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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    The remainder of this document discusses the patterns that are supported by
46    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47    From release 6.0, PCRE offers a second matching function,
48    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49    Perl-compatible. Some of the features discussed below are not available when
50    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53    .\" HREF
54    \fBpcrematching\fP
55  .\"  .\"
56  page.  page.
57    .
58    .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76    .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP or
87    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89    .sp
90      (*CR)a.b
91    .sp
92    changes the convention to CR. That pattern matches "a\enb" because LF is no
93    longer a newline. Note that these special settings, which are not
94    Perl-compatible, are recognized only at the very start of a pattern, and that
95    they must be in upper case. If more than one of them is present, the last one
96    is used.
97    .P
98    The newline convention does not affect what the \eR escape sequence matches. By
99    default, this is any Unicode newline sequence, for Perl compatibility. However,
100    this can be changed; see the description of \eR in the section entitled
101    .\" HTML <a href="#newlineseq">
102    .\" </a>
103    "Newline sequences"
104    .\"
105    below. A change of \eR setting can be combined with a change of newline
106    convention.
107    .
108    .
109    .SH "CHARACTERS AND METACHARACTERS"
110    .rs
111    .sp
112  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
113  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
114  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
115    .sp
116    The quick brown fox    The quick brown fox
117    .sp
118  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
119  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
120  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
121  \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
122    always possible. For characters with higher values, the concept of case is
123    supported if PCRE is compiled with Unicode property support, but not otherwise.
124    If you want to use caseless matching for characters 128 and above, you must
125    ensure that PCRE is compiled with Unicode property support as well as with
126    UTF-8 support.
127    .P
128    The power of regular expressions comes from the ability to include alternatives
129    and repetitions in the pattern. These are encoded in the pattern by the use of
130    \fImetacharacters\fP, which do not stand for themselves but instead are
131  interpreted in some special way.  interpreted in some special way.
132    .P
133  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
134  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
135  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
136  as follows:  are as follows:
137    .sp
138    \\      general escape character with several uses    \e      general escape character with several uses
139    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
140    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
141    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 150  as follows:
150    +      1 or more quantifier    +      1 or more quantifier
151           also "possessive quantifier"           also "possessive quantifier"
152    {      start min/max quantifier    {      start min/max quantifier
153    .sp
154  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
155  a character class the only meta-characters are:  a character class the only metacharacters are:
156    .sp
157    \\      general escape character    \e      general escape character
158    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
159    -      indicates character range    -      indicates character range
160    .\" JOIN
161    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
162             syntax)             syntax)
163    ]      terminates the character class    ]      terminates the character class
164    .sp
165  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
166    .
167    .
168  .SH BACKSLASH  .SH BACKSLASH
169  .rs  .rs
170  .sp  .sp
171  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
172  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
173  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
174  outside character classes.  outside character classes.
175    .P
176  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.
177  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
178  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
179  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
180  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
181    .P
182  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
183  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
184  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
185  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.
186  pattern.  .P
   
187  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
188  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
189  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
190  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
191    .sp
192    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
193    .sp
194    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
195      \eQabc$xyz\eE        abc$xyz        abc followed by the
196                                        contents of $xyz                                        contents of $xyz
197    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
198    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
199    .sp
200  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
201    .
202    .
203    .\" HTML <a name="digitsafterbackslash"></a>
204    .SS "Non-printing characters"
205    .rs
206    .sp
207  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
208  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
209  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
210  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
211  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
212  represents:  .sp
213      \ea        alarm, that is, the BEL character (hex 07)
214    \\a        alarm, that is, the BEL character (hex 07)    \ecx       "control-x", where x is any character
215    \\cx       "control-x", where x is any character    \ee        escape (hex 1B)
216    \\e        escape (hex 1B)    \ef        formfeed (hex 0C)
217    \\f        formfeed (hex 0C)    \en        linefeed (hex 0A)
218    \\n        newline (hex 0A)    \er        carriage return (hex 0D)
219    \\r        carriage return (hex 0D)    \et        tab (hex 09)
220    \\t        tab (hex 09)    \eddd      character with octal code ddd, or backreference
221    \\ddd      character with octal code ddd, or backreference    \exhh      character with hex code hh
222    \\xhh      character with hex code hh    \ex{hhh..} character with hex code hhh..
223    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)  .sp
224    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  
225  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.
226  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
227  7B.  7B.
