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1  .TH PCRE 3  .TH PCRE 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  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
30  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
31  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
32    .sp
33    The quick brown fox    The quick brown fox
34    .sp
35  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. The power of
36  regular expressions comes from the ability to include alternatives and  regular expressions comes from the ability to include alternatives and
37  repetitions in the pattern. These are encoded in the pattern by the use of  repetitions in the pattern. These are encoded in the pattern by the use of
38  \fImeta-characters\fR, which do not stand for themselves but instead are  \fImetacharacters\fP, which do not stand for themselves but instead are
39  interpreted in some special way.  interpreted in some special way.
40    .P
41  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
42  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
43  recognized in square brackets. Outside square brackets, the meta-characters are  recognized in square brackets. Outside square brackets, the metacharacters are
44  as follows:  as follows:
45    .sp
46    \\      general escape character with several uses    \e      general escape character with several uses
47    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
48    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
49    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 58  as follows:
58    +      1 or more quantifier    +      1 or more quantifier
59           also "possessive quantifier"           also "possessive quantifier"
60    {      start min/max quantifier    {      start min/max quantifier
61    .sp
62  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
63  a character class the only meta-characters are:  a character class the only metacharacters are:
64    .sp
65    \\      general escape character    \e      general escape character
66    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
67    -      indicates character range    -      indicates character range
68    .\" JOIN
69    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
70             syntax)             syntax)
71    ]      terminates the character class    ]      terminates the character class
72    .sp
73  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
74    .
75  .SH BACKSLASH  .SH BACKSLASH
76  .rs  .rs
77  .sp  .sp
78  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
79  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character may
80  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
81  outside character classes.  outside character classes.
82    .P
83  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.
84  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
85  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
86  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
87  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
88    .P
89  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
90  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
91  a character class and the next newline character are ignored. An escaping  a character class and the next newline character are ignored. An escaping
92  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
93  pattern.  pattern.
94    .P
95  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
96  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
97  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
98  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
99    .sp
100    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
101    .sp
102    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
103      \eQabc$xyz\eE        abc$xyz        abc followed by the
104                                        contents of $xyz                                        contents of $xyz
105    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
106    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
107    .sp
108  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
109    .
110    .
111    .\" HTML <a name="digitsafterbackslash"></a>
112    .SS "Non-printing characters"
113    .rs
114    .sp
115  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
116  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
117  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
118  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
119  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
120  represents:  represents:
121    .sp
122    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
123    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
124    \\e        escape (hex 1B)    \ee        escape (hex 1B)
125    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
126    \\n        newline (hex 0A)    \en        newline (hex 0A)
127    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
128    \\t        tab (hex 09)    \et        tab (hex 09)
129    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
130    \\xhh      character with hex code hh    \exhh      character with hex code hh
131    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh... (UTF-8 mode only)
132    .sp
133  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
134  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.
135  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
136  7B.  7B.
137    .P
138  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
139  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
140  appear between \\x{ and }, but the value of the character code must be less  appear between \ex{ and }, but the value of the character code must be less
141  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
142  other than hexadecimal digits appear between \\x{ and }, or if there is no  other than hexadecimal digits appear between \ex{ and }, or if there is no
143  terminating }, this form of escape is not recognized. Instead, the initial  terminating }, this form of escape is not recognized. Instead, the initial
144  \\x will be interpreted as a basic hexadecimal escape, with no following  \ex will be interpreted as a basic hexadecimal escape, with no following
145  digits, giving a byte whose value is zero.  digits, giving a character whose value is zero.
146    .P
147  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
148  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the
149  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc}.
150    .P
151  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
152  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
153  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
154  (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
155  character that follows is itself an octal digit.  pattern character that follows is itself an octal digit.
156    .P
157  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.
158  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
159  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
160  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
161  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
162  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
163    .\" </a>
164    later,
165    .\"
166    following the discussion of
167    .\" HTML <a href="#subpattern">
168    .\" </a>
169    parenthesized subpatterns.
170    .\"
171    .P
172  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
173  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
174  digits following the backslash, and generates a single byte from the least  digits following the backslash, and generates a single byte from the least
175  significant 8 bits of the value. Any subsequent digits stand for themselves.  significant 8 bits of the value. Any subsequent digits stand for themselves.
