128 are used for accented letters, and these are matched by \ew.
272    .P
273  These character type sequences can appear both inside and outside character  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
274  classes. They each match one character of the appropriate type. If the current  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
275  matching point is at the end of the subject string, all of them fail, since  character property support is available. The use of locales with Unicode is
276  there is no character to match.  discouraged.
277    .
278    .
279    .\" HTML <a name="uniextseq"></a>
280    .SS Unicode character properties
281    .rs
282    .sp
283    When PCRE is built with Unicode character property support, three additional
284    escape sequences to match character properties are available when UTF-8 mode
285    is selected. They are:
286    .sp
287      \ep{\fIxx\fP}   a character with the \fIxx\fP property
288      \eP{\fIxx\fP}   a character without the \fIxx\fP property
289      \eX       an extended Unicode sequence
290    .sp
291    The property names represented by \fIxx\fP above are limited to the Unicode
292    script names, the general category properties, and "Any", which matches any
293    character (including newline). Other properties such as "InMusicalSymbols" are
294    not currently supported by PCRE. Note that \eP{Any} does not match any
295    characters, so always causes a match failure.
296    .P
297    Sets of Unicode characters are defined as belonging to certain scripts. A
298    character from one of these sets can be matched using a script name. For
299    example:
300    .sp
301      \ep{Greek}
302      \eP{Han}
303    .sp
304    Those that are not part of an identified script are lumped together as
305    "Common". The current list of scripts is:
306    .P
307    Arabic,
308    Armenian,
309    Bengali,
310    Bopomofo,
311    Braille,
312    Buginese,
313    Buhid,
314    Canadian_Aboriginal,
315    Cherokee,
316    Common,
317    Coptic,
318    Cypriot,
319    Cyrillic,
320    Deseret,
321    Devanagari,
322    Ethiopic,
323    Georgian,
324    Glagolitic,
325    Gothic,
326    Greek,
327    Gujarati,
328    Gurmukhi,
329    Han,
330    Hangul,
331    Hanunoo,
332    Hebrew,
333    Hiragana,
334    Inherited,
335    Kannada,
336    Katakana,
337    Kharoshthi,
338    Khmer,
339    Lao,
340    Latin,
341    Limbu,
342    Linear_B,
343    Malayalam,
344    Mongolian,
345    Myanmar,
346    New_Tai_Lue,
347    Ogham,
348    Old_Italic,
349    Old_Persian,
350    Oriya,
351    Osmanya,
352    Runic,
353    Shavian,
354    Sinhala,
355    Syloti_Nagri,
356    Syriac,
357    Tagalog,
358    Tagbanwa,
359    Tai_Le,
360    Tamil,
361    Telugu,
362    Thaana,
363    Thai,
364    Tibetan,
365    Tifinagh,
366    Ugaritic,
367    Yi.
368    .P
369    Each character has exactly one general category property, specified by a
370    two-letter abbreviation. For compatibility with Perl, negation can be specified
371    by including a circumflex between the opening brace and the property name. For
372    example, \ep{^Lu} is the same as \eP{Lu}.
373    .P
374    If only one letter is specified with \ep or \eP, it includes all the general
375    category properties that start with that letter. In this case, in the absence
376    of negation, the curly brackets in the escape sequence are optional; these two
377    examples have the same effect:
378    .sp
379      \ep{L}
380      \epL
381    .sp
382    The following general category property codes are supported:
383    .sp
384      C     Other
385      Cc    Control
386      Cf    Format
387      Cn    Unassigned
388      Co    Private use
389      Cs    Surrogate
390    .sp
391      L     Letter
392      Ll    Lower case letter
393      Lm    Modifier letter
394      Lo    Other letter
395      Lt    Title case letter
396      Lu    Upper case letter
397    .sp
398      M     Mark
399      Mc    Spacing mark
400      Me    Enclosing mark
401      Mn    Non-spacing mark
402    .sp
403      N     Number
404      Nd    Decimal number
405      Nl    Letter number
406      No    Other number
407    .sp
408      P     Punctuation
409      Pc    Connector punctuation
410      Pd    Dash punctuation
411      Pe    Close punctuation
412      Pf    Final punctuation
413      Pi    Initial punctuation
414      Po    Other punctuation
415      Ps    Open punctuation
416    .sp
417      S     Symbol
418      Sc    Currency symbol
419      Sk    Modifier symbol
420      Sm    Mathematical symbol
421      So    Other symbol
422    .sp
423      Z     Separator
424      Zl    Line separator
425      Zp    Paragraph separator
426      Zs    Space separator
427    .sp
428    The special property L& is also supported: it matches a character that has
429    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
430    a modifier or "other".
431    .P
432    The long synonyms for these properties that Perl supports (such as \ep{Letter})
433    are not supported by PCRE. Nor is is permitted to prefix any of these
434    properties with "Is".
435    .P
436    No character that is in the Unicode table has the Cn (unassigned) property.
437    Instead, this property is assumed for any code point that is not in the
438    Unicode table.
439    .P
440    Specifying caseless matching does not affect these escape sequences. For
441    example, \ep{Lu} always matches only upper case letters.
