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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE REGULAR EXPRESSION DETAILS  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of other books, some of which have copious  .\" HREF
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  \fBpcresyntax\fP
11  O'Reilly, covers them in great detail. The description here is intended as  .\"
12  reference documentation.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14  The basic operation of PCRE is on strings of bytes. However, there is also  conflict with the Perl syntax) in order to provide some compatibility with
15  support for UTF-8 character strings. To use this support you must build PCRE to  regular expressions in Python, .NET, and Oniguruma.
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  .P
17  option. How this affects the pattern matching is mentioned in several places  Perl's regular expressions are described in its own documentation, and
18  below. There is also a summary of UTF-8 features in the  regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22    .P
23    The original operation of PCRE was on strings of one-byte characters. However,
24    there is now also support for UTF-8 character strings. To use this,
25    PCRE must be built to include UTF-8 support, and you must call
26    \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27    is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
38  .\"  .\"
39  in the main  in the main
40  .\" HREF  .\" HREF
41  \fBpcre\fR  \fBpcre\fP
42    .\"
43    page.
44    .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55    The remainder of this document discusses the patterns that are supported by
56    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57    From release 6.0, PCRE offers a second matching function,
58    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59    Perl-compatible. Some of the features discussed below are not available when
60    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61    alternative function, and how it differs from the normal function, are
62    discussed in the
63    .\" HREF
64    \fBpcrematching\fP
65  .\"  .\"
66  page.  page.
67    .
68    .
69    .SH "NEWLINE CONVENTIONS"
70    .rs
71    .sp
72    PCRE supports five different conventions for indicating line breaks in
73    strings: a single CR (carriage return) character, a single LF (linefeed)
74    character, the two-character sequence CRLF, any of the three preceding, or any
75    Unicode newline sequence. The
76    .\" HREF
77    \fBpcreapi\fP
78    .\"
79    page has
80    .\" HTML <a href="pcreapi.html#newlines">
81    .\" </a>
82    further discussion
83    .\"
84    about newlines, and shows how to set the newline convention in the
85    \fIoptions\fP arguments for the compiling and matching functions.
86    .P
87    It is also possible to specify a newline convention by starting a pattern
88    string with one of the following five sequences:
89    .sp
90      (*CR)        carriage return
91      (*LF)        linefeed
92      (*CRLF)      carriage return, followed by linefeed
93      (*ANYCRLF)   any of the three above
94      (*ANY)       all Unicode newline sequences
95    .sp
96    These override the default and the options given to \fBpcre_compile()\fP or
97    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
98    newline sequence, the pattern
99    .sp
100      (*CR)a.b
101    .sp
102    changes the convention to CR. That pattern matches "a\enb" because LF is no
103    longer a newline. Note that these special settings, which are not
104    Perl-compatible, are recognized only at the very start of a pattern, and that
105    they must be in upper case. If more than one of them is present, the last one
106    is used.
107    .P
108    The newline convention affects the interpretation of the dot metacharacter when
109    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
110    affect what the \eR escape sequence matches. By default, this is any Unicode
111    newline sequence, for Perl compatibility. However, this can be changed; see the
112    description of \eR in the section entitled
113    .\" HTML <a href="#newlineseq">
114    .\" </a>
115    "Newline sequences"
116    .\"
117    below. A change of \eR setting can be combined with a change of newline
118    convention.
119    .
120    .
121    .SH "CHARACTERS AND METACHARACTERS"
122    .rs
123    .sp
124  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
125  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
126  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
127    .sp
128    The quick brown fox    The quick brown fox
129    .sp
130  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
131  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
132  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
133  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
134    always possible. For characters with higher values, the concept of case is
135    supported if PCRE is compiled with Unicode property support, but not otherwise.
136    If you want to use caseless matching for characters 128 and above, you must
137    ensure that PCRE is compiled with Unicode property support as well as with
138    UTF-8 support.
139    .P
140    The power of regular expressions comes from the ability to include alternatives
141    and repetitions in the pattern. These are encoded in the pattern by the use of
142    \fImetacharacters\fP, which do not stand for themselves but instead are
143  interpreted in some special way.  interpreted in some special way.
144    .P
145  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
146  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
147  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
148  as follows:  are as follows:
149    .sp
150    \\      general escape character with several uses    \e      general escape character with several uses
151    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
152    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
153    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 162  as follows:
162    +      1 or more quantifier    +      1 or more quantifier
163           also "possessive quantifier"           also "possessive quantifier"
164    {      start min/max quantifier    {      start min/max quantifier
165    .sp
166  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
167  a character class the only meta-characters are:  a character class the only metacharacters are:
168    .sp
169    \\      general escape character    \e      general escape character
170    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
171    -      indicates character range    -      indicates character range
172    .\" JOIN
173    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
174             syntax)             syntax)
175    ]      terminates the character class    ]      terminates the character class
176    .sp
177  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
178    .
179    .
180  .SH BACKSLASH  .SH BACKSLASH
181  .rs  .rs
182  .sp  .sp
183  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
184  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
185  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
186  outside character classes.  outside character classes.
187    .P
188  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.
189  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
190  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
191  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
192  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
193    .P
194  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
195  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
196  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
197  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
198  pattern.  .P
   
199  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
200  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
201  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
202  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
203    .sp
204    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
205    .sp
206    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
207      \eQabc$xyz\eE        abc$xyz        abc followed by the
208                                        contents of $xyz                                        contents of $xyz
209    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
210    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
211    .sp
212  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
213    .
214    .
215    .\" HTML <a name="digitsafterbackslash"></a>
216    .SS "Non-printing characters"
217    .rs
218    .sp
219  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
220  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
221  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
222  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 often easier to use
223  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
224  represents:  .sp
225      \ea        alarm, that is, the BEL character (hex 07)
226    \\a        alarm, that is, the BEL character (hex 07)    \ecx       "control-x", where x is any character
227    \\cx       "control-x", where x is any character    \ee        escape (hex 1B)
228    \\e        escape (hex 1B)    \ef        formfeed (hex 0C)
229    \\f        formfeed (hex 0C)    \en        linefeed (hex 0A)
230    \\n        newline (hex 0A)    \er        carriage return (hex 0D)
231    \\r        carriage return (hex 0D)    \et        tab (hex 09)
232    \\t        tab (hex 09)    \eddd      character with octal code ddd, or back reference
233    \\ddd      character with octal code ddd, or backreference    \exhh      character with hex code hh
234    \\xhh      character with hex code hh    \ex{hhh..} character with hex code hhh..
235    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)  .sp
236    The precise effect of \ecx is as follows: if x is a lower case letter, it
 The precise effect of \\cx is as follows: if x is a lower case letter, it  
237  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.
238  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
239  7B.  7B.
240    .P
241  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
242  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
243  appear between \\x{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
244  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
245  other than hexadecimal digits appear between \\x{ and }, or if there is no  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
246  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
247  \\x will be interpreted as a basic hexadecimal escape, with no following  .P
248  digits, giving a byte whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
249    there is no terminating }, this form of escape is not recognized. Instead, the
250    initial \ex will be interpreted as a basic hexadecimal escape, with no
251    following digits, giving a character whose value is zero.
252    .P
253  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
254  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
255  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
256    .P
257  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. If there are fewer than two
258  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
259  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
260  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
261  character that follows is itself an octal digit.  follows is itself an octal digit.
262    .P
263  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.
264  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
265  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
266  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
267  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
268  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
269    .\" </a>
270    later,
271    .\"
272    following the discussion of
273    .\" HTML <a href="#subpattern">
274    .\" </a>
275    parenthesized subpatterns.
276    .\"
277    .P
278  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
279  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
280  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
281  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
282  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
283    to \e777 are permitted. For example:
284    \\040   is another way of writing a space  .sp
285    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
286    .\" JOIN
287      \e40    is the same, provided there are fewer than 40
288              previous capturing subpatterns              previous capturing subpatterns
289    \\7     is always a back reference    \e7     is always a back reference
290    \\11    might be a back reference, or another way of  .\" JOIN
291      \e11    might be a back reference, or another way of
292              writing a tab              writing a tab
293    \\011   is always a tab    \e011   is always a tab
294    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
295    \\113   might be a back reference, otherwise the  .\" JOIN
296      \e113   might be a back reference, otherwise the
297              character with octal code 113              character with octal code 113
298    \\377   might be a back reference, otherwise  .\" JOIN
299      \e377   might be a back reference, otherwise
300              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
301    \\81    is either a back reference, or a binary zero  .\" JOIN
302      \e81    is either a back reference, or a binary zero
303              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
304    .sp
305  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
306  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
307    .P
308  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
309  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
310  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
311  backspace character (hex 08). Outside a character class it has a different  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
312  meaning (see below).  unrecognized escape sequences, they are treated as the literal characters "B",
313    "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
314  The third use of backslash is for specifying generic character types:  set. Outside a character class, these sequences have different meanings.
315    .
316    \\d     any decimal digit  .
317    \\D     any character that is not a decimal digit  .SS "Absolute and relative back references"
318    \\s     any whitespace character  .rs
319    \\S     any character that is not a whitespace character  .sp
320    \\w     any "word" character  The sequence \eg followed by an unsigned or a negative number, optionally
321    \\W     any "non-word" character  enclosed in braces, is an absolute or relative back reference. A named back
322    reference can be coded as \eg{name}. Back references are discussed
323  Each pair of escape sequences partitions the complete set of characters into  .\" HTML <a href="#backreferences">
324  two disjoint sets. Any given character matches one, and only one, of each pair.  .\" </a>
325    later,
326  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  .\"
327  \\w, and always match \\D, \\S, and \\W.  following the discussion of
328    .\" HTML <a href="#subpattern">
329  For compatibility with Perl, \\s does not match the VT character (code 11).  .\" </a>
330  This makes it different from the the POSIX "space" class. The \\s characters  parenthesized subpatterns.
331  are HT (9), LF (10), FF (12), CR (13), and space (32).  .\"
332    .
333  A "word" character is any letter or digit or the underscore character, that is,  .
334  any character which can be part of a Perl "word". The definition of letters and  .SS "Absolute and relative subroutine calls"
335  digits is controlled by PCRE's character tables, and may vary if locale-  .rs
336  specific matching is taking place (see  .sp
337    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
338    a number enclosed either in angle brackets or single quotes, is an alternative
339    syntax for referencing a subpattern as a "subroutine". Details are discussed
340    .\" HTML <a href="#onigurumasubroutines">
341    .\" </a>
342    later.