228    .P
229  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
230  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{
231  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
232  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
233  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
234  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
235  \\x will be interpreted as a basic hexadecimal escape, with no following  .P
236  digits, giving a byte whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240    .P
241  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
242  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
243  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
244    .P
245  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
246  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
247  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
248  (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
249  character that follows is itself an octal digit.  follows is itself an octal digit.
250    .P
251  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.
252  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
253  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
254  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
255  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
256  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
257    .\" </a>
258    later,
259    .\"
260    following the discussion of
261    .\" HTML <a href="#subpattern">
262    .\" </a>
263    parenthesized subpatterns.
264    .\"
265    .P
266  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
267  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
268  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
269  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
270  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
271    to \e777 are permitted. For example:
272    \\040   is another way of writing a space  .sp
273    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
274    .\" JOIN
275      \e40    is the same, provided there are fewer than 40
276              previous capturing subpatterns              previous capturing subpatterns
277    \\7     is always a back reference    \e7     is always a back reference
278    \\11    might be a back reference, or another way of  .\" JOIN
279      \e11    might be a back reference, or another way of
280              writing a tab              writing a tab
281    \\011   is always a tab    \e011   is always a tab
282    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
283    \\113   might be a back reference, otherwise the  .\" JOIN
284      \e113   might be a back reference, otherwise the
285              character with octal code 113              character with octal code 113
286    \\377   might be a back reference, otherwise  .\" JOIN
287      \e377   might be a back reference, otherwise
288              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
289    \\81    is either a back reference, or a binary zero  .\" JOIN
290      \e81    is either a back reference, or a binary zero
291              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
292    .sp
293  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
294  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
295    .P
296  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
297  (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
298  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
299  backspace character (hex 08). Outside a character class it has a different  sequences \eR and \eX are interpreted as the characters "R" and "X",
300  meaning (see below).  respectively. Outside a character class, these sequences have different
301    meanings
302  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
303    .\" </a>
304    \\d     any decimal digit  (see below).
305    \\D     any character that is not a decimal digit  .\"
306    \\s     any whitespace character  .
307    \\S     any character that is not a whitespace character  .
308    \\w     any "word" character  .SS "Absolute and relative back references"
309    \\W     any "non-word" character  .rs
310    .sp
311    The sequence \eg followed by an unsigned or a negative number, optionally
312    enclosed in braces, is an absolute or relative back reference. A named back
313    reference can be coded as \eg{name}. Back references are discussed
314    .\" HTML <a href="#backreferences">
315    .\" </a>
316    later,
317    .\"
318    following the discussion of
319    .\" HTML <a href="#subpattern">
320    .\" </a>
321    parenthesized subpatterns.
322    .\"
323    .
324    .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342    .
343    .
344    .SS "Generic character types"
345    .rs
346    .sp
347    Another use of backslash is for specifying generic character types. The
348    following are always recognized:
349    .sp
350      \ed     any decimal digit
351      \eD     any character that is not a decimal digit
352      \eh     any horizontal whitespace character
353      \eH     any character that is not a horizontal whitespace character
354      \es     any whitespace character
355      \eS     any character that is not a whitespace character
356      \ev     any vertical whitespace character
357      \eV     any character that is not a vertical whitespace character
358      \ew     any "word" character
359      \eW     any "non-word" character
360    .sp
361  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
362  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
363    .P
364  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  These character type sequences can appear both inside and outside character
365  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
366    matching point is at the end of the subject string, all of them fail, since
367  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
368  This makes it different from the the POSIX "space" class. The \\s characters  .P
369  are HT (9), LF (10), FF (12), CR (13), and space (32).  For compatibility with Perl, \es does not match the VT character (code 11).
370    This makes it different from the the POSIX "space" class. The \es characters
371  A "word" character is any letter or digit or the underscore character, that is,  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372  any character which can be part of a Perl "word". The definition of letters and  included in a Perl script, \es may match the VT character. In PCRE, it never
373  digits is controlled by PCRE's character tables, and may vary if locale-  does.