442    .P
443    The \eX escape matches any number of Unicode characters that form an extended
444    Unicode sequence. \eX is equivalent to
445    .sp
446      (?>\ePM\epM*)
447    .sp
448    That is, it matches a character without the "mark" property, followed by zero
449    or more characters with the "mark" property, and treats the sequence as an
450    atomic group
451    .\" HTML <a href="#atomicgroup">
452    .\" </a>
453    (see below).
454    .\"
455    Characters with the "mark" property are typically accents that affect the
456    preceding character.
457    .P
458    Matching characters by Unicode property is not fast, because PCRE has to search
459    a structure that contains data for over fifteen thousand characters. That is
460    why the traditional escape sequences such as \ed and \ew do not use Unicode
461    properties in PCRE.
462    .
463    .
464    .\" HTML <a name="smallassertions"></a>
465    .SS "Simple assertions"
466    .rs
467    .sp
468  The fourth use of backslash is for certain simple assertions. An assertion  The fourth use of backslash is for certain simple assertions. An assertion
469  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,
470  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
471  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
472  assertions are  .\" HTML <a href="#bigassertions">
473    .\" </a>
474    \\b     matches at a word boundary  below.
475    \\B     matches when not at a word boundary  .\"
476    \\A     matches at start of subject  The backslashed
477    \\Z     matches at end of subject or before newline at end  assertions are:
478    \\z     matches at end of subject  .sp
479    \\G     matches at first matching position in subject    \eb     matches at a word boundary
480      \eB     matches when not at a word boundary
481  These assertions may not appear in character classes (but note that \\b has a    \eA     matches at start of subject
482      \eZ     matches at end of subject or before newline at end
483      \ez     matches at end of subject
484      \eG     matches at first matching position in subject
485    .sp
486    These assertions may not appear in character classes (but note that \eb has a
487  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
488    .P
489  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
490  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
491  \\w and the other matches \\W), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
492  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
493    .P
494  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
495  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
496  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
497  multiline mode.  independent of multiline mode. These three assertions are not affected by the
498    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
499  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
500  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
501  matching is to start at a point other than the beginning of the subject, \\A  at a point other than the beginning of the subject, \eA can never match. The
502  can never match. The difference between \\Z and \\z is that \\Z matches before  difference between \eZ and \ez is that \eZ matches before a newline that is the
503  a newline that is the last character of the string as well as at the end of the  last character of the string as well as at the end of the string, whereas \ez
504  string, whereas \\z matches only at the end.  matches only at the end.
505    .P
506  The \\G 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
507  start point of the match, as specified by the \fIstartoffset\fR argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
508  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
509  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
510  arguments, you can mimic Perl's /g option, and it is in this kind of  arguments, you can mimic Perl's /g option, and it is in this kind of
511  implementation where \\G can be useful.  implementation where \eG can be useful.
512    .P
513  Note, however, that PCRE's interpretation of \\G, as the start of the current  Note, however, that PCRE's interpretation of \eG, as the start of the current
514  match, is subtly different from Perl's, which defines it as the end of the  match, is subtly different from Perl's, which defines it as the end of the
515  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
516  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
517  reproduce this behaviour.  reproduce this behaviour.
518    .P
519  If all the alternatives of a pattern begin with \\G, the expression is anchored  If all the alternatives of a pattern begin with \eG, the expression is anchored
520  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
521  regular expression.  regular expression.
522    .
523  .SH CIRCUMFLEX AND DOLLAR  .
524    .SH "CIRCUMFLEX AND DOLLAR"
525  .rs  .rs
526  .sp  .sp
527  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
528  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
529  at the start of the subject string. If the \fIstartoffset\fR argument of  at the start of the subject string. If the \fIstartoffset\fP argument of
530  \fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
531  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
532  meaning (see below).  meaning
533    .\" HTML <a href="#characterclass">
534    .\" </a>
535    (see below).
536    .\"
537    .P
538  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
539  alternatives are involved, but it should be the first thing in each alternative  alternatives are involved, but it should be the first thing in each alternative
540  in which it appears if the pattern is ever to match that branch. If all  in which it appears if the pattern is ever to match that branch. If all
# Line 290  possible alternatives start with a circu Line 542  possible alternatives start with a circu
542  constrained to match only at the start of the subject, it is said to be an  constrained to match only at the start of the subject, it is said to be an
543  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
544  to be anchored.)  to be anchored.)
545    .P
546  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
547  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
548  character that is the last character in the string (by default). Dollar need  character that is the last character in the string (by default). Dollar need
549  not be the last character of the pattern if a number of alternatives are  not be the last character of the pattern if a number of alternatives are
550  involved, but it should be the last item in any branch in which it appears.  involved, but it should be the last item in any branch in which it appears.
551  Dollar has no special meaning in a character class.  Dollar has no special meaning in a character class.
552    .P
553  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
554  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
555  does not affect the \\Z assertion.  does not affect the \eZ assertion.
556    .P
557  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
558  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, they match immediately
559  after and immediately before an internal newline character, respectively, in  after and immediately before an internal newline character, respectively, in
560  addition to matching at the start and end of the subject string. For example,  addition to matching at the start and end of the subject string. For example,
561  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  the pattern /^abc$/ matches the subject string "def\enabc" (where \en
562  but not otherwise. Consequently, patterns that are anchored in single line mode  represents a newline character) in multiline mode, but not otherwise.