343    .\"
344    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
345    synonymous. The former is a back reference; the latter is a
346    .\" HTML <a href="#subpatternsassubroutines">
347    .\" </a>
348    subroutine
349    .\"
350    call.
351    .
352    .
353    .\" HTML <a name="genericchartypes"></a>
354    .SS "Generic character types"
355    .rs
356    .sp
357    Another use of backslash is for specifying generic character types:
358    .sp
359      \ed     any decimal digit
360      \eD     any character that is not a decimal digit
361      \eh     any horizontal whitespace character
362      \eH     any character that is not a horizontal whitespace character
363      \es     any whitespace character
364      \eS     any character that is not a whitespace character
365      \ev     any vertical whitespace character
366      \eV     any character that is not a vertical whitespace character
367      \ew     any "word" character
368      \eW     any "non-word" character
369    .sp
370    There is also the single sequence \eN, which matches a non-newline character.
371    This is the same as
372    .\" HTML <a href="#fullstopdot">
373    .\" </a>
374    the "." metacharacter
375    .\"
376    when PCRE_DOTALL is not set.
377    .P
378    Each pair of lower and upper case escape sequences partitions the complete set
379    of characters into two disjoint sets. Any given character matches one, and only
380    one, of each pair. The sequences can appear both inside and outside character
381    classes. They each match one character of the appropriate type. If the current
382    matching point is at the end of the subject string, all of them fail, because
383    there is no character to match.
384    .P
385    For compatibility with Perl, \es does not match the VT character (code 11).
386    This makes it different from the the POSIX "space" class. The \es characters
387    are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
388    included in a Perl script, \es may match the VT character. In PCRE, it never
389    does.
390    .P
391    A "word" character is an underscore or any character that is a letter or digit.
392    By default, the definition of letters and digits is controlled by PCRE's
393    low-valued character tables, and may vary if locale-specific matching is taking
394    place (see
395  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
396  .\" </a>  .\" </a>
397  "Locale support"  "Locale support"
398  .\"  .\"
399  in the  in the
400  .\" HREF  .\" HREF
401  \fBpcreapi\fR  \fBpcreapi\fP
402  .\"  .\"
403  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
404  than 128 are used for accented letters, and these are matched by \\w.  or "french" in Windows, some character codes greater than 128 are used for
405    accented letters, and these are then matched by \ew. The use of locales with
406  These character type sequences can appear both inside and outside character  Unicode is discouraged.
407  classes. They each match one character of the appropriate type. If the current  .P
408  matching point is at the end of the subject string, all of them fail, since  By default, in UTF-8 mode, characters with values greater than 128 never match
409  there is no character to match.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
410    their original meanings from before UTF-8 support was available, mainly for
411  The fourth use of backslash is for certain simple assertions. An assertion  efficiency reasons. However, if PCRE is compiled with Unicode property support,
412    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
413    properties are used to determine character types, as follows:
414    .sp
415      \ed  any character that \ep{Nd} matches (decimal digit)
416      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
417      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
418    .sp
419    The upper case escapes match the inverse sets of characters. Note that \ed
420    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
421    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
422    \eB because they are defined in terms of \ew and \eW. Matching these sequences
423    is noticeably slower when PCRE_UCP is set.
424    .P
425    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
426    other sequences, which match only ASCII characters by default, these always
427    match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
428    set. The horizontal space characters are:
429    .sp
430      U+0009     Horizontal tab
431      U+0020     Space
432      U+00A0     Non-break space
433      U+1680     Ogham space mark
434      U+180E     Mongolian vowel separator
435      U+2000     En quad
436      U+2001     Em quad
437      U+2002     En space
438      U+2003     Em space
439      U+2004     Three-per-em space
440      U+2005     Four-per-em space
441      U+2006     Six-per-em space
442      U+2007     Figure space
443      U+2008     Punctuation space
444      U+2009     Thin space
445      U+200A     Hair space
446      U+202F     Narrow no-break space
447      U+205F     Medium mathematical space
448      U+3000     Ideographic space
449    .sp
450    The vertical space characters are:
451    .sp
452      U+000A     Linefeed
453      U+000B     Vertical tab
454      U+000C     Formfeed
455      U+000D     Carriage return
456      U+0085     Next line
457      U+2028     Line separator
458      U+2029     Paragraph separator
459    .
460    .
461    .\" HTML <a name="newlineseq"></a>
462    .SS "Newline sequences"
463    .rs
464    .sp
465    Outside a character class, by default, the escape sequence \eR matches any
466    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
467    equivalent to the following:
468    .sp
469      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
470    .sp
471    This is an example of an "atomic group", details of which are given
472    .\" HTML <a href="#atomicgroup">
473    .\" </a>
474    below.
475    .\"
476    This particular group matches either the two-character sequence CR followed by
477    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
478    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
479    line, U+0085). The two-character sequence is treated as a single unit that
480    cannot be split.
481    .P
482    In UTF-8 mode, two additional characters whose codepoints are greater than 255
483    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
484    Unicode character property support is not needed for these characters to be
485    recognized.
486    .P
487    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
488    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
489    either at compile time or when the pattern is matched. (BSR is an abbrevation
490    for "backslash R".) This can be made the default when PCRE is built; if this is
491    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
492    It is also possible to specify these settings by starting a pattern string with
493    one of the following sequences:
494    .sp
495      (*BSR_ANYCRLF)   CR, LF, or CRLF only
496      (*BSR_UNICODE)   any Unicode newline sequence
497    .sp
498    These override the default and the options given to \fBpcre_compile()\fP or
499    \fBpcre_compile2()\fP, but they can be overridden by options given to
500    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
501    which are not Perl-compatible, are recognized only at the very start of a
502    pattern, and that they must be in upper case. If more than one of them is
503    present, the last one is used. They can be combined with a change of newline
504    convention; for example, a pattern can start with:
505    .sp
506      (*ANY)(*BSR_ANYCRLF)
507    .sp
508    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
509    a character class, \eR is treated as an unrecognized escape sequence, and so
510    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
511    .
512    .
513    .\" HTML <a name="uniextseq"></a>
514    .SS Unicode character properties
515    .rs
516    .sp
517    When PCRE is built with Unicode character property support, three additional
518    escape sequences that match characters with specific properties are available.
519    When not in UTF-8 mode, these sequences are of course limited to testing
520    characters whose codepoints are less than 256, but they do work in this mode.
521    The extra escape sequences are:
522    .sp
523      \ep{\fIxx\fP}   a character with the \fIxx\fP property
524      \eP{\fIxx\fP}   a character without the \fIxx\fP property
525      \eX       an extended Unicode sequence
526    .sp
527    The property names represented by \fIxx\fP above are limited to the Unicode
528    script names, the general category properties, "Any", which matches any
529    character (including newline), and some special PCRE properties (described
530    in the
531    .\" HTML <a href="#extraprops">
532    .\" </a>
533    next section).
534    .\"
535    Other Perl properties such as "InMusicalSymbols" are not currently supported by
536    PCRE. Note that \eP{Any} does not match any characters, so always causes a
537    match failure.
538    .P
539    Sets of Unicode characters are defined as belonging to certain scripts. A
540    character from one of these sets can be matched using a script name. For
541    example:
542    .sp
543      \ep{Greek}
544      \eP{Han}
545    .sp
546    Those that are not part of an identified script are lumped together as
547    "Common". The current list of scripts is:
548    .P
549    Arabic,
550    Armenian,
551    Avestan,
552    Balinese,
553    Bamum,
554    Bengali,
555    Bopomofo,
556    Braille,
557    Buginese,
558    Buhid,
559    Canadian_Aboriginal,
560    Carian,
561    Cham,
562    Cherokee,
563    Common,
564    Coptic,
565    Cuneiform,
566    Cypriot,
567    Cyrillic,
568    Deseret,
569    Devanagari,
570    Egyptian_Hieroglyphs,
571    Ethiopic,
572    Georgian,
573    Glagolitic,
574    Gothic,
575    Greek,
576    Gujarati,
577    Gurmukhi,
578    Han,
579    Hangul,
580    Hanunoo,
581    Hebrew,
582    Hiragana,
583    Imperial_Aramaic,
584    Inherited,
585    Inscriptional_Pahlavi,
586    Inscriptional_Parthian,
587    Javanese,
588    Kaithi,
589    Kannada,
590    Katakana,
591    Kayah_Li,
592    Kharoshthi,
593    Khmer,
594    Lao,
595    Latin,
596    Lepcha,
597    Limbu,
598    Linear_B,
599    Lisu,
600    Lycian,
601    Lydian,
602    Malayalam,
603    Meetei_Mayek,
604    Mongolian,
605    Myanmar,
606    New_Tai_Lue,
607    Nko,
608    Ogham,
609    Old_Italic,
610    Old_Persian,
611    Old_South_Arabian,
612    Old_Turkic,
613    Ol_Chiki,
614    Oriya,
615    Osmanya,
616    Phags_Pa,
617    Phoenician,
618    Rejang,
619    Runic,
620    Samaritan,
621    Saurashtra,
622    Shavian,
623    Sinhala,
624    Sundanese,
625    Syloti_Nagri,
626    Syriac,
627    Tagalog,
628    Tagbanwa,
629    Tai_Le,
630    Tai_Tham,
631    Tai_Viet,
632    Tamil,
633    Telugu,
634    Thaana,
635    Thai,
636    Tibetan,
637    Tifinagh,
638    Ugaritic,
639    Vai,
640    Yi.
641    .P
642    Each character has exactly one Unicode general category property, specified by
643    a two-letter abbreviation. For compatibility with Perl, negation can be
644    specified by including a circumflex between the opening brace and the property
645    name. For example, \ep{^Lu} is the same as \eP{Lu}.