176  For example:  For example:
177    .sp
178    \\040   is another way of writing a space    \e040   is another way of writing a space
179    \\40    is the same, provided there are fewer than 40  .\" JOIN
180      \e40    is the same, provided there are fewer than 40
181              previous capturing subpatterns              previous capturing subpatterns
182    \\7     is always a back reference    \e7     is always a back reference
183    \\11    might be a back reference, or another way of  .\" JOIN
184      \e11    might be a back reference, or another way of
185              writing a tab              writing a tab
186    \\011   is always a tab    \e011   is always a tab
187    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
188    \\113   might be a back reference, otherwise the  .\" JOIN
189      \e113   might be a back reference, otherwise the
190              character with octal code 113              character with octal code 113
191    \\377   might be a back reference, otherwise  .\" JOIN
192      \e377   might be a back reference, otherwise
193              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
194    \\81    is either a back reference, or a binary zero  .\" JOIN
195      \e81    is either a back reference, or a binary zero
196              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
197    .sp
198  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
199  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
200    .P
201  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
202  (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
203  addition, inside a character class, the sequence \\b is interpreted as the  addition, inside a character class, the sequence \eb is interpreted as the
204  backspace character (hex 08). Outside a character class it has a different  backspace character (hex 08), and the sequence \eX is interpreted as the
205  meaning (see below).  character "X". Outside a character class, these sequences have different
206    meanings
207  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
208    .\" </a>
209    \\d     any decimal digit  (see below).
210    \\D     any character that is not a decimal digit  .\"
211    \\s     any whitespace character  .
212    \\S     any character that is not a whitespace character  .
213    \\w     any "word" character  .SS "Generic character types"
214    \\W     any "non-word" character  .rs
215    .sp
216    The third use of backslash is for specifying generic character types. The
217    following are always recognized:
218    .sp
219      \ed     any decimal digit
220      \eD     any character that is not a decimal digit
221      \es     any whitespace character
222      \eS     any character that is not a whitespace character
223      \ew     any "word" character
224      \eW     any "non-word" character
225    .sp
226  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
227  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.
228    .P
229  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
230  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
231    matching point is at the end of the subject string, all of them fail, since
232  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
233  This makes it different from the the POSIX "space" class. The \\s characters  .P
234    For compatibility with Perl, \es does not match the VT character (code 11).
235    This makes it different from the the POSIX "space" class. The \es characters
236  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32).
237    .P
238  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
239  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
240  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
241  specific matching is taking place (see  place (see
242  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
243  .\" </a>  .\" </a>
244  "Locale support"  "Locale support"
245  .\"  .\"
246  in the  in the
247  .\" HREF  .\" HREF
248  \fBpcreapi\fR  \fBpcreapi\fP
249  .\"  .\"
250  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in the "fr_FR" (French) locale, some character codes
251  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.
252    .P
253  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
254  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
255  matching point is at the end of the subject string, all of them fail, since  character property support is available.
256  there is no character to match.  .
257    .
258    .\" HTML <a name="uniextseq"></a>
259    .SS Unicode character properties
260    .rs
261    .sp
262    When PCRE is built with Unicode character property support, three additional
263    escape sequences to match generic character types are available when UTF-8 mode
264    is selected. They are:
265    .sp
266     \ep{\fIxx\fP}   a character with the \fIxx\fP property
267     \eP{\fIxx\fP}   a character without the \fIxx\fP property
268     \eX       an extended Unicode sequence
269    .sp
270    The property names represented by \fIxx\fP above are limited to the
271    Unicode general category properties. Each character has exactly one such
272    property, specified by a two-letter abbreviation. For compatibility with Perl,
273    negation can be specified by including a circumflex between the opening brace
274    and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.
275    .P
276    If only one letter is specified with \ep or \eP, it includes all the properties
277    that start with that letter. In this case, in the absence of negation, the
278    curly brackets in the escape sequence are optional; these two examples have
279    the same effect:
280    .sp
281      \ep{L}
282      \epL
283    .sp
284    The following property codes are supported:
285    .sp
286      C     Other
287      Cc    Control
288      Cf    Format
289      Cn    Unassigned
290      Co    Private use
291      Cs    Surrogate
292    .sp
293      L     Letter
294      Ll    Lower case letter
295      Lm    Modifier letter
296      Lo    Other letter
297      Lt    Title case letter
298      Lu    Upper case letter
299    .sp
300      M     Mark
301      Mc    Spacing mark
302      Me    Enclosing mark
303      Mn    Non-spacing mark
304    .sp
305      N     Number
306      Nd    Decimal number
307      Nl    Letter number
308      No    Other number
309    .sp
310      P     Punctuation
311      Pc    Connector punctuation
312      Pd    Dash punctuation
313      Pe    Close punctuation
314      Pf    Final punctuation
315      Pi    Initial punctuation
316      Po    Other punctuation
317      Ps    Open punctuation
318    .sp
319      S     Symbol
320      Sc    Currency symbol
321      Sk    Modifier symbol
322      Sm    Mathematical symbol
323      So    Other symbol
324    .sp
325      Z     Separator
326      Zl    Line separator
327      Zp    Paragraph separator
328      Zs    Space separator
329    .sp
330    Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
331    PCRE.
332    .P
333    Specifying caseless matching does not affect these escape sequences. For
334    example, \ep{Lu} always matches only upper case letters.