563  because all branches start with ^ are not anchored in multiline mode, and a  Consequently, patterns that are anchored in single line mode because all
564  match for circumflex is possible when the \fIstartoffset\fR argument of  branches start with ^ are not anchored in multiline mode, and a match for
565  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP
566  PCRE_MULTILINE is set.  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
567    set.
568  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  .P
569    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
570  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
571  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether PCRE_MULTILINE is set or not.
572    .
573  .SH FULL STOP (PERIOD, DOT)  .
574    .SH "FULL STOP (PERIOD, DOT)"
575  .rs  .rs
576  .sp  .sp
577  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
578  the subject, including a non-printing character, but not (by default) newline.  the subject, including a non-printing character, but not (by default) newline.
579  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
580  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  byte long, except (by default) newline. If the PCRE_DOTALL option is set,
581  dots match newlines as well. The handling of dot is entirely independent of the  dots match newlines as well. The handling of dot is entirely independent of the
582  handling of circumflex and dollar, the only relationship being that they both  handling of circumflex and dollar, the only relationship being that they both
583  involve newline characters. Dot has no special meaning in a character class.  involve newline characters. Dot has no special meaning in a character class.
584    .
585  .SH MATCHING A SINGLE BYTE  .
586    .SH "MATCHING A SINGLE BYTE"
587  .rs  .rs
588  .sp  .sp
589  Outside a character class, the escape sequence \\C matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
590  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
591  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  provided in Perl in order to match individual bytes in UTF-8 mode. Because it
592  Because it breaks up UTF-8 characters into individual bytes, what remains in  breaks up UTF-8 characters into individual bytes, what remains in the string
593  the string may be a malformed UTF-8 string. For this reason it is best avoided.  may be a malformed UTF-8 string. For this reason, the \eC escape sequence is
594    best avoided.
595  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  .P
596  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  PCRE does not allow \eC to appear in lookbehind assertions
597    .\" HTML <a href="#lookbehind">
598  .SH SQUARE BRACKETS  .\" </a>
599    (described below),
600    .\"
601    because in UTF-8 mode this would make it impossible to calculate the length of
602    the lookbehind.
603    .
604    .
605    .\" HTML <a name="characterclass"></a>
606    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
607  .rs  .rs
608  .sp  .sp
609  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
# Line 348  square bracket. A closing square bracket Line 611  square bracket. A closing square bracket
611  closing square bracket is required as a member of the class, it should be the  closing square bracket is required as a member of the class, it should be the
612  first data character in the class (after an initial circumflex, if present) or  first data character in the class (after an initial circumflex, if present) or
613  escaped with a backslash.  escaped with a backslash.
614    .P
615  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
616  character may occupy more than one byte. A matched character must be in the set  character may occupy more than one byte. A matched character must be in the set
617  of characters defined by the class, unless the first character in the class  of characters defined by the class, unless the first character in the class
# Line 356  definition is a circumflex, in which cas Line 619  definition is a circumflex, in which cas
619  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
620  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
621  backslash.  backslash.
622    .P
623  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
624  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
625  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
626  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
627  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion: it still consumes a character from the subject
628  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
629    string.
630    .P
631  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
632  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  class as a literal string of bytes, or by using the \ex{ escaping mechanism.
633    .P
634  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
635  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
636  "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
637  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
638  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
639    always possible. For characters with higher values, the concept of case is
640    supported if PCRE is compiled with Unicode property support, but not otherwise.
641    If you want to use caseless matching for characters 128 and above, you must
642    ensure that PCRE is compiled with Unicode property support as well as with
643    UTF-8 support.
644    .P
645  The newline character is never treated in any special way in character classes,  The newline character is never treated in any special way in character classes,
646  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
647  such as [^a] will always match a newline.  such as [^a] will always match a newline.
648    .P
649  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
650  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,
651  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
652  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
653  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class.
654    .P
655  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
656  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
657  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
658  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
659  the end of range, so [W-\\]46] is interpreted as a single class containing a  the end of range, so [W-\e]46] is interpreted as a class containing a range
660  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
661  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
662    .P
663  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
664  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
665  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
666  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
667    .P
668  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
669  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
670  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
671  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E
672    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
673  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a  characters with values greater than 128 only when it is compiled with Unicode
674  character class, and add the characters that they match to the class. For  property support.
675  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
676    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
677    in a character class, and add the characters that they match to the class. For
678    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
679  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
680  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
681  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
682    .P
683  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
684  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
685  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
686    introducing a POSIX class name - see the next section), and the terminating
687  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
688    does no harm.
689    .
690    .
691    .SH "POSIX CHARACTER CLASSES"
692  .rs  .rs
693  .sp  .sp
694  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
695  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
696  this notation. For example,  this notation. For example,
697    .sp
698    [01[:alpha:]%]    [01[:alpha:]%]
699    .sp
700  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
701  are  are
702    .sp
703    alnum    letters and digits    alnum    letters and digits
704    alpha    letters    alpha    letters
705    ascii    character codes 0 - 127    ascii    character codes 0 - 127
706    blank    space or tab only    blank    space or tab only
707    cntrl    control characters    cntrl    control characters
708    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
709    graph    printing characters, excluding space    graph    printing characters, excluding space
710    lower    lower case letters    lower    lower case letters
711    print    printing characters, including space    print    printing characters, including space
712    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
713    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
714    upper    upper case letters    upper    upper case letters
715    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
716    xdigit   hexadecimal digits    xdigit   hexadecimal digits
717    .sp
718  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
719  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
720  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
721  compatibility).  compatibility).