374  specific matching is taking place (see  .P
375    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377    character property support is available. These sequences retain their original
378    meanings from before UTF-8 support was available, mainly for efficiency
379    reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381    .P
382    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384    The horizontal space characters are:
385    .sp
386      U+0009     Horizontal tab
387      U+0020     Space
388      U+00A0     Non-break space
389      U+1680     Ogham space mark
390      U+180E     Mongolian vowel separator
391      U+2000     En quad
392      U+2001     Em quad
393      U+2002     En space
394      U+2003     Em space
395      U+2004     Three-per-em space
396      U+2005     Four-per-em space
397      U+2006     Six-per-em space
398      U+2007     Figure space
399      U+2008     Punctuation space
400      U+2009     Thin space
401      U+200A     Hair space
402      U+202F     Narrow no-break space
403      U+205F     Medium mathematical space
404      U+3000     Ideographic space
405    .sp
406    The vertical space characters are:
407    .sp
408      U+000A     Linefeed
409      U+000B     Vertical tab
410      U+000C     Formfeed
411      U+000D     Carriage return
412      U+0085     Next line
413      U+2028     Line separator
414      U+2029     Paragraph separator
415    .P
416    A "word" character is an underscore or any character less than 256 that is a
417    letter or digit. The definition of letters and digits is controlled by PCRE's
418    low-valued character tables, and may vary if locale-specific matching is taking
419    place (see
420  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
421  .\" </a>  .\" </a>
422  "Locale support"  "Locale support"
423  .\"  .\"
424  in the  in the
425  .\" HREF  .\" HREF
426  \fBpcreapi\fR  \fBpcreapi\fP
427  .\"  .\"
428  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,
429  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
430    accented letters, and these are matched by \ew. The use of locales with Unicode
431  These character type sequences can appear both inside and outside character  is discouraged.
432  classes. They each match one character of the appropriate type. If the current  .
433  matching point is at the end of the subject string, all of them fail, since  .
434  there is no character to match.  .\" HTML <a name="newlineseq"></a>
435    .SS "Newline sequences"
436  The fourth use of backslash is for certain simple assertions. An assertion  .rs
437    .sp
438    Outside a character class, by default, the escape sequence \eR matches any
439    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440    equivalent to the following:
441    .sp
442      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443    .sp
444    This is an example of an "atomic group", details of which are given
445    .\" HTML <a href="#atomicgroup">
446    .\" </a>
447    below.
448    .\"
449    This particular group matches either the two-character sequence CR followed by
450    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
451    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
452    line, U+0085). The two-character sequence is treated as a single unit that
453    cannot be split.
454    .P
455    In UTF-8 mode, two additional characters whose codepoints are greater than 255
456    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
457    Unicode character property support is not needed for these characters to be
458    recognized.
459    .P
460    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462    either at compile time or when the pattern is matched. (BSR is an abbrevation
463    for "backslash R".) This can be made the default when PCRE is built; if this is
464    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465    It is also possible to specify these settings by starting a pattern string with
466    one of the following sequences:
467    .sp
468      (*BSR_ANYCRLF)   CR, LF, or CRLF only
469      (*BSR_UNICODE)   any Unicode newline sequence
470    .sp
471    These override the default and the options given to \fBpcre_compile()\fP or
472    \fBpcre_compile2()\fP, but they can be overridden by options given to
473    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474    which are not Perl-compatible, are recognized only at the very start of a
475    pattern, and that they must be in upper case. If more than one of them is
476    present, the last one is used. They can be combined with a change of newline
477    convention, for example, a pattern can start with:
478    .sp
479      (*ANY)(*BSR_ANYCRLF)
480    .sp
481    Inside a character class, \eR matches the letter "R".
482    .
483    .
484    .\" HTML <a name="uniextseq"></a>
485    .SS Unicode character properties
486    .rs
487    .sp
488    When PCRE is built with Unicode character property support, three additional
489    escape sequences that match characters with specific properties are available.
490    When not in UTF-8 mode, these sequences are of course limited to testing
491    characters whose codepoints are less than 256, but they do work in this mode.
492    The extra escape sequences are:
493    .sp
494      \ep{\fIxx\fP}   a character with the \fIxx\fP property
495      \eP{\fIxx\fP}   a character without the \fIxx\fP property
496      \eX       an extended Unicode sequence
497    .sp
498    The property names represented by \fIxx\fP above are limited to the Unicode
499    script names, the general category properties, and "Any", which matches any
500    character (including newline). Other properties such as "InMusicalSymbols" are
501    not currently supported by PCRE. Note that \eP{Any} does not match any
502    characters, so always causes a match failure.