335    .P
336    The \eX escape matches any number of Unicode characters that form an extended
337    Unicode sequence. \eX is equivalent to
338    .sp
339      (?>\ePM\epM*)
340    .sp
341    That is, it matches a character without the "mark" property, followed by zero
342    or more characters with the "mark" property, and treats the sequence as an
343    atomic group
344    .\" HTML <a href="#atomicgroup">
345    .\" </a>
346    (see below).
347    .\"
348    Characters with the "mark" property are typically accents that affect the
349    preceding character.
350    .P
351    Matching characters by Unicode property is not fast, because PCRE has to search
352    a structure that contains data for over fifteen thousand characters. That is
353    why the traditional escape sequences such as \ed and \ew do not use Unicode
354    properties in PCRE.
355    .
356    .
357    .\" HTML <a name="smallassertions"></a>
358    .SS "Simple assertions"
359    .rs
360    .sp
361  The fourth use of backslash is for certain simple assertions. An assertion  The fourth use of backslash is for certain simple assertions. An assertion
362  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,
363  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
364  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
365  assertions are  .\" HTML <a href="#bigassertions">
366    .\" </a>
367    \\b     matches at a word boundary  below.
368    \\B     matches when not at a word boundary  .\"
369    \\A     matches at start of subject  The backslashed
370    \\Z     matches at end of subject or before newline at end  assertions are:
371    \\z     matches at end of subject  .sp
372    \\G     matches at first matching position in subject    \eb     matches at a word boundary
373      \eB     matches when not at a word boundary
374  These assertions may not appear in character classes (but note that \\b has a    \eA     matches at start of subject
375      \eZ     matches at end of subject or before newline at end
376      \ez     matches at end of subject
377      \eG     matches at first matching position in subject
378    .sp
379    These assertions may not appear in character classes (but note that \eb has a
380  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
381    .P
382  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
383  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
384  \\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
385  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
386    .P
387  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
388  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
389  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
390  multiline mode.  independent of multiline mode. These three assertions are not affected by the
391    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
392  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
393  \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
394  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
395  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
396  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
397  string, whereas \\z matches only at the end.  matches only at the end.
398    .P
399  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
400  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
401  \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
402  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
403  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
404  implementation where \\G can be useful.  implementation where \eG can be useful.
405    .P
406  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
407  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
408  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
409  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
410  reproduce this behaviour.  reproduce this behaviour.
411    .P
412  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
413  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
414  regular expression.  regular expression.
415    .
416  .SH CIRCUMFLEX AND DOLLAR  .
417    .SH "CIRCUMFLEX AND DOLLAR"
418  .rs  .rs
419  .sp  .sp
420  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
421  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
422  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
423  \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
424  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
425  meaning (see below).  meaning
426    .\" HTML <a href="#characterclass">
427    .\" </a>
428    (see below).
429    .\"
430    .P
431  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
432  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
433  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 435  possible alternatives start with a circu
435  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
436  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
437  to be anchored.)  to be anchored.)
438    .P
439  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
440  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
441  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
442  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
443  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.
444  Dollar has no special meaning in a character class.  Dollar has no special meaning in a character class.
445    .P
446  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
447  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
448  does not affect the \\Z assertion.  does not affect the \eZ assertion.
449    .P
450  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
451  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
452  after and immediately before an internal newline character, respectively, in  after and immediately before an internal newline character, respectively, in
453  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,
454  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  the pattern /^abc$/ matches the subject string "def\enabc" (where \en
455  but not otherwise. Consequently, patterns that are anchored in single line mode  represents a newline character) in multiline mode, but not otherwise.
456  because all branches start with ^ are not anchored in multiline mode, and a  Consequently, patterns that are anchored in single line mode because all
457  match for circumflex is possible when the \fIstartoffset\fR argument of  branches start with ^ are not anchored in multiline mode, and a match for
458  \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
459  PCRE_MULTILINE is set.  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
460    set.
461  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  .P
462    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
463  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
464  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether PCRE_MULTILINE is set or not.
465    .
466  .SH FULL STOP (PERIOD, DOT)  .
467    .SH "FULL STOP (PERIOD, DOT)"
468  .rs  .rs
469  .sp  .sp
470  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
471  the subject, including a non-printing character, but not (by default) newline.  the subject, including a non-printing character, but not (by default) newline.
472  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
473  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,
474  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
475  handling of circumflex and dollar, the only relationship being that they both  handling of circumflex and dollar, the only relationship being that they both
476  involve newline characters. Dot has no special meaning in a character class.  involve newline characters. Dot has no special meaning in a character class.
477    .
478  .SH MATCHING A SINGLE BYTE  .
479    .SH "MATCHING A SINGLE BYTE"
480  .rs  .rs
481  .sp  .sp
482  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
483  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
484  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
485  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
486  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
487    best avoided.