722    .P
723  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
724  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
725  after the colon. For example,  after the colon. For example,
726    .sp
727    [12[:^digit:]]    [12[:^digit:]]
728    .sp
729  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
730  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
731  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
732    .P
733  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
734  the POSIX character classes.  the POSIX character classes.
735    .
736  .SH VERTICAL BAR  .
737    .SH "VERTICAL BAR"
738  .rs  .rs
739  .sp  .sp
740  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
741  the pattern  the pattern
742    .sp
743    gilbert|sullivan    gilbert|sullivan
744    .sp
745  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
746  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string).
747  The matching process tries each alternative in turn, from left to right,  The matching process tries each alternative in turn, from left to right,
748  and the first one that succeeds is used. If the alternatives are within a  and the first one that succeeds is used. If the alternatives are within a
749  subpattern (defined below), "succeeds" means matching the rest of the main  subpattern
750  pattern as well as the alternative in the subpattern.  .\" HTML <a href="#subpattern">
751    .\" </a>
752  .SH INTERNAL OPTION SETTING  (defined below),
753    .\"
754    "succeeds" means matching the rest of the main pattern as well as the
755    alternative in the subpattern.
756    .
757    .
758    .SH "INTERNAL OPTION SETTING"
759  .rs  .rs
760  .sp  .sp
761  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
762  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
763  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
764    .sp
765    i  for PCRE_CASELESS    i  for PCRE_CASELESS
766    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
767    s  for PCRE_DOTALL    s  for PCRE_DOTALL
768    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
769    .sp
770  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
771  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
772  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
773  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
774  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
775  unset.  unset.
776    .P
777  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
778  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
779  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
780  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
781  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
782    .P
783  An option change within a subpattern affects only that part of the current  An option change within a subpattern affects only that part of the current
784  pattern that follows it, so  pattern that follows it, so
785    .sp
786    (a(?i)b)c    (a(?i)b)c
787    .sp
788  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
789  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
790  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
791  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
792    .sp
793    (a(?i)b|c)    (a(?i)b|c)
794    .sp
795  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
796  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
797  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
798  behaviour otherwise.  behaviour otherwise.
799    .P
800  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
801  same way as the Perl-compatible options by using the characters U and X  same way as the Perl-compatible options by using the characters U and X
802  respectively. The (?X) flag setting is special in that it must always occur  respectively. The (?X) flag setting is special in that it must always occur
803  earlier in the pattern than any of the additional features it turns on, even  earlier in the pattern than any of the additional features it turns on, even
804  when it is at top level. It is best put at the start.  when it is at top level. It is best to put it at the start.
805    .
806    .
807    .\" HTML <a name="subpattern"></a>
808  .SH SUBPATTERNS  .SH SUBPATTERNS
809  .rs  .rs
810  .sp  .sp
811  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
812  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
813    .sp
814  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
815    .sp
816    cat(aract|erpillar|)    cat(aract|erpillar|)
817    .sp
818  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
819  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or the empty string.
820    .sp
821  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
822  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
823  the subpattern is passed back to the caller via the \fIovector\fR argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
824  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
825  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
826    .P
827  For example, if the string "the red king" is matched against the pattern  For example, if the string "the red king" is matched against the pattern
828    .sp
829    the ((red|white) (king|queen))    the ((red|white) (king|queen))
830    .sp
831  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
832  2, and 3, respectively.  2, and 3, respectively.
833    .P
834  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
835  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
836  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
837  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
838  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
839  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
840    .sp
841    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
842    .sp
843  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
844  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535, and the maximum depth
845  of nesting of all subpatterns, both capturing and non-capturing, is 200.  of nesting of all subpatterns, both capturing and non-capturing, is 200.
846    .P
847  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
848  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
849  the ":". Thus the two patterns  the ":". Thus the two patterns
850    .sp
851    (?i:saturday|sunday)    (?i:saturday|sunday)
852    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
853    .sp
854  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
855  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
856  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
857  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
858    .
859  .SH NAMED SUBPATTERNS  .
860    .SH "NAMED SUBPATTERNS"
861  .rs  .rs
862  .sp  .sp
863  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
864  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
865  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
866  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns, something that Perl does
867  not provide. The Python syntax (?P<name>...) is used. Names consist of  not provide. The Python syntax (?P<name>...) is used. Names consist of
868  alphanumeric characters and underscores, and must be unique within a pattern.  alphanumeric characters and underscores, and must be unique within a pattern.
869    .P
870  Named capturing parentheses are still allocated numbers as well as names. The  Named capturing parentheses are still allocated numbers as well as names. The
871  PCRE API provides function calls for extracting the name-to-number translation  PCRE API provides function calls for extracting the name-to-number translation
872  table from a compiled pattern. For further details see the  table from a compiled pattern. There is also a convenience function for
873    extracting a captured substring by name. For further details see the
874  .\" HREF  .\" HREF
875  \fBpcreapi\fR  \fBpcreapi\fP
876  .\"  .\"
877  documentation.  documentation.