503    .P
504    Sets of Unicode characters are defined as belonging to certain scripts. A
505    character from one of these sets can be matched using a script name. For
506    example:
507    .sp
508      \ep{Greek}
509      \eP{Han}
510    .sp
511    Those that are not part of an identified script are lumped together as
512    "Common". The current list of scripts is:
513    .P
514    Arabic,
515    Armenian,
516    Balinese,
517    Bengali,
518    Bopomofo,
519    Braille,
520    Buginese,
521    Buhid,
522    Canadian_Aboriginal,
523    Cherokee,
524    Common,
525    Coptic,
526    Cuneiform,
527    Cypriot,
528    Cyrillic,
529    Deseret,
530    Devanagari,
531    Ethiopic,
532    Georgian,
533    Glagolitic,
534    Gothic,
535    Greek,
536    Gujarati,
537    Gurmukhi,
538    Han,
539    Hangul,
540    Hanunoo,
541    Hebrew,
542    Hiragana,
543    Inherited,
544    Kannada,
545    Katakana,
546    Kharoshthi,
547    Khmer,
548    Lao,
549    Latin,
550    Limbu,
551    Linear_B,
552    Malayalam,
553    Mongolian,
554    Myanmar,
555    New_Tai_Lue,
556    Nko,
557    Ogham,
558    Old_Italic,
559    Old_Persian,
560    Oriya,
561    Osmanya,
562    Phags_Pa,
563    Phoenician,
564    Runic,
565    Shavian,
566    Sinhala,
567    Syloti_Nagri,
568    Syriac,
569    Tagalog,
570    Tagbanwa,
571    Tai_Le,
572    Tamil,
573    Telugu,
574    Thaana,
575    Thai,
576    Tibetan,
577    Tifinagh,
578    Ugaritic,
579    Yi.
580    .P
581    Each character has exactly one general category property, specified by a
582    two-letter abbreviation. For compatibility with Perl, negation can be specified
583    by including a circumflex between the opening brace and the property name. For
584    example, \ep{^Lu} is the same as \eP{Lu}.
585    .P
586    If only one letter is specified with \ep or \eP, it includes all the general
587    category properties that start with that letter. In this case, in the absence
588    of negation, the curly brackets in the escape sequence are optional; these two
589    examples have the same effect:
590    .sp
591      \ep{L}
592      \epL
593    .sp
594    The following general category property codes are supported:
595    .sp
596      C     Other
597      Cc    Control
598      Cf    Format
599      Cn    Unassigned
600      Co    Private use
601      Cs    Surrogate
602    .sp
603      L     Letter
604      Ll    Lower case letter
605      Lm    Modifier letter
606      Lo    Other letter
607      Lt    Title case letter
608      Lu    Upper case letter
609    .sp
610      M     Mark
611      Mc    Spacing mark
612      Me    Enclosing mark
613      Mn    Non-spacing mark
614    .sp
615      N     Number
616      Nd    Decimal number
617      Nl    Letter number
618      No    Other number
619    .sp
620      P     Punctuation
621      Pc    Connector punctuation
622      Pd    Dash punctuation
623      Pe    Close punctuation
624      Pf    Final punctuation
625      Pi    Initial punctuation
626      Po    Other punctuation
627      Ps    Open punctuation
628    .sp
629      S     Symbol
630      Sc    Currency symbol
631      Sk    Modifier symbol
632      Sm    Mathematical symbol
633      So    Other symbol
634    .sp
635      Z     Separator
636      Zl    Line separator
637      Zp    Paragraph separator
638      Zs    Space separator
639    .sp
640    The special property L& is also supported: it matches a character that has
641    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
642    a modifier or "other".
643    .P
644    The Cs (Surrogate) property applies only to characters in the range U+D800 to
645    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
646    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
647    (see the discussion of PCRE_NO_UTF8_CHECK in the
648    .\" HREF
649    \fBpcreapi\fP
650    .\"
651    page). Perl does not support the Cs property.
652    .P
653    The long synonyms for property names that Perl supports (such as \ep{Letter})
654    are not supported by PCRE, nor is it permitted to prefix any of these
655    properties with "Is".
656    .P
657    No character that is in the Unicode table has the Cn (unassigned) property.
658    Instead, this property is assumed for any code point that is not in the
659    Unicode table.
660    .P
661    Specifying caseless matching does not affect these escape sequences. For
662    example, \ep{Lu} always matches only upper case letters.
663    .P
664    The \eX escape matches any number of Unicode characters that form an extended
665    Unicode sequence. \eX is equivalent to
666    .sp
667      (?>\ePM\epM*)
668    .sp
669    That is, it matches a character without the "mark" property, followed by zero
670    or more characters with the "mark" property, and treats the sequence as an
671    atomic group
672    .\" HTML <a href="#atomicgroup">
673    .\" </a>
674    (see below).