488  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  .P
489  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
490    .\" HTML <a href="#lookbehind">
491  .SH SQUARE BRACKETS  .\" </a>
492    (described below),
493    .\"
494    because in UTF-8 mode this would make it impossible to calculate the length of
495    the lookbehind.
496    .
497    .
498    .\" HTML <a name="characterclass"></a>
499    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
500  .rs  .rs
501  .sp  .sp
502  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 504  square bracket. A closing square bracket
504  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
505  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
506  escaped with a backslash.  escaped with a backslash.
507    .P
508  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
509  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
510  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 512  definition is a circumflex, in which cas
512  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
513  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
514  backslash.  backslash.
515    .P
516  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
517  [^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
518  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
519  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
520  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
521  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
522    string.
523    .P
524  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
525  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.
526    .P
527  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
528  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
529  "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
530  caseful version would. PCRE does not support the concept of case for characters  caseful version would. When running in UTF-8 mode, PCRE supports the concept of
531  with values greater than 255.  case for characters with values greater than 128 only when it is compiled with
532    Unicode property support.
533    .P
534  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,
535  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
536  such as [^a] will always match a newline.  such as [^a] will always match a newline.
537    .P
538  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
539  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,
540  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
541  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
542  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.
543    .P
544  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
545  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
546  ("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
547  "-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
548  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
549  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
550  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
551    .P
552  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
553  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
554  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
555  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
556    .P
557  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
558  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
559  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
560  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
561    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
562  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
563  character class, and add the characters that they match to the class. For  property support.
564  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
565    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
566    in a character class, and add the characters that they match to the class. For
567    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
568  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
569  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
570  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
571    .P
572  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
573  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
574  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
575    introducing a POSIX class name - see the next section), and the terminating
576  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
577    does no harm.
578    .
579    .
580    .SH "POSIX CHARACTER CLASSES"
581  .rs  .rs
582  .sp  .sp
583  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
584  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
585  this notation. For example,  this notation. For example,
586    .sp
587    [01[:alpha:]%]    [01[:alpha:]%]
588    .sp
589  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
590  are  are
591    .sp
592    alnum    letters and digits    alnum    letters and digits
593    alpha    letters    alpha    letters
594    ascii    character codes 0 - 127    ascii    character codes 0 - 127
595    blank    space or tab only    blank    space or tab only
596    cntrl    control characters    cntrl    control characters
597    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
598    graph    printing characters, excluding space    graph    printing characters, excluding space
599    lower    lower case letters    lower    lower case letters
600    print    printing characters, including space    print    printing characters, including space
601    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
602    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
603    upper    upper case letters    upper    upper case letters
604    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
605    xdigit   hexadecimal digits    xdigit   hexadecimal digits
606    .sp
607  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
608  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
609  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
610  compatibility).  compatibility).
611    .P
612  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
613  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
614  after the colon. For example,  after the colon. For example,
615    .sp
616    [12[:^digit:]]    [12[:^digit:]]
617    .sp
618  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
619  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
620  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
621    .P
622  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
623  the POSIX character classes.  the POSIX character classes.
624    .
625  .SH VERTICAL BAR  .
626    .SH "VERTICAL BAR"
627  .rs  .rs
628  .sp  .sp
629  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
630  the pattern  the pattern
631    .sp
632    gilbert|sullivan    gilbert|sullivan
633    .sp
634  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
635  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string).
636  The matching process tries each alternative in turn, from left to right,  The matching process tries each alternative in turn, from left to right,
637  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
638  subpattern (defined below), "succeeds" means matching the rest of the main  subpattern
639  pattern as well as the alternative in the subpattern.  .\" HTML <a href="#subpattern">
640    .\" </a>
641  .SH INTERNAL OPTION SETTING  (defined below),
642    .\"
643    "succeeds" means matching the rest of the main pattern as well as the
644    alternative in the subpattern.
645    .
646    .
647    .SH "INTERNAL OPTION SETTING"
648  .rs  .rs
649  .sp  .sp
650  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
651  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
652  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
653    .sp
654    i  for PCRE_CASELESS    i  for PCRE_CASELESS
655    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
656    s  for PCRE_DOTALL    s  for PCRE_DOTALL
657    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
658    .sp
659  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
660  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
661  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
662  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
663  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
664  unset.  unset.
665    .P
666  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
667  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
668  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
669  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
670  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
671    .P
672  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
673  pattern that follows it, so  pattern that follows it, so
674    .sp
675    (a(?i)b)c    (a(?i)b)c
676    .sp
677  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).
678  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
679  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
680  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
681    .sp
682    (a(?i)b|c)    (a(?i)b|c)
683    .sp
684  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
685  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
686  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
687  behaviour otherwise.  behaviour otherwise.
688    .P
689  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
690  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
691  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
692  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
693  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.
694    .
695    .