646    .P
647    If only one letter is specified with \ep or \eP, it includes all the general
648    category properties that start with that letter. In this case, in the absence
649    of negation, the curly brackets in the escape sequence are optional; these two
650    examples have the same effect:
651    .sp
652      \ep{L}
653      \epL
654    .sp
655    The following general category property codes are supported:
656    .sp
657      C     Other
658      Cc    Control
659      Cf    Format
660      Cn    Unassigned
661      Co    Private use
662      Cs    Surrogate
663    .sp
664      L     Letter
665      Ll    Lower case letter
666      Lm    Modifier letter
667      Lo    Other letter
668      Lt    Title case letter
669      Lu    Upper case letter
670    .sp
671      M     Mark
672      Mc    Spacing mark
673      Me    Enclosing mark
674      Mn    Non-spacing mark
675    .sp
676      N     Number
677      Nd    Decimal number
678      Nl    Letter number
679      No    Other number
680    .sp
681      P     Punctuation
682      Pc    Connector punctuation
683      Pd    Dash punctuation
684      Pe    Close punctuation
685      Pf    Final punctuation
686      Pi    Initial punctuation
687      Po    Other punctuation
688      Ps    Open punctuation
689    .sp
690      S     Symbol
691      Sc    Currency symbol
692      Sk    Modifier symbol
693      Sm    Mathematical symbol
694      So    Other symbol
695    .sp
696      Z     Separator
697      Zl    Line separator
698      Zp    Paragraph separator
699      Zs    Space separator
700    .sp
701    The special property L& is also supported: it matches a character that has
702    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
703    a modifier or "other".
704    .P
705    The Cs (Surrogate) property applies only to characters in the range U+D800 to
706    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
707    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
708    (see the discussion of PCRE_NO_UTF8_CHECK in the
709    .\" HREF
710    \fBpcreapi\fP
711    .\"
712    page). Perl does not support the Cs property.
713    .P
714    The long synonyms for property names that Perl supports (such as \ep{Letter})
715    are not supported by PCRE, nor is it permitted to prefix any of these
716    properties with "Is".
717    .P
718    No character that is in the Unicode table has the Cn (unassigned) property.
719    Instead, this property is assumed for any code point that is not in the
720    Unicode table.
721    .P
722    Specifying caseless matching does not affect these escape sequences. For
723    example, \ep{Lu} always matches only upper case letters.
724    .P
725    The \eX escape matches any number of Unicode characters that form an extended
726    Unicode sequence. \eX is equivalent to
727    .sp
728      (?>\ePM\epM*)
729    .sp
730    That is, it matches a character without the "mark" property, followed by zero
731    or more characters with the "mark" property, and treats the sequence as an
732    atomic group
733    .\" HTML <a href="#atomicgroup">
734    .\" </a>
735    (see below).
736    .\"
737    Characters with the "mark" property are typically accents that affect the
738    preceding character. None of them have codepoints less than 256, so in
739    non-UTF-8 mode \eX matches any one character.
740    .P
741    Matching characters by Unicode property is not fast, because PCRE has to search
742    a structure that contains data for over fifteen thousand characters. That is
743    why the traditional escape sequences such as \ed and \ew do not use Unicode
744    properties in PCRE by default, though you can make them do so by setting the
745    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
746    (*UCP).
747    .
748    .
749    .\" HTML <a name="extraprops"></a>
750    .SS PCRE's additional properties
751    .rs
752    .sp
753    As well as the standard Unicode properties described in the previous
754    section, PCRE supports four more that make it possible to convert traditional
755    escape sequences such as \ew and \es and POSIX character classes to use Unicode
756    properties. PCRE uses these non-standard, non-Perl properties internally when
757    PCRE_UCP is set. They are:
758    .sp
759      Xan   Any alphanumeric character
760      Xps   Any POSIX space character
761      Xsp   Any Perl space character
762      Xwd   Any Perl "word" character
763    .sp
764    Xan matches characters that have either the L (letter) or the N (number)
765    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
766    carriage return, and any other character that has the Z (separator) property.
767    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
768    same characters as Xan, plus underscore.
769    .
770    .
771    .\" HTML <a name="resetmatchstart"></a>
772    .SS "Resetting the match start"
773    .rs
774    .sp
775    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
776    matched characters not to be included in the final matched sequence. For
777    example, the pattern:
778    .sp
779      foo\eKbar
780    .sp
781    matches "foobar", but reports that it has matched "bar". This feature is
782    similar to a lookbehind assertion
783    .\" HTML <a href="#lookbehind">
784    .\" </a>
785    (described below).
786    .\"
787    However, in this case, the part of the subject before the real match does not
788    have to be of fixed length, as lookbehind assertions do. The use of \eK does
789    not interfere with the setting of
790    .\" HTML <a href="#subpattern">
791    .\" </a>
792    captured substrings.
793    .\"
794    For example, when the pattern
795    .sp
796      (foo)\eKbar
797    .sp
798    matches "foobar", the first substring is still set to "foo".
799    .P
800    Perl documents that the use of \eK within assertions is "not well defined". In
801    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
802    ignored in negative assertions.
803    .
804    .
805    .\" HTML <a name="smallassertions"></a>
806    .SS "Simple assertions"
807    .rs
808    .sp
809    The final use of backslash is for certain simple assertions. An assertion
810  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,
811  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
812  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
813  assertions are  .\" HTML <a href="#bigassertions">
814    .\" </a>
815    \\b     matches at a word boundary  below.
816    \\B     matches when not at a word boundary  .\"
817    \\A     matches at start of subject  The backslashed assertions are:
818    \\Z     matches at end of subject or before newline at end  .sp
819    \\z     matches at end of subject    \eb     matches at a word boundary
820    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
821      \eA     matches at the start of the subject
822  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
823  different meaning, namely the backspace character, inside a character class).            also matches before a newline at the end of the subject
824      \ez     matches only at the end of the subject
825      \eG     matches at the first matching position in the subject
826    .sp
827    Inside a character class, \eb has a different meaning; it matches the backspace
828    character. If any other of these assertions appears in a character class, by
829    default it matches the corresponding literal character (for example, \eB
830    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
831    escape sequence" error is generated instead.
832    .P
833  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
834  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
835  \\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
836  first or last character matches \\w, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
837    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
838  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
839  dollar (described below) in that they only ever match at the very start and end  of word" or "end of word" metasequence. However, whatever follows \eb normally
840  of the subject string, whatever options are set. Thus, they are independent of  determines which it is. For example, the fragment \eba matches "a" at the start
841  multiline mode.  of a word.
842    .P
843  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
844  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  dollar (described in the next section) in that they only ever match at the very
845  matching is to start at a point other than the beginning of the subject, \\A  start and end of the subject string, whatever options are set. Thus, they are
846  can never match. The difference between \\Z and \\z is that \\Z matches before  independent of multiline mode. These three assertions are not affected by the
847  a newline that is the last character of the string as well as at the end of the  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
848  string, whereas \\z matches only at the end.  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
849    argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
850  The \\G assertion is true only when the current matching position is at the  at a point other than the beginning of the subject, \eA can never match. The
851  start point of the match, as specified by the \fIstartoffset\fR argument of  difference between \eZ and \ez is that \eZ matches before a newline at the end
852  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  of the string as well as at the very end, whereas \ez matches only at the end.
853  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  .P
854    The \eG assertion is true only when the current matching position is at the
855    start point of the match, as specified by the \fIstartoffset\fP argument of
856    \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
857    non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
858  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
859  implementation where \\G can be useful.  implementation where \eG can be useful.
860    .P
861  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
862  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
863  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
864  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
865  reproduce this behaviour.  reproduce this behaviour.
866    .P
867  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
868  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
869  regular expression.  regular expression.
870    .
871  .SH CIRCUMFLEX AND DOLLAR  .
872    .SH "CIRCUMFLEX AND DOLLAR"
873  .rs  .rs
874  .sp  .sp
875  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
876  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
877  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
878  \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
879  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
880  meaning (see below).  meaning
881    .\" HTML <a href="#characterclass">
882    .\" </a>
883    (see below).
884    .\"
885    .P
886  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
887  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
888  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 890  possible alternatives start with a circu
890  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
891  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
892  to be anchored.)  to be anchored.)
893    .P
894  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
895  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
896  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
897  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
898  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
899  Dollar has no special meaning in a character class.  character class.
900    .P
901  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
902  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
903  does not affect the \\Z assertion.  does not affect the \eZ assertion.
904    .P
905  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
906  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
907  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
908  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
909  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
910  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
911  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
912  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
913  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
914  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
915    patterns that are anchored in single line mode because all branches start with
916  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  ^ are not anchored in multiline mode, and a match for circumflex is possible
917    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
918    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
919    .P
920    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
921  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
922  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
923    .
924  .SH FULL STOP (PERIOD, DOT)  .
925    .\" HTML <a name="fullstopdot"></a>
926    .SH "FULL STOP (PERIOD, DOT) AND \eN"
927  .rs  .rs
928  .sp  .sp
929  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
930  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
931  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
932  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
933  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
934  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
935  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
936    (including isolated CRs and LFs). When any Unicode line endings are being
937  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
938  .rs  characters.
939  .sp  .P
940  Outside a character class, the escape sequence \\C matches any one byte, both  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
941  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  option is set, a dot matches any one character, without exception. If the
942  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  two-character sequence CRLF is present in the subject string, it takes two dots
943  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
944  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
945    The handling of dot is entirely independent of the handling of circumflex and
946  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  dollar, the only relationship being that they both involve newlines. Dot has no
947  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
948    .P
949  .SH SQUARE BRACKETS  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
950    set. In other words, it matches any one character except one that signifies the
951    end of a line.
952    .
953    .
954    .SH "MATCHING A SINGLE BYTE"
955    .rs
956    .sp
957    Outside a character class, the escape sequence \eC matches any one byte, both
958    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
959    characters. The feature is provided in Perl in order to match individual bytes
960    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
961    what remains in the string may be a malformed UTF-8 string. For this reason,
962    the \eC escape sequence is best avoided.
963    .P
964    PCRE does not allow \eC to appear in lookbehind assertions
965    .\" HTML <a href="#lookbehind">
966    .\" </a>
967    (described below),
968    .\"
969    because in UTF-8 mode this would make it impossible to calculate the length of
970    the lookbehind.
971    .
972    .
973    .\" HTML <a name="characterclass"></a>
974    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
975  .rs  .rs
976  .sp  .sp
977  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
978  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
979  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
980  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
981  escaped with a backslash.  a member of the class, it should be the first data character in the class
982    (after an initial circumflex, if present) or escaped with a backslash.