675    .\"
676    Characters with the "mark" property are typically accents that affect the
677    preceding character. None of them have codepoints less than 256, so in
678    non-UTF-8 mode \eX matches any one character.
679    .P
680    Matching characters by Unicode property is not fast, because PCRE has to search
681    a structure that contains data for over fifteen thousand characters. That is
682    why the traditional escape sequences such as \ed and \ew do not use Unicode
683    properties in PCRE.
684    .
685    .
686    .\" HTML <a name="resetmatchstart"></a>
687    .SS "Resetting the match start"
688    .rs
689    .sp
690    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
691    matched characters not to be included in the final matched sequence. For
692    example, the pattern:
693    .sp
694      foo\eKbar
695    .sp
696    matches "foobar", but reports that it has matched "bar". This feature is
697    similar to a lookbehind assertion
698    .\" HTML <a href="#lookbehind">
699    .\" </a>
700    (described below).
701    .\"
702    However, in this case, the part of the subject before the real match does not
703    have to be of fixed length, as lookbehind assertions do. The use of \eK does
704    not interfere with the setting of
705    .\" HTML <a href="#subpattern">
706    .\" </a>
707    captured substrings.
708    .\"
709    For example, when the pattern
710    .sp
711      (foo)\eKbar
712    .sp
713    matches "foobar", the first substring is still set to "foo".
714    .
715    .
716    .\" HTML <a name="smallassertions"></a>
717    .SS "Simple assertions"
718    .rs
719    .sp
720    The final use of backslash is for certain simple assertions. An assertion
721  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,
722  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
723  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
724  assertions are  .\" HTML <a href="#bigassertions">
725    .\" </a>
726    \\b     matches at a word boundary  below.
727    \\B     matches when not at a word boundary  .\"
728    \\A     matches at start of subject  The backslashed assertions are:
729    \\Z     matches at end of subject or before newline at end  .sp
730    \\z     matches at end of subject    \eb     matches at a word boundary
731    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
732      \eA     matches at the start of the subject
733  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
734              also matches before a newline at the end of the subject
735      \ez     matches only at the end of the subject
736      \eG     matches at the first matching position in the subject
737    .sp
738    These assertions may not appear in character classes (but note that \eb has a
739  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
740    .P
741  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
742  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
743  \\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
744  first or last character matches \\w, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745    separte "start of word" or "end of word" metasequence. However, whatever
746  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  follows \eb normally determines which it is. For example, the fragment
747  dollar (described below) in that they only ever match at the very start and end  \eba matches "a" at the start of a word.
748  of the subject string, whatever options are set. Thus, they are independent of  .P
749  multiline mode.  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
750    dollar (described in the next section) in that they only ever match at the very
751  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  start and end of the subject string, whatever options are set. Thus, they are
752  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  independent of multiline mode. These three assertions are not affected by the
753  matching is to start at a point other than the beginning of the subject, \\A  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
754  can never match. The difference between \\Z and \\z is that \\Z matches before  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
755  a newline that is the last character of the string as well as at the end of the  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
756  string, whereas \\z matches only at the end.  at a point other than the beginning of the subject, \eA can never match. The
757    difference between \eZ and \ez is that \eZ matches before a newline at the end
758  The \\G assertion is true only when the current matching position is at the  of the string as well as at the very end, whereas \ez matches only at the end.
759  start point of the match, as specified by the \fIstartoffset\fR argument of  .P
760  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  The \eG assertion is true only when the current matching position is at the
761  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  start point of the match, as specified by the \fIstartoffset\fP argument of
762    \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
763    non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
764  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
765  implementation where \\G can be useful.  implementation where \eG can be useful.
766    .P
767  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
768  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
769  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
770  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
771  reproduce this behaviour.  reproduce this behaviour.
772    .P
773  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
774  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
775  regular expression.  regular expression.
776    .
777  .SH CIRCUMFLEX AND DOLLAR  .
778    .SH "CIRCUMFLEX AND DOLLAR"
779  .rs  .rs
780  .sp  .sp
781  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
782  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
783  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
784  \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
785  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
786  meaning (see below).  meaning
787    .\" HTML <a href="#characterclass">
788    .\" </a>
789    (see below).
790    .\"
791    .P
792  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
793  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
794  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 796  possible alternatives start with a circu
796  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
797  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
798  to be anchored.)  to be anchored.)