696    .\" HTML <a name="subpattern"></a>
697  .SH SUBPATTERNS  .SH SUBPATTERNS
698  .rs  .rs
699  .sp  .sp
700  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
701  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
702    .sp
703  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
704    .sp
705    cat(aract|erpillar|)    cat(aract|erpillar|)
706    .sp
707  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
708  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or the empty string.
709    .sp
710  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
711  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
712  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
713  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
714  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
715    .P
716  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
717    .sp
718    the ((red|white) (king|queen))    the ((red|white) (king|queen))
719    .sp
720  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,
721  2, and 3, respectively.  2, and 3, respectively.
722    .P
723  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
724  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
725  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
726  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
727  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
728  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
729    .sp
730    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
731    .sp
732  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
733  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
734  of nesting of all subpatterns, both capturing and non-capturing, is 200.  of nesting of all subpatterns, both capturing and non-capturing, is 200.
735    .P
736  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
737  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
738  the ":". Thus the two patterns  the ":". Thus the two patterns
739    .sp
740    (?i:saturday|sunday)    (?i:saturday|sunday)
741    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
742    .sp
743  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
744  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
745  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
746  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
747    .
748  .SH NAMED SUBPATTERNS  .
749    .SH "NAMED SUBPATTERNS"
750  .rs  .rs
751  .sp  .sp
752  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
753  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
754  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
755  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns, something that Perl does
756  not provide. The Python syntax (?P<name>...) is used. Names consist of  not provide. The Python syntax (?P<name>...) is used. Names consist of
757  alphanumeric characters and underscores, and must be unique within a pattern.  alphanumeric characters and underscores, and must be unique within a pattern.
758    .P
759  Named capturing parentheses are still allocated numbers as well as names. The  Named capturing parentheses are still allocated numbers as well as names. The
760  PCRE API provides function calls for extracting the name-to-number translation  PCRE API provides function calls for extracting the name-to-number translation
761  table from a compiled pattern. For further details see the  table from a compiled pattern. There is also a convenience function for
762    extracting a captured substring by name. For further details see the
763  .\" HREF  .\" HREF
764  \fBpcreapi\fR  \fBpcreapi\fP
765  .\"  .\"
766  documentation.  documentation.
767    .
768    .
769  .SH REPETITION  .SH REPETITION
770  .rs  .rs
771  .sp  .sp
772  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
773  items:  items:
774    .sp
775    a literal data character    a literal data character
776    the . metacharacter    the . metacharacter
777    the \\C escape sequence    the \eC escape sequence
778    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
779      an escape such as \ed that matches a single character
780    a character class    a character class
781    a back reference (see next section)    a back reference (see next section)
782    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
783    .sp
784  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
785  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
786  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
787  be less than or equal to the second. For example:  be less than or equal to the second. For example:
788    .sp
789    z{2,4}    z{2,4}
790    .sp
791  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
792  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
793  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
794  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
795    .sp
796    [aeiou]{3,}    [aeiou]{3,}
797    .sp
798  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
799    .sp
800    \\d{8}    \ed{8}
801    .sp
802  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
803  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
804  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
805  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
806    .P
807  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
808  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
809  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
810    support is available, \eX{3} matches three Unicode extended sequences, each of
811    which may be several bytes long (and they may be of different lengths).
812    .P
813  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
814  previous item and the quantifier were not present.  previous item and the quantifier were not present.
815    .P
816  For convenience (and historical compatibility) the three most common  For convenience (and historical compatibility) the three most common
817  quantifiers have single-character abbreviations:  quantifiers have single-character abbreviations:
818    .sp
819    *    is equivalent to {0,}    *    is equivalent to {0,}
820    +    is equivalent to {1,}    +    is equivalent to {1,}
821    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
822    .sp
823  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
824  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:
825    .sp
826    (a?)*    (a?)*
827    .sp
828  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
829  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
830  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
831  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
832    .P
833  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
834  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
835  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
836  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 */
837  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
838  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
839    .sp
840    /\\*.*\\*/    /\e*.*\e*/
841    .sp
842  to the string  to the string
843    .sp
844    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
845    .sp
846  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 .*
847  item.  item.
848    .P
849  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
850  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
851  pattern  pattern
852    .sp
853    /\\*.*?\\*/    /\e*.*?\e*/
854    .sp
855  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
856  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
857  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
858  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
859    .sp
860    \\d??\\d    \ed??\ed
861    .sp
862  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
863  way the rest of the pattern matches.  way the rest of the pattern matches.
864    .P
865  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),
866  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
867  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
868  default behaviour.  default behaviour.