983    .P
984  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
985  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
986  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
987  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
988  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
989  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
990  backslash.  backslash.
991    .P
992  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
993  [^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
994  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
995  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
996  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
997  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
998    string.
999    .P
1000  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
1001  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.
1002    .P
1003  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
1004  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
1005  "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
1006  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
1007  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
1008    always possible. For characters with higher values, the concept of case is
1009  The newline character is never treated in any special way in character classes,  supported if PCRE is compiled with Unicode property support, but not otherwise.
1010  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1011  such as [^a] will always match a newline.  you must ensure that PCRE is compiled with Unicode property support as well as
1012    with UTF-8 support.
1013    .P
1014    Characters that might indicate line breaks are never treated in any special way
1015    when matching character classes, whatever line-ending sequence is in use, and
1016    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1017    such as [^a] always matches one of these characters.
1018    .P
1019  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
1020  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,
1021  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
1022  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
1023  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.
1024    .P
1025  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
1026  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
1027  ("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
1028  "-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
1029  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
1030  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
1031  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
1032    .P
1033  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
1034  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1035  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
1036  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
1037    .P
1038  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
1039  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
1040  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1041  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\exc8-\excb] matches accented E
1042    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1043  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
1044  character class, and add the characters that they match to the class. For  property support.
1045  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
1046  conveniently be used with the upper case character types to specify a more  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1047  restricted set of characters than the matching lower case type. For example,  \eW may also appear in a character class, and add the characters that they
1048  the class [^\\W_] matches any letter or digit, but not underscore.  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1049    circumflex can conveniently be used with the upper case character types to
1050  All non-alphameric characters other than \\, -, ^ (at the start) and the  specify a more restricted set of characters than the matching lower case type.
1051  terminating ] are non-special in character classes, but it does no harm if they  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1052  are escaped.  .P
1053    The only metacharacters that are recognized in character classes are backslash,
1054  .SH POSIX CHARACTER CLASSES  hyphen (only where it can be interpreted as specifying a range), circumflex
1055    (only at the start), opening square bracket (only when it can be interpreted as
1056    introducing a POSIX class name - see the next section), and the terminating
1057    closing square bracket. However, escaping other non-alphanumeric characters
1058    does no harm.
1059    .
1060    .
1061    .SH "POSIX CHARACTER CLASSES"
1062  .rs  .rs
1063  .sp  .sp
1064  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
1065  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1066  this notation. For example,  this notation. For example,
1067    .sp
1068    [01[:alpha:]%]    [01[:alpha:]%]
1069    .sp
1070  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1071  are  are:
1072    .sp
1073    alnum    letters and digits    alnum    letters and digits
1074    alpha    letters    alpha    letters
1075    ascii    character codes 0 - 127    ascii    character codes 0 - 127
1076    blank    space or tab only    blank    space or tab only
1077    cntrl    control characters    cntrl    control characters
1078    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
1079    graph    printing characters, excluding space    graph    printing characters, excluding space
1080    lower    lower case letters    lower    lower case letters
1081    print    printing characters, including space    print    printing characters, including space
1082    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1083    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
1084    upper    upper case letters    upper    upper case letters
1085    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
1086    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1087    .sp
1088  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
1089  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
1090  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
1091  compatibility).  compatibility).
1092    .P
1093  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
1094  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
1095  after the colon. For example,  after the colon. For example,
1096    .sp
1097    [12[:^digit:]]    [12[:^digit:]]
1098    .sp
1099  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
1100  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
1101  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1102    .P
1103  In UTF-8 mode, characters with values greater than 255 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1104  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1105    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1106  .SH VERTICAL BAR  character properties are used. This is achieved by replacing the POSIX classes
1107    by other sequences, as follows:
1108    .sp
1109      [:alnum:]  becomes  \ep{Xan}
1110      [:alpha:]  becomes  \ep{L}
1111      [:blank:]  becomes  \eh
1112      [:digit:]  becomes  \ep{Nd}
1113      [:lower:]  becomes  \ep{Ll}
1114      [:space:]  becomes  \ep{Xps}
1115      [:upper:]  becomes  \ep{Lu}
1116      [:word:]   becomes  \ep{Xwd}
1117    .sp
1118    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1119    classes are unchanged, and match only characters with code points less than
1120    128.
1121    .
1122    .
1123    .SH "VERTICAL BAR"
1124  .rs  .rs
1125  .sp  .sp
1126  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1127  the pattern  the pattern
1128    .sp
1129    gilbert|sullivan    gilbert|sullivan
1130    .sp
1131  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1132  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1133  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1134  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
1135  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
1136  pattern as well as the alternative in the subpattern.  .\" </a>
1137    (defined below),
1138  .SH INTERNAL OPTION SETTING  .\"
1139    "succeeds" means matching the rest of the main pattern as well as the
1140    alternative in the subpattern.
1141    .
1142    .
1143    .SH "INTERNAL OPTION SETTING"
1144  .rs  .rs
1145  .sp  .sp
1146  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1147  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1148  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1149    The option letters are
1150    .sp
1151    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1152    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
1153    s  for PCRE_DOTALL    s  for PCRE_DOTALL
1154    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
1155    .sp
1156  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
1157  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
1158  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1159  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1160  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
1161  unset.  unset.
1162    .P
1163  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1164  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1165  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1166  the global options (and it will therefore show up in data extracted by the  .P
1167  \fBpcre_fullinfo()\fR function).  When one of these option changes occurs at top level (that is, not inside
1168    subpattern parentheses), the change applies to the remainder of the pattern
1169  An option change within a subpattern affects only that part of the current  that follows. If the change is placed right at the start of a pattern, PCRE
1170  pattern that follows it, so  extracts it into the global options (and it will therefore show up in data
1171    extracted by the \fBpcre_fullinfo()\fP function).
1172    .P
1173    An option change within a subpattern (see below for a description of
1174    subpatterns) affects only that part of the current pattern that follows it, so
1175    .sp
1176    (a(?i)b)c    (a(?i)b)c
1177    .sp
1178  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).
1179  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
1180  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
1181  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
1182    .sp
1183    (a(?i)b|c)    (a(?i)b|c)
1184    .sp
1185  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
1186  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
1187  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1188  behaviour otherwise.  behaviour otherwise.
1189    .P
1190  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1191  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1192  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1193  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1194  when it is at top level. It is best put at the start.  section entitled
1195    .\" HTML <a href="#newlineseq">
1196    .\" </a>
1197    "Newline sequences"
1198    .\"
1199    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1200    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1201    PCRE_UTF8 and the PCRE_UCP options, respectively.
1202    .
1203    .
1204    .\" HTML <a name="subpattern"></a>
1205  .SH SUBPATTERNS  .SH SUBPATTERNS
1206  .rs  .rs
1207  .sp  .sp
1208  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1209  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1210    .sp
1211  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
1212    .sp
1213    cat(aract|erpillar|)    cat(aract|erpillar|)
1214    .sp
1215  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1216  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1217    .sp
1218  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
1219  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
1220  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
1221  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1222  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
1223    .P
1224  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
1225    .sp
1226    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1227    .sp
1228  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,
1229  2, and 3, respectively.  2, and 3, respectively.
1230    .P
1231  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
1232  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
1233  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
1234  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
1235  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1236  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1237    .sp
1238    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1239    .sp
1240  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
1241  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1242  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1243  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
1244  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1245  the ":". Thus the two patterns  the ":". Thus the two patterns
1246    .sp
1247    (?i:saturday|sunday)    (?i:saturday|sunday)
1248    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1249    .sp
1250  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1251  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
1252  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
1253  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1254    .
1255  .SH NAMED SUBPATTERNS  .
1256    .\" HTML <a name="dupsubpatternnumber"></a>
1257    .SH "DUPLICATE SUBPATTERN NUMBERS"
1258    .rs
1259    .sp
1260    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1261    the same numbers for its capturing parentheses. Such a subpattern starts with
1262    (?| and is itself a non-capturing subpattern. For example, consider this
1263    pattern:
1264    .sp
1265      (?|(Sat)ur|(Sun))day
1266    .sp
1267    Because the two alternatives are inside a (?| group, both sets of capturing
1268    parentheses are numbered one. Thus, when the pattern matches, you can look
1269    at captured substring number one, whichever alternative matched. This construct
1270    is useful when you want to capture part, but not all, of one of a number of
1271    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1272    number is reset at the start of each branch. The numbers of any capturing
1273    buffers that follow the subpattern start after the highest number used in any
1274    branch. The following example is taken from the Perl documentation.
1275    The numbers underneath show in which buffer the captured content will be
1276    stored.
1277    .sp
1278      # before  ---------------branch-reset----------- after
1279      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1280      # 1            2         2  3        2     3     4
1281    .sp
1282    A back reference to a numbered subpattern uses the most recent value that is
1283    set for that number by any subpattern. The following pattern matches "abcabc"
1284    or "defdef":
1285    .sp
1286      /(?|(abc)|(def))\e1/
1287    .sp
1288    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1289    refers to the first one in the pattern with the given number. The following
1290    pattern matches "abcabc" or "defabc":
1291    .sp
1292      /(?|(abc)|(def))(?1)/
1293    .sp
1294    If a
1295    .\" HTML <a href="#conditions">
1296    .\" </a>
1297    condition test
1298    .\"
1299    for a subpattern's having matched refers to a non-unique number, the test is
1300    true if any of the subpatterns of that number have matched.
1301    .P
1302    An alternative approach to using this "branch reset" feature is to use
1303    duplicate named subpatterns, as described in the next section.
1304    .
1305    .
1306    .SH "NAMED SUBPATTERNS"
1307  .rs  .rs
1308  .sp  .sp
1309  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
1310  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1311  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
1312  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1313  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1314  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1315    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1316  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1317  PCRE API provides function calls for extracting the name-to-number translation  .P
1318  table from a compiled pattern. For further details see the  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1319    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1320    parentheses from other parts of the pattern, such as
1321    .\" HTML <a href="#backreferences">
1322    .\" </a>
1323    back references,
1324    .\"
1325    .\" HTML <a href="#recursion">
1326    .\" </a>
1327    recursion,
1328    .\"
1329    and
1330    .\" HTML <a href="#conditions">
1331    .\" </a>
1332    conditions,
1333    .\"
1334    can be made by name as well as by number.