799    .P
800  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
801  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
802  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
803  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
804  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
805  Dollar has no special meaning in a character class.  character class.
806    .P
807  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
808  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
809  does not affect the \\Z assertion.  does not affect the \eZ assertion.
810    .P
811  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
812  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
813  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
814  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
815  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
816  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
817  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
818  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
819  \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
820  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
821    patterns that are anchored in single line mode because all branches start with
822  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
823    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
824    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
825    .P
826    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
827  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
828  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
829    .
830  .SH FULL STOP (PERIOD, DOT)  .
831    .SH "FULL STOP (PERIOD, DOT)"
832  .rs  .rs
833  .sp  .sp
834  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
835  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
836  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.
837  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
838  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
839  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
840  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
841    (including isolated CRs and LFs). When any Unicode line endings are being
842  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
843  .rs  characters.
844  .sp  .P
845  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
846  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
847  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
848  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
849  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
850    The handling of dot is entirely independent of the handling of circumflex and
851  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
852  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
853    .
854  .SH SQUARE BRACKETS  .
855    .SH "MATCHING A SINGLE BYTE"
856    .rs
857    .sp
858    Outside a character class, the escape sequence \eC matches any one byte, both
859    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
860    characters. The feature is provided in Perl in order to match individual bytes
861    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
862    what remains in the string may be a malformed UTF-8 string. For this reason,
863    the \eC escape sequence is best avoided.
864    .P
865    PCRE does not allow \eC to appear in lookbehind assertions
866    .\" HTML <a href="#lookbehind">
867    .\" </a>
868    (described below),
869    .\"
870    because in UTF-8 mode this would make it impossible to calculate the length of
871    the lookbehind.
872    .
873    .
874    .\" HTML <a name="characterclass"></a>
875    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
876  .rs  .rs
877  .sp  .sp
878  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
879  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.
880  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
881  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
882  escaped with a backslash.  a member of the class, it should be the first data character in the class
883    (after an initial circumflex, if present) or escaped with a backslash.
884    .P
885  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
886  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
887  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
888  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
889  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
890  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
891  backslash.  backslash.
892    .P
893  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
894  [^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
895  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
896  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
897  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
898  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
899    string.
900    .P
901  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
902  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.
903    .P
904  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
905  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
906  "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
907  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
908  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
909    always possible. For characters with higher values, the concept of case is
910  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.
911  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,
912  such as [^a] will always match a newline.  you must ensure that PCRE is compiled with Unicode property support as well as
913    with UTF-8 support.
914    .P
915    Characters that might indicate line breaks are never treated in any special way
916    when matching character classes, whatever line-ending sequence is in use, and
917    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
918    such as [^a] always matches one of these characters.
919    .P
920  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
921  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,
922  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
923  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
924  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.
925    .P
926  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
927  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
928  ("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
929  "-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
930  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
931  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
932  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
933    .P
934  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
935  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
936  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
937  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
938    .P
939  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
940  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
941  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
942  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
943    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
944  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
945  character class, and add the characters that they match to the class. For  property support.
946  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
947    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
948    in a character class, and add the characters that they match to the class. For
949    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
950  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
951  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
952  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
953    .P
954  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
955  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
956  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
957    introducing a POSIX class name - see the next section), and the terminating
958  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
959    does no harm.
960    .
961    .
962    .SH "POSIX CHARACTER CLASSES"
963  .rs  .rs
964  .sp  .sp
965  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
966  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
967  this notation. For example,  this notation. For example,
968    .sp
969    [01[:alpha:]%]    [01[:alpha:]%]
970    .sp
971  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
972  are  are
973    .sp
974    alnum    letters and digits    alnum    letters and digits
975    alpha    letters    alpha    letters
976    ascii    character codes 0 - 127    ascii    character codes 0 - 127
977    blank    space or tab only    blank    space or tab only
978    cntrl    control characters    cntrl    control characters
979    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
980    graph    printing characters, excluding space    graph    printing characters, excluding space
981    lower    lower case letters    lower    lower case letters
982    print    printing characters, including space    print    printing characters, including space
983    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
984    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
985    upper    upper case letters    upper    upper case letters
986    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
987    xdigit   hexadecimal digits    xdigit   hexadecimal digits
988    .sp
989  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
990  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
991  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
992  compatibility).  compatibility).
993    .P
994  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
995  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
996  after the colon. For example,  after the colon. For example,
997    .sp
998    [12[:^digit:]]    [12[:^digit:]]
999    .sp
1000  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
1001  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
1002  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1003    .P
1004  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
1005  the POSIX character classes.  the POSIX character classes.