869    .P
870  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
871  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
872  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
873    .P
874  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
875  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
876  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
877  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
878  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
879  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
880    .P
881  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
882  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
883  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
884    .P
885  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
886  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
887  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
888  succeed. Consider, for example:  succeed. Consider, for example:
889    .sp
890    (.*)abc\\1    (.*)abc\e1
891    .sp
892  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
893  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
894    .P
895  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
896  that matched the final iteration. For example, after  that matched the final iteration. For example, after
897    .sp
898    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
899    .sp
900  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
901  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
902  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
903  example, after  example, after
904    .sp
905    /(a|(b))+/    /(a|(b))+/
906    .sp
907  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
908    .
909  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
910    .\" HTML <a name="atomicgroup"></a>
911    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
912  .rs  .rs
913  .sp  .sp
914  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 917  number of repeats allows the rest of the
917  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
918  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
919  there is no point in carrying on.  there is no point in carrying on.
920    .P
921  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
922    .sp
923    123456bar    123456bar
924    .sp
925  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
926  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+
927  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"
928  (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
929  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.
930    .P
931  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
932  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
933  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
934    .sp
935    (?>\\d+)bar    (?>\ed+)foo
936    .sp
937  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
938  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
939  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
940  normal.  normal.
941    .P
942  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
943  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
944  the current point in the subject string.  the current point in the subject string.
945    .P
946  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
947  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
948  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
949  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,
950  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
951    .P
952  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
953  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
954  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
955  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
956  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
957  previous example can be rewritten as  previous example can be rewritten as
958    .sp
959    \\d++bar    \ed++foo
960    .sp
961  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
962  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
963  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
964  possessive quantifier and the equivalent atomic group.  possessive quantifier and the equivalent atomic group.
965    .P
966  The possessive quantifier syntax is an extension to the Perl syntax. It  The possessive quantifier syntax is an extension to the Perl syntax. It
967  originates in Sun's Java package.  originates in Sun's Java package.
968    .P
969  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
970  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
971  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
972  pattern  pattern
973    .sp
974    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
975    .sp
976  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
977  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
978  quickly. However, if it is applied to  quickly. However, if it is applied to
979    .sp
980    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
981    .sp
982  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
983  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
984  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
985  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
986  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
987  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
988  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
989    an atomic group, like this:
990    ((?>\\D+)|<\\d+>)*[!?]  .sp
991      ((?>\eD+)|<\ed+>)*[!?]
992    .sp
993  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
994    .
995  .SH BACK REFERENCES  .
996    .\" HTML <a name="backreferences"></a>
997    .SH "BACK REFERENCES"
998  .rs  .rs
999  .sp  .sp
1000  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
1001  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1002  (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
1003  previous capturing left parentheses.  previous capturing left parentheses.
1004    .P
1005  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
1006  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
1007  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
1008  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
1009  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. See the subsection entitled "Non-printing characters"
1010  details of the handling of digits following a backslash.  .\" HTML <a href="#digitsafterbackslash">
1011    .\" </a>
1012    above
1013    .\"
1014    for further details of the handling of digits following a backslash.
1015    .P
1016  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1017  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1018  itself (see  itself (see
# Line 832  itself (see Line 1021  itself (see
1021  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1022  .\"  .\"
1023  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1024    .sp
1025    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1026    .sp
1027  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1028  "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
1029  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1030    .sp
1031    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1032    .sp
1033  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
1034  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1035    .P
1036  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
1037  rewrite the above example as follows:  rewrite the above example as follows:
1038    .sp
1039    (?<p1>(?i)rah)\\s+(?P=p1)    (?<p1>(?i)rah)\es+(?P=p1)
1040    .sp
1041  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
1042  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1043  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1044    .sp
1045    (a|(bc))\\2    (a|(bc))\e2
1046    .sp
1047  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
1048  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1049  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
1050  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
1051  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1052  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1053    .\" HTML <a href="#comments">
1054    .\" </a>
1055    "Comments"
1056    .\"
1057    below) can be used.
1058    .P
1059  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
1060  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.
1061  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1062  example, the pattern  example, the pattern
1063    .sp
1064    (a|b\\1)+    (a|b\e1)+
1065    .sp
1066  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
1067  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1068  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
1069  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
1070  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
1071  minimum of zero.  minimum of zero.
1072    .
1073    .
1074    .\" HTML <a name="bigassertions"></a>
1075  .SH ASSERTIONS  .SH ASSERTIONS
1076  .rs  .rs
1077  .sp  .sp
1078  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
1079  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1080  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
1081    .\" HTML <a href="#smallassertions">
1082    .\" </a>
1083    above.
1084    .\"
1085    .P
1086  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1087  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
1088  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1089    except that it does not cause the current matching position to be changed.
1090  An assertion subpattern is matched in the normal way, except that it does not  .P
1091  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1092    because it makes no sense to assert the same thing several times. If any kind
1093    of assertion contains capturing subpatterns within it, these are counted for
1094    the purposes of numbering the capturing subpatterns in the whole pattern.
1095    However, substring capturing is carried out only for positive assertions,
1096    because it does not make sense for negative assertions.
1097    .
1098    .