878    .
879    .
880  .SH REPETITION  .SH REPETITION
881  .rs  .rs
882  .sp  .sp
883  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
884  items:  items:
885    .sp
886    a literal data character    a literal data character
887    the . metacharacter    the . metacharacter
888    the \\C escape sequence    the \eC escape sequence
889    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
890      an escape such as \ed that matches a single character
891    a character class    a character class
892    a back reference (see next section)    a back reference (see next section)
893    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
894    .sp
895  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
896  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
897  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
898  be less than or equal to the second. For example:  be less than or equal to the second. For example:
899    .sp
900    z{2,4}    z{2,4}
901    .sp
902  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
903  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
904  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
905  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
906    .sp
907    [aeiou]{3,}    [aeiou]{3,}
908    .sp
909  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
910    .sp
911    \\d{8}    \ed{8}
912    .sp
913  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
914  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
915  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
916  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
917    .P
918  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
919  bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
920  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
921    support is available, \eX{3} matches three Unicode extended sequences, each of
922    which may be several bytes long (and they may be of different lengths).
923    .P
924  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
925  previous item and the quantifier were not present.  previous item and the quantifier were not present.
926    .P
927  For convenience (and historical compatibility) the three most common  For convenience (and historical compatibility) the three most common
928  quantifiers have single-character abbreviations:  quantifiers have single-character abbreviations:
929    .sp
930    *    is equivalent to {0,}    *    is equivalent to {0,}
931    +    is equivalent to {1,}    +    is equivalent to {1,}
932    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
933    .sp
934  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
935  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
936    .sp
937    (a?)*    (a?)*
938    .sp
939  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
940  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
941  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
942  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
943    .P
944  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
945  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
946  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
947  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
948  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
949  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
950    .sp
951    /\\*.*\\*/    /\e*.*\e*/
952    .sp
953  to the string  to the string
954    .sp
955    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
956    .sp
957  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
958  item.  item.
959    .P
960  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
961  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
962  pattern  pattern
963    .sp
964    /\\*.*?\\*/    /\e*.*?\e*/
965    .sp
966  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
967  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
968  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
969  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
970    .sp
971    \\d??\\d    \ed??\ed
972    .sp
973  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
974  way the rest of the pattern matches.  way the rest of the pattern matches.
975    .P
976  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
977  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
978  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
979  default behaviour.  default behaviour.
980    .P
981  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
982  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
983  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
984    .P
985  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
986  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
987  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
988  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
989  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
990  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
991    .P
992  In cases where it is known that the subject string contains no newlines, it is  In cases where it is known that the subject string contains no newlines, it is
993  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
994  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
995    .P
996  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
997  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
998  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, and a later one
999  succeed. Consider, for example:  succeed. Consider, for example:
1000    .sp
1001    (.*)abc\\1    (.*)abc\e1
1002    .sp
1003  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1004  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1005    .P
1006  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1007  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1008    .sp
1009    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1010    .sp
1011  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1012  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1013  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1014  example, after  example, after
1015    .sp
1016    /(a|(b))+/    /(a|(b))+/
1017    .sp
1018  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1019    .
1020  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1021    .\" HTML <a name="atomicgroup"></a>
1022    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1023  .rs  .rs
1024  .sp  .sp
1025  With both maximizing and minimizing repetition, failure of what follows  With both maximizing and minimizing repetition, failure of what follows
# Line 735  number of repeats allows the rest of the Line 1028  number of repeats allows the rest of the
1028  useful to prevent this, either to change the nature of the match, or to cause  useful to prevent this, either to change the nature of the match, or to cause
1029  it fail earlier than it otherwise might, when the author of the pattern knows  it fail earlier than it otherwise might, when the author of the pattern knows
1030  there is no point in carrying on.  there is no point in carrying on.
1031    .P
1032  Consider, for example, the pattern \\d+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1033    .sp
1034    123456bar    123456bar
1035    .sp
1036  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1037  action of the matcher is to try again with only 5 digits matching the \\d+  action of the matcher is to try again with only 5 digits matching the \ed+
1038  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1039  (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
1040  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.
1041    .P
1042  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 would give up
1043  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
1044  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1045    .sp
1046    (?>\\d+)bar    (?>\ed+)foo
1047    .sp
1048  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1049  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1050  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1051  normal.  normal.
1052    .P
1053  An alternative description is that a subpattern of this type matches the string  An alternative description is that a subpattern of this type matches the string
1054  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1055  the current point in the subject string.  the current point in the subject string.
1056    .P
1057  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1058  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1059  everything it can. So, while both \\d+ and \\d+? are prepared to adjust the  everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1060  number of digits they match in order to make the rest of the pattern match,  number of digits they match in order to make the rest of the pattern match,
1061  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1062    .P
1063  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1064  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1065  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1066  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1067  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1068  previous example can be rewritten as  previous example can be rewritten as
1069    .sp
1070    \\d++bar    \ed++foo
1071    .sp
1072  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1073  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
1074  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning or processing of a
1075  possessive quantifier and the equivalent atomic group.  possessive quantifier and the equivalent atomic group.
1076    .P
1077  The possessive quantifier syntax is an extension to the Perl syntax. It  The possessive quantifier syntax is an extension to the Perl syntax. It
1078  originates in Sun's Java package.  originates in Sun's Java package.