1335    .P
1336    Names consist of up to 32 alphanumeric characters and underscores. Named
1337    capturing parentheses are still allocated numbers as well as names, exactly as
1338    if the names were not present. The PCRE API provides function calls for
1339    extracting the name-to-number translation table from a compiled pattern. There
1340    is also a convenience function for extracting a captured substring by name.
1341    .P
1342    By default, a name must be unique within a pattern, but it is possible to relax
1343    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1344    names are also always permitted for subpatterns with the same number, set up as
1345    described in the previous section.) Duplicate names can be useful for patterns
1346    where only one instance of the named parentheses can match. Suppose you want to
1347    match the name of a weekday, either as a 3-letter abbreviation or as the full
1348    name, and in both cases you want to extract the abbreviation. This pattern
1349    (ignoring the line breaks) does the job:
1350    .sp
1351      (?<DN>Mon|Fri|Sun)(?:day)?|
1352      (?<DN>Tue)(?:sday)?|
1353      (?<DN>Wed)(?:nesday)?|
1354      (?<DN>Thu)(?:rsday)?|
1355      (?<DN>Sat)(?:urday)?
1356    .sp
1357    There are five capturing substrings, but only one is ever set after a match.
1358    (An alternative way of solving this problem is to use a "branch reset"
1359    subpattern, as described in the previous section.)
1360    .P
1361    The convenience function for extracting the data by name returns the substring
1362    for the first (and in this example, the only) subpattern of that name that
1363    matched. This saves searching to find which numbered subpattern it was.
1364    .P
1365    If you make a back reference to a non-unique named subpattern from elsewhere in
1366    the pattern, the one that corresponds to the first occurrence of the name is
1367    used. In the absence of duplicate numbers (see the previous section) this is
1368    the one with the lowest number. If you use a named reference in a condition
1369    test (see the
1370    .\"
1371    .\" HTML <a href="#conditions">
1372    .\" </a>
1373    section about conditions
1374    .\"
1375    below), either to check whether a subpattern has matched, or to check for
1376    recursion, all subpatterns with the same name are tested. If the condition is
1377    true for any one of them, the overall condition is true. This is the same
1378    behaviour as testing by number. For further details of the interfaces for
1379    handling named subpatterns, see the
1380  .\" HREF  .\" HREF
1381  \fBpcreapi\fR  \fBpcreapi\fP
1382  .\"  .\"
1383  documentation.  documentation.
1384    .P
1385    \fBWarning:\fP You cannot use different names to distinguish between two
1386    subpatterns with the same number because PCRE uses only the numbers when
1387    matching. For this reason, an error is given at compile time if different names
1388    are given to subpatterns with the same number. However, you can give the same
1389    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1390    .
1391    .
1392  .SH REPETITION  .SH REPETITION
1393  .rs  .rs
1394  .sp  .sp
1395  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1396  items:  items:
1397    .sp
1398    a literal data character    a literal data character
1399    the . metacharacter    the dot metacharacter
1400    the \\C escape sequence    the \eC escape sequence
1401    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1402      the \eR escape sequence
1403      an escape such as \ed that matches a single character
1404    a character class    a character class
1405    a back reference (see next section)    a back reference (see next section)
1406    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1407      a recursive or "subroutine" call to a subpattern
1408    .sp
1409  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1410  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1411  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
1412  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1413    .sp
1414    z{2,4}    z{2,4}
1415    .sp
1416  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
1417  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
1418  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
1419  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1420    .sp
1421    [aeiou]{3,}    [aeiou]{3,}
1422    .sp
1423  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1424    .sp
1425    \\d{8}    \ed{8}
1426    .sp
1427  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
1428  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
1429  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
1430  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1431    .P
1432  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
1433  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
1434  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1435    support is available, \eX{3} matches three Unicode extended sequences, each of
1436    which may be several bytes long (and they may be of different lengths).
1437    .P
1438  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
1439  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1440    subpatterns that are referenced as
1441  For convenience (and historical compatibility) the three most common  .\" HTML <a href="#subpatternsassubroutines">
1442  quantifiers have single-character abbreviations:  .\" </a>
1443    subroutines
1444    .\"
1445    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1446    quantifier are omitted from the compiled pattern.
1447    .P
1448    For convenience, the three most common quantifiers have single-character
1449    abbreviations:
1450    .sp
1451    *    is equivalent to {0,}    *    is equivalent to {0,}
1452    +    is equivalent to {1,}    +    is equivalent to {1,}
1453    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1454    .sp
1455  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
1456  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:
1457    .sp
1458    (a?)*    (a?)*
1459    .sp
1460  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
1461  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
1462  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
1463  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1464    .P
1465  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
1466  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1467  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
1468  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 */
1469  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1470  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1471    .sp
1472    /\\*.*\\*/    /\e*.*\e*/
1473    .sp
1474  to the string  to the string
1475    .sp
1476    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1477    .sp
1478  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 .*
1479  item.  item.
1480    .P
1481  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
1482  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1483  pattern  pattern
1484    .sp
1485    /\\*.*?\\*/    /\e*.*?\e*/
1486    .sp
1487  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
1488  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1489  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
1490  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
1491    .sp
1492    \\d??\\d    \ed??\ed
1493    .sp
1494  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
1495  way the rest of the pattern matches.  way the rest of the pattern matches.
1496    .P
1497  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1498  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
1499  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
1500  default behaviour.  default behaviour.
1501    .P
1502  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1503  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
1504  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1505    .P
1506  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
1507  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1508  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1509  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
1510  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
1511  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1512    .P
1513  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
1514  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1515  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1516    .P
1517  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1518  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1519  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1520  succeed. Consider, for example:  succeeds. Consider, for example:
1521    .sp
1522    (.*)abc\\1    (.*)abc\e1
1523    .sp
1524  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
1525  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1526    .P
1527  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1528  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1529    .sp
1530    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1531    .sp
1532  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1533  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1534  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1535  example, after  example, after
1536    .sp
1537    /(a|(b))+/    /(a|(b))+/
1538    .sp
1539  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1540    .
1541  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1542    .\" HTML <a name="atomicgroup"></a>
1543    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1544  .rs  .rs
1545  .sp  .sp
1546  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1547  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1548  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1549  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1550  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1551  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1552    .P
1553  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
1554    .sp
1555    123456bar    123456bar
1556    .sp
1557  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
1558  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+
1559  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"
1560  (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
1561  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.
1562    .P
1563  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1564  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
1565  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1566    .sp
1567    (?>\\d+)foo    (?>\ed+)foo
1568    .sp
1569  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
1570  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
1571  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1572  normal.  normal.
1573    .P
1574  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
1575  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1576  the current point in the subject string.  the current point in the subject string.
1577    .P
1578  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1579  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
1580  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
1581  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,
1582  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1583    .P
1584  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1585  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1586  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
1587  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1588  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1589  previous example can be rewritten as  previous example can be rewritten as
1590    .sp
1591    \\d++bar    \ed++foo
1592    .sp
1593    Note that a possessive quantifier can be used with an entire group, for
1594    example:
1595    .sp
1596      (abc|xyz){2,3}+
1597    .sp
1598  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1599  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
1600  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1601  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1602    difference; possessive quantifiers should be slightly faster.
1603  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1604  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1605    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1606    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1607    package, and PCRE copied it from there. It ultimately found its way into Perl
1608    at release 5.10.
1609    .P
1610    PCRE has an optimization that automatically "possessifies" certain simple
1611    pattern constructs. For example, the sequence A+B is treated as A++B because
1612    there is no point in backtracking into a sequence of A's when B must follow.
1613    .P
1614  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
1615  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
1616  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
1617  pattern  pattern
1618    .sp
1619    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1620    .sp
1621  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
1622  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1623  quickly. However, if it is applied to  quickly. However, if it is applied to
1624    .sp
1625    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1626    .sp
1627  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
1628  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
1629  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
1630  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
1631  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
1632  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
1633  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1634    an atomic group, like this:
1635    ((?>\\D+)|<\\d+>)*[!?]  .sp
1636      ((?>\eD+)|<\ed+>)*[!?]
1637    .sp
1638  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1639    .
1640  .SH BACK REFERENCES  .
1641    .\" HTML <a name="backreferences"></a>
1642    .SH "BACK REFERENCES"
1643  .rs  .rs
1644  .sp  .sp
1645  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
1646  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1647  (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
1648  previous capturing left parentheses.  previous capturing left parentheses.
1649    .P
1650  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
1651  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
1652  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
1653  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
1654  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1655  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1656    in an earlier iteration.
1657    .P
1658    It is not possible to have a numerical "forward back reference" to a subpattern
1659    whose number is 10 or more using this syntax because a sequence such as \e50 is
1660    interpreted as a character defined in octal. See the subsection entitled
1661    "Non-printing characters"
1662    .\" HTML <a href="#digitsafterbackslash">
1663    .\" </a>
1664    above
1665    .\"
1666    for further details of the handling of digits following a backslash. There is
1667    no such problem when named parentheses are used. A back reference to any
1668    subpattern is possible using named parentheses (see below).
1669    .P
1670    Another way of avoiding the ambiguity inherent in the use of digits following a
1671    backslash is to use the \eg escape sequence, which is a feature introduced in
1672    Perl 5.10. This escape must be followed by an unsigned number or a negative
1673    number, optionally enclosed in braces. These examples are all identical:
1674    .sp
1675      (ring), \e1
1676      (ring), \eg1
1677      (ring), \eg{1}
1678    .sp
1679    An unsigned number specifies an absolute reference without the ambiguity that
1680    is present in the older syntax. It is also useful when literal digits follow
1681    the reference. A negative number is a relative reference. Consider this
1682    example:
1683    .sp
1684      (abc(def)ghi)\eg{-1}
1685    .sp
1686    The sequence \eg{-1} is a reference to the most recently started capturing
1687    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1688    would be equivalent to \e1. The use of relative references can be helpful in
1689    long patterns, and also in patterns that are created by joining together
1690    fragments that contain references within themselves.