1099    .SS "Lookahead assertions"
1100    .rs
1101    .sp
1102    Lookahead assertions start
1103  with (?= for positive assertions and (?! for negative assertions. For example,  with (?= for positive assertions and (?! for negative assertions. For example,
1104    .sp
1105    \\w+(?=;)    \ew+(?=;)
1106    .sp
1107  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
1108  the match, and  the match, and
1109    .sp
1110    foo(?!bar)    foo(?!bar)
1111    .sp
1112  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
1113  apparently similar pattern  apparently similar pattern
1114    .sp
1115    (?!foo)bar    (?!foo)bar
1116    .sp
1117  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
1118  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1119  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1120  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1121    .P
1122  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
1123  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
1124  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.
1125    .
1126    .
1127    .\" HTML <a name="lookbehind"></a>
1128    .SS "Lookbehind assertions"
1129    .rs
1130    .sp
1131  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1132  negative assertions. For example,  negative assertions. For example,
1133    .sp
1134    (?<!foo)bar    (?<!foo)bar
1135    .sp
1136  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
1137  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
1138  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
1139  all have to have the same fixed length. Thus  all have to have the same fixed length. Thus
1140    .sp
1141    (?<=bullock|donkey)    (?<=bullock|donkey)
1142    .sp
1143  is permitted, but  is permitted, but
1144    .sp
1145    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1146    .sp
1147  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1148  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
1149  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
1150  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1151    .sp
1152    (?<=ab(c|de))    (?<=ab(c|de))
1153    .sp
1154  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
1155  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:
1156    .sp
1157    (?<=abc|abde)    (?<=abc|abde)
1158    .sp
1159  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1160  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
1161  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1162  match is deemed to fail.  match is deemed to fail.
1163    .P
1164  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)
1165  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1166  the length of the lookbehind.  the length of the lookbehind. The \eX escape, which can match different numbers
1167    of bytes, is also not permitted.
1168    .P
1169  Atomic groups can be used in conjunction with lookbehind assertions to specify  Atomic groups can be used in conjunction with lookbehind assertions to specify
1170  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
1171  such as  such as
1172    .sp
1173    abcd$    abcd$
1174    .sp
1175  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
1176  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
1177  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
1178    .sp
1179    ^.*abcd$    ^.*abcd$
1180    .sp
1181  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
1182  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,
1183  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"
1184  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,
1185  if the pattern is written as  if the pattern is written as
1186    .sp
1187    ^(?>.*)(?<=abcd)    ^(?>.*)(?<=abcd)
1188    .sp
1189  or, equivalently,  or, equivalently, using the possessive quantifier syntax,
1190    .sp
1191    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1192    .sp
1193  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
1194  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
1195  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1196  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1197    .
1198    .
1199    .SS "Using multiple assertions"
1200    .rs
1201    .sp
1202  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1203    .sp
1204    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1205    .sp
1206  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
1207  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1208  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
1209  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".
1210  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
1211  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
1212  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1213    .sp
1214    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1215    .sp
1216  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1217  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
1218  preceding three characters are not "999".  preceding three characters are not "999".
1219    .P
1220  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1221    .sp
1222    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1223    .sp
1224  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
1225  preceded by "foo", while  preceded by "foo", while
1226    .sp
1227    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1228    .sp
1229  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
1230  characters that are not "999".  characters that are not "999".
1231    .
1232  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1233  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  
1234  .rs  .rs
1235  .sp  .sp
1236  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1237  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1238  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1239  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1240    .sp
1241    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1242    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1243    .sp
1244  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1245  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
1246  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1247    .P
1248  There are three kinds of condition. If the text between the parentheses  There are three kinds of condition. If the text between the parentheses
1249  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
1250  subpattern of that number has previously matched. The number must be greater  subpattern of that number has previously matched. The number must be greater
1251  than zero. Consider the following pattern, which contains non-significant white  than zero. Consider the following pattern, which contains non-significant white
1252  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
1253  it into three parts for ease of discussion:  it into three parts for ease of discussion:
1254    .sp
1255    ( \\( )?    [^()]+    (?(1) \\) )    ( \e( )?    [^()]+    (?(1) \e) )
1256    .sp
1257  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1258  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
1259  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 1263  the condition is true, and so the yes-pa
1263  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1264  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1265  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1266    .P
1267  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
1268  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.
1269  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.
1270    .P
1271  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.
1272  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1273  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1274  alternatives on the second line:  alternatives on the second line:
1275    .sp
1276    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1277    \\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} )
1278    .sp
1279  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1280  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
1281  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
1282  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1283  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
1284  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.
1285    .
1286    .
1287    .\" HTML <a name="comments"></a>
1288  .SH COMMENTS  .SH COMMENTS
1289  .rs  .rs
1290  .sp  .sp
1291  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
1292  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1293  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.