1079    .P
1080  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
1081  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
1082  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1083  pattern  pattern
1084    .sp
1085    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1086    .sp
1087  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1088  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1089  quickly. However, if it is applied to  quickly. However, if it is applied to
1090    .sp
1091    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1092    .sp
1093  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1094  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \eD+ repeat and the external * repeat in a
1095  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1096  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1097  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1098  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1099  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1100    an atomic group, like this:
1101    ((?>\\D+)|<\\d+>)*[!?]  .sp
1102      ((?>\eD+)|<\ed+>)*[!?]
1103    .sp
1104  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1105    .
1106  .SH BACK REFERENCES  .
1107    .\" HTML <a name="backreferences"></a>
1108    .SH "BACK REFERENCES"
1109  .rs  .rs
1110  .sp  .sp
1111  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1112  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1113  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1114  previous capturing left parentheses.  previous capturing left parentheses.
1115    .P
1116  However, if the decimal number following the backslash is less than 10, it is  However, if the decimal number following the backslash is less than 10, it is
1117  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
1118  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
1119  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
1120  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. See the subsection entitled "Non-printing characters"
1121  details of the handling of digits following a backslash.  .\" HTML <a href="#digitsafterbackslash">
1122    .\" </a>
1123    above
1124    .\"
1125    for further details of the handling of digits following a backslash.
1126    .P
1127  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1128  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1129  itself (see  itself (see
# Line 832  itself (see Line 1132  itself (see
1132  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1133  .\"  .\"
1134  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1135    .sp
1136    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1137    .sp
1138  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1139  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1140  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1141    .sp
1142    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1143    .sp
1144  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
1145  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1146    .P
1147  Back references to named subpatterns use the Python syntax (?P=name). We could  Back references to named subpatterns use the Python syntax (?P=name). We could
1148  rewrite the above example as follows:  rewrite the above example as follows:
1149    .sp
1150    (?<p1>(?i)rah)\\s+(?P=p1)    (?<p1>(?i)rah)\es+(?P=p1)
1151    .sp
1152  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
1153  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1154  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1155    .sp
1156    (a|(bc))\\2    (a|(bc))\e2
1157    .sp
1158  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". Because there may be
1159  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1160  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1161  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1162  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1163  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1164    .\" HTML <a href="#comments">
1165    .\" </a>
1166    "Comments"
1167    .\"
1168    below) can be used.
1169    .P
1170  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1171  when the subpattern is first used, so, for example, (a\\1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1172  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1173  example, the pattern  example, the pattern
1174    .sp
1175    (a|b\\1)+    (a|b\e1)+
1176    .sp
1177  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1178  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1179  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1180  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1181  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1182  minimum of zero.  minimum of zero.
1183    .
1184    .
1185    .\" HTML <a name="bigassertions"></a>
1186  .SH ASSERTIONS  .SH ASSERTIONS
1187  .rs  .rs
1188  .sp  .sp
1189  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1190  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1191  assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.  assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1192    .\" HTML <a href="#smallassertions">
1193    .\" </a>
1194    above.
1195    .\"
1196    .P
1197  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1198  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1199  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1200    except that it does not cause the current matching position to be changed.
1201  An assertion subpattern is matched in the normal way, except that it does not  .P
1202  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1203    because it makes no sense to assert the same thing several times. If any kind
1204    of assertion contains capturing subpatterns within it, these are counted for
1205    the purposes of numbering the capturing subpatterns in the whole pattern.
1206    However, substring capturing is carried out only for positive assertions,
1207    because it does not make sense for negative assertions.
1208    .
1209    .
1210    .SS "Lookahead assertions"
1211    .rs
1212    .sp
1213    Lookahead assertions start
1214  with (?= for positive assertions and (?! for negative assertions. For example,  with (?= for positive assertions and (?! for negative assertions. For example,
1215    .sp
1216    \\w+(?=;)    \ew+(?=;)
1217    .sp
1218  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1219  the match, and  the match, and
1220    .sp
1221    foo(?!bar)    foo(?!bar)
1222    .sp
1223  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1224  apparently similar pattern  apparently similar pattern
1225    .sp
1226    (?!foo)bar    (?!foo)bar
1227    .sp
1228  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1229  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1230  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1231  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1232    .P
1233  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1234  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1235  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1236    .
1237    .
1238    .\" HTML <a name="lookbehind"></a>
1239    .SS "Lookbehind assertions"
1240    .rs
1241    .sp
1242  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1243  negative assertions. For example,  negative assertions. For example,
1244    .sp
1245    (?<!foo)bar    (?<!foo)bar
1246    .sp
1247  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
1248  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
1249  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several alternatives, they do not
1250  all have to have the same fixed length. Thus  all have to have the same fixed length. Thus
1251    .sp
1252    (?<=bullock|donkey)    (?<=bullock|donkey)
1253    .sp
1254  is permitted, but  is permitted, but
1255    .sp
1256    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1257    .sp
1258  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1259  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1260  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1261  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1262    .sp
1263    (?<=ab(c|de))    (?<=ab(c|de))
1264    .sp
1265  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1266  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
1267    .sp
1268    (?<=abc|abde)    (?<=abc|abde)
1269    .sp
1270  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1271  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed width and then try to
1272  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1273  match is deemed to fail.  match is deemed to fail.