1691    .P
1692  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1693  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1694  itself (see  itself (see
# Line 832  itself (see Line 1697  itself (see
1697  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1698  .\"  .\"
1699  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1700    .sp
1701    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1702    .sp
1703  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1704  "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
1705  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1706    .sp
1707    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1708    .sp
1709  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
1710  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1711    .P
1712  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1713  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1714    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1715    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1716    references, is also supported. We could rewrite the above example in any of
1717    the following ways:
1718    .sp
1719      (?<p1>(?i)rah)\es+\ek<p1>
1720      (?'p1'(?i)rah)\es+\ek{p1}
1721      (?P<p1>(?i)rah)\es+(?P=p1)
1722      (?<p1>(?i)rah)\es+\eg{p1}
1723    .sp
1724    A subpattern that is referenced by name may appear in the pattern before or
1725    after the reference.
1726    .P
1727  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
1728  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1729  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1730    .sp
1731    (a|(bc))\\2    (a|(bc))\e2
1732    .sp
1733  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". However, if the
1734  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1735  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1736  with a digit character, some delimiter must be used to terminate the back  .P
1737  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1738  Otherwise an empty comment can be used.  following a backslash are taken as part of a potential back reference number.
1739    If the pattern continues with a digit character, some delimiter must be used to
1740    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1741    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1742    .\" HTML <a href="#comments">
1743    .\" </a>
1744    "Comments"
1745    .\"
1746    below) can be used.
1747    .
1748    .SS "Recursive back references"
1749    .rs
1750    .sp
1751  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
1752  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.
1753  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1754  example, the pattern  example, the pattern
1755    .sp
1756    (a|b\\1)+    (a|b\e1)+
1757    .sp
1758  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
1759  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1760  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
1761  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
1762  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
1763  minimum of zero.  minimum of zero.
1764    .P
1765    Back references of this type cause the group that they reference to be treated
1766    as an
1767    .\" HTML <a href="#atomicgroup">
1768    .\" </a>
1769    atomic group.
1770    .\"
1771    Once the whole group has been matched, a subsequent matching failure cannot
1772    cause backtracking into the middle of the group.
1773    .
1774    .
1775    .\" HTML <a name="bigassertions"></a>
1776  .SH ASSERTIONS  .SH ASSERTIONS
1777  .rs  .rs
1778  .sp  .sp
1779  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
1780  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1781  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
1782    .\" HTML <a href="#smallassertions">
1783    .\" </a>
1784    above.
1785    .\"
1786    .P
1787  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1788  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
1789  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1790    except that it does not cause the current matching position to be changed.
1791  An assertion subpattern is matched in the normal way, except that it does not  .P
1792  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1793  with (?= for positive assertions and (?! for negative assertions. For example,  because it makes no sense to assert the same thing several times. If any kind
1794    of assertion contains capturing subpatterns within it, these are counted for
1795    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1796    However, substring capturing is carried out only for positive assertions,
1797    because it does not make sense for negative assertions.
1798    .
1799    .
1800    .SS "Lookahead assertions"
1801    .rs
1802    .sp
1803    Lookahead assertions start with (?= for positive assertions and (?! for
1804    negative assertions. For example,
1805    .sp
1806      \ew+(?=;)
1807    .sp
1808  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
1809  the match, and  the match, and
1810    .sp
1811    foo(?!bar)    foo(?!bar)
1812    .sp
1813  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
1814  apparently similar pattern  apparently similar pattern
1815    .sp
1816    (?!foo)bar    (?!foo)bar
1817    .sp
1818  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
1819  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1820  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1821  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1822    .P
1823  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
1824  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
1825  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.
1826    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1827    synonym for (?!).
1828    .
1829    .
1830    .\" HTML <a name="lookbehind"></a>
1831    .SS "Lookbehind assertions"
1832    .rs
1833    .sp
1834  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1835  negative assertions. For example,  negative assertions. For example,
1836    .sp
1837    (?<!foo)bar    (?<!foo)bar
1838    .sp
1839  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
1840  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
1841  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1842  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1843    .sp
1844    (?<=bullock|donkey)    (?<=bullock|donkey)
1845    .sp
1846  is permitted, but  is permitted, but
1847    .sp
1848    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1849    .sp
1850  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1851  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
1852  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1853  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1854    .sp
1855    (?<=ab(c|de))    (?<=ab(c|de))
1856    .sp
1857  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
1858  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1859    branches:
1860    .sp
1861    (?<=abc|abde)    (?<=abc|abde)
1862    .sp
1863    In some cases, the Perl 5.10 escape sequence \eK
1864    .\" HTML <a href="#resetmatchstart">
1865    .\" </a>
1866    (see above)
1867    .\"
1868    can be used instead of a lookbehind assertion to get round the fixed-length
1869    restriction.
1870    .P
1871  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1872  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1873  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1874  match is deemed to fail.  assertion fails.
1875    .P
1876  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)
1877  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1878  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1879    different numbers of bytes, are also not permitted.
1880  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1881  efficient matching at the end of the subject string. Consider a simple pattern  .\" HTML <a href="#subpatternsassubroutines">
1882  such as  .\" </a>
1883    "Subroutine"
1884    .\"
1885    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1886    as the subpattern matches a fixed-length string.
1887    .\" HTML <a href="#recursion">
1888    .\" </a>
1889    Recursion,
1890    .\"
1891    however, is not supported.
1892    .P
1893    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1894    specify efficient matching of fixed-length strings at the end of subject
1895    strings. Consider a simple pattern such as
1896    .sp
1897    abcd$    abcd$
1898    .sp
1899  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
1900  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
1901  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
1902    .sp
1903    ^.*abcd$    ^.*abcd$
1904    .sp
1905  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
1906  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,
1907  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"
1908  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,
1909  if the pattern is written as  if the pattern is written as
1910    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1911    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1912    .sp
1913  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
1914  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
1915  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1916  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1917    .
1918    .
1919    .SS "Using multiple assertions"
1920    .rs
1921    .sp
1922  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1923    .sp
1924    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1925    .sp
1926  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
1927  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1928  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
1929  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".
1930  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
1931  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
1932  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1933    .sp
1934    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1935    .sp
1936  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1937  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
1938  preceding three characters are not "999".  preceding three characters are not "999".
1939    .P
1940  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1941    .sp
1942    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1943    .sp
1944  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
1945  preceded by "foo", while  preceded by "foo", while
1946    .sp
1947    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1948    .sp
1949  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
1950  characters that are not "999".  characters that are not "999".
1951    .
1952  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1953  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1954  of assertion contains capturing subpatterns within it, these are counted for  .SH "CONDITIONAL SUBPATTERNS"
 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  
1955  .rs  .rs
1956  .sp  .sp
1957  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1958  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1959  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1960  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1961    .sp
1962    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1963    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1964    .sp
1965  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1966  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
1967  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1968    .P
1969  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1970  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1971  subpattern of that number has previously matched. The number must be greater  .
1972  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1973  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1974  it into three parts for ease of discussion:  .sp
1975    If the text between the parentheses consists of a sequence of digits, the
1976    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if a capturing subpattern of that number has previously
1977    matched. If there is more than one capturing subpattern with the same number
1978    (see the earlier
1979    .\"
1980    .\" HTML <a href="#recursion">
1981    .\" </a>
1982    section about duplicate subpattern numbers),
1983    .\"
1984    the condition is true if any of them have been set. An alternative notation is
1985    to precede the digits with a plus or minus sign. In this case, the subpattern
1986    number is relative rather than absolute. The most recently opened parentheses
1987    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1988    looping constructs it can also make sense to refer to subsequent groups with
1989    constructs such as (?(+2).
1990    .P
1991    Consider the following pattern, which contains non-significant white space to
1992    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1993    three parts for ease of discussion:
1994    .sp
1995      ( \e( )?    [^()]+    (?(1) \e) )
1996    .sp
1997  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1998  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
1999  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 2003  the condition is true, and so the yes-pa
2003  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2004  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2005  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2006    .P
2007  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2008  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2009  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2010      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2011  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
2012    This makes the fragment independent of the parentheses in the larger pattern.
2013    .
2014    .SS "Checking for a used subpattern by name"
2015    .rs
2016    .sp
2017    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2018    subpattern by name. For compatibility with earlier versions of PCRE, which had
2019    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2020    there is a possible ambiguity with this syntax, because subpattern names may
2021    consist entirely of digits. PCRE looks first for a named subpattern; if it
2022    cannot find one and the name consists entirely of digits, PCRE looks for a
2023    subpattern of that number, which must be greater than zero. Using subpattern
2024    names that consist entirely of digits is not recommended.
2025    .P
2026    Rewriting the above example to use a named subpattern gives this:
2027    .sp
2028      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2029    .sp
2030    If the name used in a condition of this kind is a duplicate, the test is
2031    applied to all subpatterns of the same name, and is true if any one of them has
2032    matched.
2033    .
2034    .SS "Checking for pattern recursion"
2035    .rs
2036    .sp
2037    If the condition is the string (R), and there is no subpattern with the name R,
2038    the condition is true if a recursive call to the whole pattern or any
2039    subpattern has been made. If digits or a name preceded by ampersand follow the
2040    letter R, for example:
2041    .sp
2042      (?(R3)...) or (?(R&name)...)
2043    .sp
2044    the condition is true if the most recent recursion is into a subpattern whose
2045    number or name is given. This condition does not check the entire recursion
2046    stack. If the name used in a condition of this kind is a duplicate, the test is
2047    applied to all subpatterns of the same name, and is true if any one of them is
2048    the most recent recursion.
2049    .P
2050    At "top level", all these recursion test conditions are false.
2051    .\" HTML <a href="#recursion">
2052    .\" </a>
2053    The syntax for recursive patterns
2054    .\"
2055    is described below.
2056    .