1006    .
1007  .SH VERTICAL BAR  .
1008    .SH "VERTICAL BAR"
1009  .rs  .rs
1010  .sp  .sp
1011  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1012  the pattern  the pattern
1013    .sp
1014    gilbert|sullivan    gilbert|sullivan
1015    .sp
1016  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1017  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1018  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
1019  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
1020  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
1021  pattern as well as the alternative in the subpattern.  .\" </a>
1022    (defined below),
1023  .SH INTERNAL OPTION SETTING  .\"
1024    "succeeds" means matching the rest of the main pattern as well as the
1025    alternative in the subpattern.
1026    .
1027    .
1028    .SH "INTERNAL OPTION SETTING"
1029  .rs  .rs
1030  .sp  .sp
1031  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1032  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
1033  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1034    The option letters are
1035    .sp
1036    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1037    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
1038    s  for PCRE_DOTALL    s  for PCRE_DOTALL
1039    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
1040    .sp
1041  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
1042  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
1043  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1044  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1045  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
1046  unset.  unset.
1047    .P
1048  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
1049  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
1050  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1051  the global options (and it will therefore show up in data extracted by the  .P
1052  \fBpcre_fullinfo()\fR function).  When one of these option changes occurs at top level (that is, not inside
1053    subpattern parentheses), the change applies to the remainder of the pattern
1054  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
1055  pattern that follows it, so  extracts it into the global options (and it will therefore show up in data
1056    extracted by the \fBpcre_fullinfo()\fP function).
1057    .P
1058    An option change within a subpattern (see below for a description of
1059    subpatterns) affects only that part of the current pattern that follows it, so
1060    .sp
1061    (a(?i)b)c    (a(?i)b)c
1062    .sp
1063  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).
1064  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
1065  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
1066  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
1067    .sp
1068    (a(?i)b|c)    (a(?i)b|c)
1069    .sp
1070  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
1071  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
1072  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1073  behaviour otherwise.  behaviour otherwise.
1074    .P
1075  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
1076  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
1077  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
1078  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
1079  when it is at top level. It is best put at the start.  section entitled
1080    .\" HTML <a href="#newlineseq">
1081    .\" </a>
1082    "Newline sequences"
1083    .\"
1084    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1085    mode; this is equivalent to setting the PCRE_UTF8 option.
1086    .
1087    .
1088    .\" HTML <a name="subpattern"></a>
1089  .SH SUBPATTERNS  .SH SUBPATTERNS
1090  .rs  .rs
1091  .sp  .sp
1092  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1093  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1094    .sp
1095  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
1096    .sp
1097    cat(aract|erpillar|)    cat(aract|erpillar|)
1098    .sp
1099  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1100  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1101    .sp
1102  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
1103  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
1104  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
1105  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1106  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
1107    .P
1108  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
1109    .sp
1110    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1111    .sp
1112  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,
1113  2, and 3, respectively.  2, and 3, respectively.
1114    .P
1115  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
1116  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
1117  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
1118  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
1119  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1120  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1121    .sp
1122    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1123    .sp
1124  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
1125  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1126  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1127  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
1128  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1129  the ":". Thus the two patterns  the ":". Thus the two patterns
1130    .sp
1131    (?i:saturday|sunday)    (?i:saturday|sunday)
1132    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1133    .sp
1134  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1135  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
1136  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
1137  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1138    .
1139  .SH NAMED SUBPATTERNS  .
1140    .\" HTML <a name="dupsubpatternnumber"></a>
1141    .SH "DUPLICATE SUBPATTERN NUMBERS"
1142    .rs
1143    .sp
1144    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1145    the same numbers for its capturing parentheses. Such a subpattern starts with
1146    (?| and is itself a non-capturing subpattern. For example, consider this
1147    pattern:
1148    .sp
1149      (?|(Sat)ur|(Sun))day
1150    .sp
1151    Because the two alternatives are inside a (?| group, both sets of capturing
1152    parentheses are numbered one. Thus, when the pattern matches, you can look
1153    at captured substring number one, whichever alternative matched. This construct
1154    is useful when you want to capture part, but not all, of one of a number of
1155    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1156    number is reset at the start of each branch. The numbers of any capturing
1157    buffers that follow the subpattern start after the highest number used in any
1158    branch. The following example is taken from the Perl documentation.