1274    .P
1275  PCRE does not allow the \\C 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)
1276  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1277  the length of the lookbehind.  the length of the lookbehind. The \eX escape, which can match different numbers
1278    of bytes, is also not permitted.
1279    .P
1280  Atomic groups can be used in conjunction with lookbehind assertions to specify  Atomic groups can be used in conjunction with lookbehind assertions to specify
1281  efficient matching at the end of the subject string. Consider a simple pattern  efficient matching at the end of the subject string. Consider a simple pattern
1282  such as  such as
1283    .sp
1284    abcd$    abcd$
1285    .sp
1286  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1287  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1288  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
1289    .sp
1290    ^.*abcd$    ^.*abcd$
1291    .sp
1292  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1293  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1294  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1295  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,
1296  if the pattern is written as  if the pattern is written as
1297    .sp
1298    ^(?>.*)(?<=abcd)    ^(?>.*)(?<=abcd)
1299    .sp
1300  or, equivalently,  or, equivalently, using the possessive quantifier syntax,
1301    .sp
1302    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1303    .sp
1304  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
1305  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
1306  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1307  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1308    .
1309    .
1310    .SS "Using multiple assertions"
1311    .rs
1312    .sp
1313  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1314    .sp
1315    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1316    .sp
1317  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1318  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1319  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1320  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1321  This pattern does \fInot\fR match "foo" preceded by six characters, the first  This pattern does \fInot\fP match "foo" preceded by six characters, the first
1322  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1323  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1324    .sp
1325    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1326    .sp
1327  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1328  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1329  preceding three characters are not "999".  preceding three characters are not "999".
1330    .P
1331  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1332    .sp
1333    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1334    .sp
1335  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1336  preceded by "foo", while  preceded by "foo", while
1337    .sp
1338    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1339    .sp
1340  is another pattern which matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
1341  characters that are not "999".  characters that are not "999".
1342    .
1343  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1344  because it makes no sense to assert the same thing several times. If any kind  .SH "CONDITIONAL SUBPATTERNS"
 of assertion contains capturing subpatterns within it, these are counted for  
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
   
 .SH CONDITIONAL SUBPATTERNS  
1345  .rs  .rs
1346  .sp  .sp
1347  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1348  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1349  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1350  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1351    .sp
1352    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1353    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1354    .sp
1355  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1356  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
1357  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1358    .P
1359  There are three kinds of condition. If the text between the parentheses  There are three kinds of condition. If the text between the parentheses
1360  consists of a sequence of digits, the condition is satisfied if the capturing  consists of a sequence of digits, the condition is satisfied if the capturing
1361  subpattern of that number has previously matched. The number must be greater  subpattern of that number has previously matched. The number must be greater
1362  than zero. Consider the following pattern, which contains non-significant white  than zero. Consider the following pattern, which contains non-significant white
1363  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  space to make it more readable (assume the PCRE_EXTENDED option) and to divide
1364  it into three parts for ease of discussion:  it into three parts for ease of discussion:
1365    .sp
1366    ( \\( )?    [^()]+    (?(1) \\) )    ( \e( )?    [^()]+    (?(1) \e) )
1367    .sp
1368  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1369  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
1370  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
# Line 1047  the condition is true, and so the yes-pa Line 1374  the condition is true, and so the yes-pa
1374  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1375  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1376  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1377    .P
1378  If the condition is the string (R), it is satisfied if a recursive call to the  If the condition is the string (R), it is satisfied if a recursive call to the
1379  pattern or subpattern has been made. At "top level", the condition is false.  pattern or subpattern has been made. At "top level", the condition is false.
1380  This is a PCRE extension. Recursive patterns are described in the next section.  This is a PCRE extension. Recursive patterns are described in the next section.
1381    .P
1382  If the condition is not a sequence of digits or (R), it must be an assertion.  If the condition is not a sequence of digits or (R), it must be an assertion.
1383  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1384  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1385  alternatives on the second line:  alternatives on the second line:
1386    .sp
1387    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1388    \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )    \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
1389    .sp
1390  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1391  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1392  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1393  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1394  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1395  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1396    .
1397    .
1398    .\" HTML <a name="comments"></a>
1399  .SH COMMENTS  .SH COMMENTS
1400  .rs  .rs
1401  .sp  .sp
1402  The sequence (?# marks the start of a comment which continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1403  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1404  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.
1405    .P
1406  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1407  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues up to the next newline
1408  character in the pattern.  character in the pattern.
1409    .
1410  .SH RECURSIVE PATTERNS  .
1411    .SH "RECURSIVE PATTERNS"
1412  .rs  .rs
1413  .sp  .sp
1414  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1415  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1416  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
1417  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth. Perl provides a facility
1418  experimental facility that allows regular expressions to recurse (amongst other  that allows regular expressions to recurse (amongst other things). It does this
1419  things). It does this by interpolating Perl code in the expression at run time,  by interpolating Perl code in the expression at run time, and the code can
1420  and the code can refer to the expression itself. A Perl pattern to solve the  refer to the expression itself. A Perl pattern to solve the parentheses problem
1421  parentheses problem can be created like this:  can be created like this:
1422    .sp
1423    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1424    .sp
1425  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
1426  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears. Obviously, PCRE cannot support
1427  the interpolation of Perl code. Instead, it supports some special syntax for  the interpolation of Perl code. Instead, it supports some special syntax for
1428  recursion of the entire pattern, and also for individual subpattern recursion.  recursion of the entire pattern, and also for individual subpattern recursion.