2057    .SS "Defining subpatterns for use by reference only"
2058    .rs
2059    .sp
2060    If the condition is the string (DEFINE), and there is no subpattern with the
2061    name DEFINE, the condition is always false. In this case, there may be only one
2062    alternative in the subpattern. It is always skipped if control reaches this
2063    point in the pattern; the idea of DEFINE is that it can be used to define
2064    "subroutines" that can be referenced from elsewhere. (The use of
2065    .\" HTML <a href="#subpatternsassubroutines">
2066    .\" </a>
2067    "subroutines"
2068    .\"
2069    is described below.) For example, a pattern to match an IPv4 address could be
2070    written like this (ignore whitespace and line breaks):
2071    .sp
2072      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2073      \eb (?&byte) (\e.(?&byte)){3} \eb
2074    .sp
2075    The first part of the pattern is a DEFINE group inside which a another group
2076    named "byte" is defined. This matches an individual component of an IPv4
2077    address (a number less than 256). When matching takes place, this part of the
2078    pattern is skipped because DEFINE acts like a false condition. The rest of the
2079    pattern uses references to the named group to match the four dot-separated
2080    components of an IPv4 address, insisting on a word boundary at each end.
2081    .
2082    .SS "Assertion conditions"
2083    .rs
2084    .sp
2085    If the condition is not in any of the above formats, it must be an assertion.
2086  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2087  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2088  alternatives on the second line:  alternatives on the second line:
2089    .sp
2090    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
2091    \\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} )
2092    .sp
2093  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
2094  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
2095  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
2096  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
2097  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
2098  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.
2099    .
2100    .
2101    .\" HTML <a name="comments"></a>
2102  .SH COMMENTS  .SH COMMENTS
2103  .rs  .rs
2104  .sp  .sp
2105  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
2106  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
2107  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.
2108    .P
2109  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
2110  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
2111  character in the pattern.  next newline in the pattern.
2112    .
2113  .SH RECURSIVE PATTERNS  .
2114    .\" HTML <a name="recursion"></a>
2115    .SH "RECURSIVE PATTERNS"
2116  .rs  .rs
2117  .sp  .sp
2118  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2119  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2120  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
2121  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
2122  experimental facility that allows regular expressions to recurse (amongst other  .P
2123  things). It does this by interpolating Perl code in the expression at run time,  For some time, Perl has provided a facility that allows regular expressions to
2124  and the code can refer to the expression itself. A Perl pattern to solve the  recurse (amongst other things). It does this by interpolating Perl code in the
2125  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2126    pattern using code interpolation to solve the parentheses problem can be
2127    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
2128    .sp
2129      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2130    .sp
2131  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
2132  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2133  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2134  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2135    supports special syntax for recursion of the entire pattern, and also for
2136  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2137  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2138  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2139  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2140  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2141    provided that it occurs inside that subpattern. (If not, it is a
2142  For example, this PCRE pattern solves the nested parentheses problem (assume  .\" HTML <a href="#subpatternsassubroutines">
2143  the PCRE_EXTENDED option is set so that white space is ignored):  .\" </a>
2144    "subroutine"
2145    \\( ( (?>[^()]+) | (?R) )* \\)  .\"
2146    call, which is described in the next section.) The special item (?R) or (?0) is
2147    a recursive call of the entire regular expression.
2148    .P
2149    This PCRE pattern solves the nested parentheses problem (assume the
2150    PCRE_EXTENDED option is set so that white space is ignored):
2151    .sp
2152      \e( ( [^()]++ | (?R) )* \e)
2153    .sp
2154  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2155  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
2156  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2157  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2158    to avoid backtracking into sequences of non-parentheses.
2159    .P
2160  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
2161  pattern, so instead you could use this:  pattern, so instead you could use this:
2162    .sp
2163    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( [^()]++ | (?1) )* \e) )
2164    .sp
2165  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
2166  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2167  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2168  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2169  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references (a Perl 5.10 feature).
2170  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2171    most recently opened parentheses preceding the recursion. In other words, a
2172    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
2173    it is encountered.
2174  This particular example pattern contains nested unlimited repeats, and so the  .P
2175  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2176  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
2177  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2178    .\" HTML <a href="#subpatternsassubroutines">
2179    .\" </a>
2180    "subroutine"
2181    .\"
2182    calls, as described in the next section.
2183    .P
2184    An alternative approach is to use named parentheses instead. The Perl syntax
2185    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2186    could rewrite the above example as follows:
2187    .sp
2188      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2189    .sp
2190    If there is more than one subpattern with the same name, the earliest one is
2191    used.
2192    .P
2193    This particular example pattern that we have been looking at contains nested
2194    unlimited repeats, and so the use of a possessive quantifier for matching
2195    strings of non-parentheses is important when applying the pattern to strings
2196    that do not match. For example, when this pattern is applied to
2197    .sp
2198    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2199    .sp
2200  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2201  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
2202  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
2203  before failure can be reported.  before failure can be reported.
2204    .P
2205  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2206  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2207  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2208  .\" HREF  .\" HREF
2209  \fBpcrecallout\fR  \fBpcrecallout\fP
2210  .\"  .\"
2211  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2212    .sp
2213    (ab(cd)ef)    (ab(cd)ef)
2214    .sp
2215  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2216  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2217    matched at the top level, its final value is unset, even if it is (temporarily)
2218    \\( ( ( (?>[^()]+) | (?R) )* ) \\)  set at a deeper level.
2219       ^                        ^  .P
2220       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2221    obtain extra memory to store data during a recursion, which it does by using
2222  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2223  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2224  has to obtain extra memory to store data during a recursion, which it does by  .P
 using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
   
2225  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.
2226  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
2227  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2228  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
2229    .sp
2230    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
2231    .sp
2232  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
2233  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2234  is the actual recursive call.  is the actual recursive call.
2235    .
2236    .
2237    .\" HTML <a name="recursiondifference"></a>
2238    .SS "Recursion difference from Perl"
2239    .rs
2240    .sp
2241    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2242    treated as an atomic group. That is, once it has matched some of the subject
2243    string, it is never re-entered, even if it contains untried alternatives and
2244    there is a subsequent matching failure. This can be illustrated by the
2245    following pattern, which purports to match a palindromic string that contains
2246    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2247    .sp
2248      ^(.|(.)(?1)\e2)$
2249    .sp
2250    The idea is that it either matches a single character, or two identical
2251    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2252    it does not if the pattern is longer than three characters. Consider the
2253    subject string "abcba":
2254    .P
2255    At the top level, the first character is matched, but as it is not at the end
2256    of the string, the first alternative fails; the second alternative is taken
2257    and the recursion kicks in. The recursive call to subpattern 1 successfully
2258    matches the next character ("b"). (Note that the beginning and end of line
2259    tests are not part of the recursion).
2260    .P
2261    Back at the top level, the next character ("c") is compared with what
2262    subpattern 2 matched, which was "a". This fails. Because the recursion is
2263    treated as an atomic group, there are now no backtracking points, and so the
2264    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2265    try the second alternative.) However, if the pattern is written with the
2266    alternatives in the other order, things are different:
2267    .sp
2268      ^((.)(?1)\e2|.)$
2269    .sp
2270    This time, the recursing alternative is tried first, and continues to recurse
2271    until it runs out of characters, at which point the recursion fails. But this
2272    time we do have another alternative to try at the higher level. That is the big
2273    difference: in the previous case the remaining alternative is at a deeper
2274    recursion level, which PCRE cannot use.
2275    .P
2276    To change the pattern so that matches all palindromic strings, not just those
2277    with an odd number of characters, it is tempting to change the pattern to this:
2278    .sp
2279      ^((.)(?1)\e2|.?)$
2280    .sp
2281    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2282    deeper recursion has matched a single character, it cannot be entered again in
2283    order to match an empty string. The solution is to separate the two cases, and
2284    write out the odd and even cases as alternatives at the higher level:
2285    .sp
2286      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2287    .sp
2288    If you want to match typical palindromic phrases, the pattern has to ignore all
2289    non-word characters, which can be done like this:
2290    .sp
2291      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2292    .sp
2293    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2294    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2295    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2296    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2297    more) to match typical phrases, and Perl takes so long that you think it has
2298    gone into a loop.
2299    .P
2300    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2301    string does not start with a palindrome that is shorter than the entire string.
2302    For example, although "abcba" is correctly matched, if the subject is "ababa",
2303    PCRE finds the palindrome "aba" at the start, then fails at top level because
2304    the end of the string does not follow. Once again, it cannot jump back into the
2305    recursion to try other alternatives, so the entire match fails.
2306    .
2307    .
2308  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2309  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
2310  .rs  .rs
2311  .sp  .sp
2312  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
2313  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
2314  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2315  pattern  before or after the reference. A numbered reference can be absolute or
2316    relative, as in these examples:
2317    (sens|respons)e and \\1ibility  .sp
2318      (...(absolute)...)...(?2)...
2319      (...(relative)...)...(?-1)...
2320      (...(?+1)...(relative)...
2321    .sp
2322    An earlier example pointed out that the pattern
2323    .sp
2324      (sens|respons)e and \e1ibility
2325    .sp
2326  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
2327  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
2328    .sp
2329    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2330    .sp
2331  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
2332  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2333  refer.  .P
2334    Like recursive subpatterns, a subroutine call is always treated as an atomic
2335    group. That is, once it has matched some of the subject string, it is never
2336    re-entered, even if it contains untried alternatives and there is a subsequent
2337    matching failure. Any capturing parentheses that are set during the subroutine
2338    call revert to their previous values afterwards.
2339    .P
2340    When a subpattern is used as a subroutine, processing options such as
2341    case-independence are fixed when the subpattern is defined. They cannot be
2342    changed for different calls. For example, consider this pattern:
2343    .sp
2344      (abc)(?i:(?-1))
2345    .sp
2346    It matches "abcabc". It does not match "abcABC" because the change of
2347    processing option does not affect the called subpattern.
2348    .
2349    .
2350    .\" HTML <a name="onigurumasubroutines"></a>
2351    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2352    .rs
2353    .sp
2354    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2355    a number enclosed either in angle brackets or single quotes, is an alternative
2356    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2357    are two of the examples used above, rewritten using this syntax:
2358    .sp
2359      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2360      (sens|respons)e and \eg'1'ibility
2361    .sp
2362    PCRE supports an extension to Oniguruma: if a number is preceded by a
2363    plus or a minus sign it is taken as a relative reference. For example:
2364    .sp
2365      (abc)(?i:\eg<-1>)
2366    .sp
2367    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2368    synonymous. The former is a back reference; the latter is a subroutine call.
2369    .
2370    .
2371  .SH CALLOUTS  .SH CALLOUTS
2372  .rs  .rs
2373  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 2375  Perl has a feature whereby using the seq
2375  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
2376  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
2377  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
2378    .P
2379  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
2380  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
2381  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.