1294    .P
1295  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1296  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues up to the next newline
1297  character in the pattern.  character in the pattern.
1298    .
1299  .SH RECURSIVE PATTERNS  .
1300    .SH "RECURSIVE PATTERNS"
1301  .rs  .rs
1302  .sp  .sp
1303  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1304  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1305  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
1306  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
1307  experimental facility that allows regular expressions to recurse (amongst other  that allows regular expressions to recurse (amongst other things). It does this
1308  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
1309  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
1310  parentheses problem can be created like this:  can be created like this:
1311    .sp
1312    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1313    .sp
1314  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
1315  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears. Obviously, PCRE cannot support
1316  the interpolation of Perl code. Instead, it supports some special syntax for  the interpolation of Perl code. Instead, it supports some special syntax for
1317  recursion of the entire pattern, and also for individual subpattern recursion.  recursion of the entire pattern, and also for individual subpattern recursion.
1318    .P
1319  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
1320  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
1321  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
1322  "subroutine" call, which is described in the next section.) The special item  "subroutine" call, which is described in the next section.) The special item
1323  (?R) is a recursive call of the entire regular expression.  (?R) is a recursive call of the entire regular expression.
1324    .P
1325  For example, this PCRE pattern solves the nested parentheses problem (assume  For example, this PCRE pattern solves the nested parentheses problem (assume
1326  the PCRE_EXTENDED option is set so that white space is ignored):  the PCRE_EXTENDED option is set so that white space is ignored):
1327    .sp
1328    \\( ( (?>[^()]+) | (?R) )* \\)    \e( ( (?>[^()]+) | (?R) )* \e)
1329    .sp
1330  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1331  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
1332  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is a correctly parenthesized substring).
1333  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1334    .P
1335  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
1336  pattern, so instead you could use this:  pattern, so instead you could use this:
1337    .sp
1338    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1339    .sp
1340  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
1341  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
1342  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
1343  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
1344  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
1345  named parentheses). We could rewrite the above example as follows:  named parentheses). We could rewrite the above example as follows:
1346    .sp
1347    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
1348    .sp
1349  This particular example pattern contains nested unlimited repeats, and so the  This particular example pattern contains nested unlimited repeats, and so the
1350  use of atomic grouping for matching strings of non-parentheses is important  use of atomic grouping for matching strings of non-parentheses is important
1351  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
1352  pattern is applied to  pattern is applied to
1353    .sp
1354    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1355    .sp
1356  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1357  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
1358  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
1359  before failure can be reported.  before failure can be reported.
1360    .P
1361  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
1362  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.
1363  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
1364  below and the  the next section and the
1365  .\" HREF  .\" HREF
1366  \fBpcrecallout\fR  \fBpcrecallout\fP
1367  .\"  .\"
1368  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1369    .sp
1370    (ab(cd)ef)    (ab(cd)ef)
1371    .sp
1372  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
1373  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1374    .sp
1375    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1376       ^                        ^       ^                        ^
1377       ^                        ^       ^                        ^
1378    .sp
1379  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
1380  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
1381  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
1382  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
1383  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.
1384    .P
1385  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.
1386  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1387  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1388  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1389    .sp
1390    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1391    .sp
1392  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
1393  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1394  is the actual recursive call.  is the actual recursive call.
1395    .
1396    .
1397  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1398  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1399  .rs  .rs
1400  .sp  .sp
1401  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
1402  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
1403  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. An earlier example pointed out that the
1404  pattern  pattern
1405    .sp
1406    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1407    .sp
1408  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1409  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1410    .sp
1411    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1412    .sp
1413  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
1414  strings. Such references must, however, follow the subpattern to which they  strings. Such references must, however, follow the subpattern to which they
1415  refer.  refer.
1416    .
1417    .
1418  .SH CALLOUTS  .SH CALLOUTS
1419  .rs  .rs
1420  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1422  Perl has a feature whereby using the seq
1422  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
1423  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1424  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1425    .P
1426  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
1427  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
1428  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.
1429  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1430    .P
1431  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1432  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
1433  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.
1434  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1435    .sp
1436    (?C1)\dabc(?C2)def    (?C1)\dabc(?C2)def
1437    .sp
1438  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
1439    automatically installed before each item in the pattern. They are all numbered
1440    255.
1441    .P
1442    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1443  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
1444  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
1445  \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
1446  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1447  function is given in the  description of the interface to the callout function is given in the
1448  .\" HREF  .\" HREF
1449  \fBpcrecallout\fR  \fBpcrecallout\fP
1450  .\"  .\"
1451  documentation.  documentation.
1452    .P
1453  .in 0  .in 0
1454  Last updated: 03 February 2003  Last updated: 09 September 2004
1455  .br  .br
1456  Copyright (c) 1997-2003 University of Cambridge.  Copyright (c) 1997-2004 University of Cambridge.
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