1429    .P
1430  The special item that consists of (? followed by a number greater than zero and  The special item that consists of (? followed by a number greater than zero and
1431  a closing parenthesis is a recursive call of the subpattern of the given  a closing parenthesis is a recursive call of the subpattern of the given
1432  number, provided that it occurs inside that subpattern. (If not, it is a  number, provided that it occurs inside that subpattern. (If not, it is a
1433  "subroutine" call, which is described in the next section.) The special item  "subroutine" call, which is described in the next section.) The special item
1434  (?R) is a recursive call of the entire regular expression.  (?R) is a recursive call of the entire regular expression.
1435    .P
1436  For example, this PCRE pattern solves the nested parentheses problem (assume  A recursive subpattern call is always treated as an atomic group. That is, once
1437  the PCRE_EXTENDED option is set so that white space is ignored):  it has matched some of the subject string, it is never re-entered, even if
1438    it contains untried alternatives and there is a subsequent matching failure.
1439    \\( ( (?>[^()]+) | (?R) )* \\)  .P
1440    This PCRE pattern solves the nested parentheses problem (assume the
1441    PCRE_EXTENDED option is set so that white space is ignored):
1442    .sp
1443      \e( ( (?>[^()]+) | (?R) )* \e)
1444    .sp
1445  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1446  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
1447  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1448  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1449    .P
1450  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
1451  pattern, so instead you could use this:  pattern, so instead you could use this:
1452    .sp
1453    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1454    .sp
1455  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
1456  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
1457  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
1458  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  parentheses instead. For this, PCRE uses (?P>name), which is an extension to
1459  the Python syntax that PCRE uses for named parentheses (Perl does not provide  the Python syntax that PCRE uses for named parentheses (Perl does not provide
1460  named parentheses). We could rewrite the above example as follows:  named parentheses). We could rewrite the above example as follows:
1461    .sp
1462    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
1463    .sp
1464  This particular example pattern contains nested unlimited repeats, and so the  This particular example pattern contains nested unlimited repeats, and so the
1465  use of atomic grouping for matching strings of non-parentheses is important  use of atomic grouping for matching strings of non-parentheses is important
1466  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
1467  pattern is applied to  pattern is applied to
1468    .sp
1469    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1470    .sp
1471  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1472  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
1473  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
1474  before failure can be reported.  before failure can be reported.
1475    .P
1476  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
1477  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.
1478  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
1479  below and the  the next section and the
1480  .\" HREF  .\" HREF
1481  \fBpcrecallout\fR  \fBpcrecallout\fP
1482  .\"  .\"
1483  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1484    .sp
1485    (ab(cd)ef)    (ab(cd)ef)
1486    .sp
1487  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1488  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1489    .sp
1490    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1491       ^                        ^       ^                        ^
1492       ^                        ^       ^                        ^
1493    .sp
1494  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1495  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1496  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
1497  using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1498  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1499    .P
1500  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
1501  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1502  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1503  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1504    .sp
1505    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1506    .sp
1507  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
1508  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1509  is the actual recursive call.  is the actual recursive call.
1510    .
1511    .
1512  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1513  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1514  .rs  .rs
1515  .sp  .sp
1516  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
1517  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
1518  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. An earlier example pointed out that the
1519  pattern  pattern
1520    .sp
1521    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1522    .sp
1523  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1524  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1525    .sp
1526    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1527    .sp
1528  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
1529  strings. Such references must, however, follow the subpattern to which they  strings. Such references must, however, follow the subpattern to which they
1530  refer.  refer.
1531    .P
1532    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1533    group. That is, once it has matched some of the subject string, it is never
1534    re-entered, even if it contains untried alternatives and there is a subsequent
1535    matching failure.
1536    .
1537    .
1538  .SH CALLOUTS  .SH CALLOUTS
1539  .rs  .rs
1540  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1542  Perl has a feature whereby using the seq
1542  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
1543  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1544  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1545    .P
1546  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1547  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
1548  function by putting its entry point in the global variable \fIpcre_callout\fR.  function by putting its entry point in the global variable \fIpcre_callout\fP.
1549  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1550    .P
1551  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1552  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
1553  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.
1554  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1555    .sp
1556    (?C1)\dabc(?C2)def    (?C1)\dabc(?C2)def
1557    .sp
1558  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1559    automatically installed before each item in the pattern. They are all numbered
1560    255.
1561    .P
1562    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1563  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
1564  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
1565  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1566  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1567  function is given in the  description of the interface to the callout function is given in the
1568  .\" HREF  .\" HREF
1569  \fBpcrecallout\fR  \fBpcrecallout\fP
1570  .\"  .\"
1571  documentation.  documentation.
1572    .P
1573  .in 0  .in 0
1574  Last updated: 03 February 2003  Last updated: 24 January 2006
1575  .br  .br
1576  Copyright (c) 1997-2003 University of Cambridge.  Copyright (c) 1997-2006 University of Cambridge.

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