2382  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2383    .P
2384  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2385  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
2386  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.
2387  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2388    .sp
2389    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2390    .sp
2391  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
2392    automatically installed before each item in the pattern. They are all numbered
2393    255.
2394    .P
2395    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2396  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
2397  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
2398  \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
2399  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
2400  function is given in the  description of the interface to the callout function is given in the
2401  .\" HREF  .\" HREF
2402  \fBpcrecallout\fR  \fBpcrecallout\fP
2403  .\"  .\"
2404  documentation.  documentation.
2405    .
2406  .in 0  .
2407  Last updated: 03 February 2003  .\" HTML <a name="backtrackcontrol"></a>
2408  .br  .SH "BACKTRACKING CONTROL"
2409  Copyright (c) 1997-2003 University of Cambridge.  .rs
2410    .sp
2411    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2412    are described in the Perl documentation as "experimental and subject to change
2413    or removal in a future version of Perl". It goes on to say: "Their usage in
2414    production code should be noted to avoid problems during upgrades." The same
2415    remarks apply to the PCRE features described in this section.
2416    .P
2417    Since these verbs are specifically related to backtracking, most of them can be
2418    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2419    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2420    failing negative assertion, they cause an error if encountered by
2421    \fBpcre_dfa_exec()\fP.
2422    .P
2423    If any of these verbs are used in an assertion or subroutine subpattern
2424    (including recursive subpatterns), their effect is confined to that subpattern;
2425    it does not extend to the surrounding pattern. Note that such subpatterns are
2426    processed as anchored at the point where they are tested.
2427    .P
2428    The new verbs make use of what was previously invalid syntax: an opening
2429    parenthesis followed by an asterisk. They are generally of the form
2430    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2431    depending on whether or not an argument is present. An name is a sequence of
2432    letters, digits, and underscores. If the name is empty, that is, if the closing
2433    parenthesis immediately follows the colon, the effect is as if the colon were
2434    not there. Any number of these verbs may occur in a pattern.
2435    .P
2436    PCRE contains some optimizations that are used to speed up matching by running
2437    some checks at the start of each match attempt. For example, it may know the
2438    minimum length of matching subject, or that a particular character must be
2439    present. When one of these optimizations suppresses the running of a match, any
2440    included backtracking verbs will not, of course, be processed. You can suppress
2441    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2442    when calling \fBpcre_exec()\fP.
2443    .
2444    .
2445    .SS "Verbs that act immediately"
2446    .rs
2447    .sp
2448    The following verbs act as soon as they are encountered. They may not be
2449    followed by a name.
2450    .sp
2451       (*ACCEPT)
2452    .sp
2453    This verb causes the match to end successfully, skipping the remainder of the
2454    pattern. When inside a recursion, only the innermost pattern is ended
2455    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2456    captured. (This feature was added to PCRE at release 8.00.) For example:
2457    .sp
2458      A((?:A|B(*ACCEPT)|C)D)
2459    .sp
2460    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2461    the outer parentheses.
2462    .sp
2463      (*FAIL) or (*F)
2464    .sp
2465    This verb causes the match to fail, forcing backtracking to occur. It is
2466    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2467    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2468    Perl features that are not present in PCRE. The nearest equivalent is the
2469    callout feature, as for example in this pattern:
2470    .sp
2471      a+(?C)(*FAIL)
2472    .sp
2473    A match with the string "aaaa" always fails, but the callout is taken before
2474    each backtrack happens (in this example, 10 times).
2475    .
2476    .
2477    .SS "Recording which path was taken"
2478    .rs
2479    .sp
2480    There is one verb whose main purpose is to track how a match was arrived at,
2481    though it also has a secondary use in conjunction with advancing the match
2482    starting point (see (*SKIP) below).
2483    .sp
2484      (*MARK:NAME) or (*:NAME)
2485    .sp
2486    A name is always required with this verb. There may be as many instances of
2487    (*MARK) as you like in a pattern, and their names do not have to be unique.
2488    .P
2489    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2490    to the caller via the \fIpcre_extra\fP data structure, as described in the
2491    .\" HTML <a href="pcreapi.html#extradata">
2492    .\" </a>
2493    section on \fIpcre_extra\fP
2494    .\"
2495    in the
2496    .\" HREF
2497    \fBpcreapi\fP
2498    .\"
2499    documentation. No data is returned for a partial match. Here is an example of
2500    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2501    outputting of (*MARK) data:
2502    .sp
2503      /X(*MARK:A)Y|X(*MARK:B)Z/K
2504      XY
2505       0: XY
2506      MK: A
2507      XZ
2508       0: XZ
2509      MK: B
2510    .sp
2511    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2512    indicates which of the two alternatives matched. This is a more efficient way
2513    of obtaining this information than putting each alternative in its own
2514    capturing parentheses.
2515    .P
2516    A name may also be returned after a failed match if the final path through the
2517    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2518    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2519    starting point for matching is advanced, the final check is often with an empty
2520    string, causing a failure before (*MARK) is reached. For example:
2521    .sp
2522      /X(*MARK:A)Y|X(*MARK:B)Z/K
2523      XP
2524      No match
2525    .sp
2526    There are three potential starting points for this match (starting with X,
2527    starting with P, and with an empty string). If the pattern is anchored, the
2528    result is different:
2529    .sp
2530      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2531      XP
2532      No match, mark = B
2533    .sp
2534    PCRE's start-of-match optimizations can also interfere with this. For example,
2535    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2536    subject length for a match, a shorter subject will not be scanned at all.
2537    .P
2538    Note that similar anomalies (though different in detail) exist in Perl, no
2539    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2540    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2541    .
2542    .
2543    .SS "Verbs that act after backtracking"
2544    .rs
2545    .sp
2546    The following verbs do nothing when they are encountered. Matching continues
2547    with what follows, but if there is no subsequent match, causing a backtrack to
2548    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2549    the verb. However, when one of these verbs appears inside an atomic group, its
2550    effect is confined to that group, because once the group has been matched,
2551    there is never any backtracking into it. In this situation, backtracking can
2552    "jump back" to the left of the entire atomic group. (Remember also, as stated
2553    above, that this localization also applies in subroutine calls and assertions.)
2554    .P
2555    These verbs differ in exactly what kind of failure occurs when backtracking
2556    reaches them.
2557    .sp
2558      (*COMMIT)
2559    .sp
2560    This verb, which may not be followed by a name, causes the whole match to fail
2561    outright if the rest of the pattern does not match. Even if the pattern is
2562    unanchored, no further attempts to find a match by advancing the starting point
2563    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2564    finding a match at the current starting point, or not at all. For example:
2565    .sp
2566      a+(*COMMIT)b
2567    .sp
2568    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2569    dynamic anchor, or "I've started, so I must finish." The name of the most
2570    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2571    match failure.
2572    .P
2573    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2574    unless PCRE's start-of-match optimizations are turned off, as shown in this
2575    \fBpcretest\fP example:
2576    .sp
2577      /(*COMMIT)abc/
2578      xyzabc
2579       0: abc
2580      xyzabc\eY
2581      No match
2582    .sp
2583    PCRE knows that any match must start with "a", so the optimization skips along
2584    the subject to "a" before running the first match attempt, which succeeds. When
2585    the optimization is disabled by the \eY escape in the second subject, the match
2586    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2587    starting points.
2588    .sp
2589      (*PRUNE) or (*PRUNE:NAME)
2590    .sp
2591    This verb causes the match to fail at the current starting position in the
2592    subject if the rest of the pattern does not match. If the pattern is
2593    unanchored, the normal "bumpalong" advance to the next starting character then
2594    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2595    reached, or when matching to the right of (*PRUNE), but if there is no match to
2596    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2597    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2598    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2599    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2600    match fails completely; the name is passed back if this is the final attempt.
2601    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2602    pattern (*PRUNE) has the same effect as (*COMMIT).
2603    .sp
2604      (*SKIP)
2605    .sp
2606    This verb, when given without a name, is like (*PRUNE), except that if the
2607    pattern is unanchored, the "bumpalong" advance is not to the next character,
2608    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2609    signifies that whatever text was matched leading up to it cannot be part of a
2610    successful match. Consider:
2611    .sp
2612      a+(*SKIP)b
2613    .sp
2614    If the subject is "aaaac...", after the first match attempt fails (starting at
2615    the first character in the string), the starting point skips on to start the
2616    next attempt at "c". Note that a possessive quantifer does not have the same
2617    effect as this example; although it would suppress backtracking during the
2618    first match attempt, the second attempt would start at the second character
2619    instead of skipping on to "c".
2620    .sp
2621      (*SKIP:NAME)
2622    .sp
2623    When (*SKIP) has an associated name, its behaviour is modified. If the
2624    following pattern fails to match, the previous path through the pattern is
2625    searched for the most recent (*MARK) that has the same name. If one is found,
2626    the "bumpalong" advance is to the subject position that corresponds to that
2627    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2628    matching name is found, normal "bumpalong" of one character happens (the
2629    (*SKIP) is ignored).
2630    .sp
2631      (*THEN) or (*THEN:NAME)
2632    .sp
2633    This verb causes a skip to the next alternation if the rest of the pattern does
2634    not match. That is, it cancels pending backtracking, but only within the
2635    current alternation. Its name comes from the observation that it can be used
2636    for a pattern-based if-then-else block:
2637    .sp
2638      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2639    .sp
2640    If the COND1 pattern matches, FOO is tried (and possibly further items after
2641    the end of the group if FOO succeeds); on failure the matcher skips to the
2642    second alternative and tries COND2, without backtracking into COND1. The
2643    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2644    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2645    like (*PRUNE).
2646    .
2647    .
2648    .SH "SEE ALSO"
2649    .rs
2650    .sp
2651    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2652    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2653    .
2654    .
2655    .SH AUTHOR
2656    .rs
2657    .sp
2658    .nf
2659    Philip Hazel
2660    University Computing Service
2661    Cambridge CB2 3QH, England.
2662    .fi
2663    .
2664    .
2665    .SH REVISION
2666    .rs
2667    .sp
2668    .nf
2669    Last updated: 18 May 2010
2670    Copyright (c) 1997-2010 University of Cambridge.
2671    .fi

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