1159    The numbers underneath show in which buffer the captured content will be
1160    stored.
1161    .sp
1162      # before  ---------------branch-reset----------- after
1163      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164      # 1            2         2  3        2     3     4
1165    .sp
1166    A backreference to a numbered subpattern uses the most recent value that is set
1167    for that number by any subpattern. The following pattern matches "abcabc" or
1168    "defdef":
1169    .sp
1170      /(?|(abc)|(def))\e1/
1171    .sp
1172    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173    refers to the first one in the pattern with the given number. The following
1174    pattern matches "abcabc" or "defabc":
1175    .sp
1176      /(?|(abc)|(def))(?1)/
1177    .sp
1178    If a
1179    .\" HTML <a href="#conditions">
1180    .\" </a>
1181    condition test
1182    .\"
1183    for a subpattern's having matched refers to a non-unique number, the test is
1184    true if any of the subpatterns of that number have matched.
1185    .P
1186    An alternative approach to using this "branch reset" feature is to use
1187    duplicate named subpatterns, as described in the next section.
1188    .
1189    .
1190    .SH "NAMED SUBPATTERNS"
1191  .rs  .rs
1192  .sp  .sp
1193  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
1194  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1195  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
1196  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1197  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
1198  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
1199    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1200  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1201  PCRE API provides function calls for extracting the name-to-number translation  .P
1202  table from a compiled pattern. For further details see the  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1203    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1204    parentheses from other parts of the pattern, such as
1205    .\" HTML <a href="#backreferences">
1206    .\" </a>
1207    backreferences,
1208    .\"
1209    .\" HTML <a href="#recursion">
1210    .\" </a>
1211    recursion,
1212    .\"
1213    and
1214    .\" HTML <a href="#conditions">
1215    .\" </a>
1216    conditions,
1217    .\"
1218    can be made by name as well as by number.
1219    .P
1220    Names consist of up to 32 alphanumeric characters and underscores. Named
1221    capturing parentheses are still allocated numbers as well as names, exactly as
1222    if the names were not present. The PCRE API provides function calls for
1223    extracting the name-to-number translation table from a compiled pattern. There
1224    is also a convenience function for extracting a captured substring by name.
1225    .P
1226    By default, a name must be unique within a pattern, but it is possible to relax
1227    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1228    names are also always permitted for subpatterns with the same number, set up as
1229    described in the previous section.) Duplicate names can be useful for patterns
1230    where only one instance of the named parentheses can match. Suppose you want to
1231    match the name of a weekday, either as a 3-letter abbreviation or as the full
1232    name, and in both cases you want to extract the abbreviation. This pattern
1233    (ignoring the line breaks) does the job:
1234    .sp
1235      (?<DN>Mon|Fri|Sun)(?:day)?|
1236      (?<DN>Tue)(?:sday)?|
1237      (?<DN>Wed)(?:nesday)?|
1238      (?<DN>Thu)(?:rsday)?|
1239      (?<DN>Sat)(?:urday)?
1240    .sp
1241    There are five capturing substrings, but only one is ever set after a match.
1242    (An alternative way of solving this problem is to use a "branch reset"
1243    subpattern, as described in the previous section.)
1244    .P
1245    The convenience function for extracting the data by name returns the substring
1246    for the first (and in this example, the only) subpattern of that name that
1247    matched. This saves searching to find which numbered subpattern it was.
1248    .P
1249    If you make a backreference to a non-unique named subpattern from elsewhere in
1250    the pattern, the one that corresponds to the first occurrence of the name is
1251    used. In the absence of duplicate numbers (see the previous section) this is
1252    the one with the lowest number. If you use a named reference in a condition
1253    test (see the
1254    .\"
1255    .\" HTML <a href="#conditions">
1256    .\" </a>
1257    section about conditions
1258    .\"
1259    below), either to check whether a subpattern has matched, or to check for
1260    recursion, all subpatterns with the same name are tested. If the condition is
1261    true for any one of them, the overall condition is true. This is the same
1262    behaviour as testing by number. For further details of the interfaces for
1263    handling named subpatterns, see the
1264  .\" HREF  .\" HREF
1265  \fBpcreapi\fR  \fBpcreapi\fP
1266  .\"  .\"
1267  documentation.  documentation.
1268    .P
1269    \fBWarning:\fP You cannot use different names to distinguish between two
1270    subpatterns with the same number because PCRE uses only the numbers when
1271    matching. For this reason, an error is given at compile time if different names
1272    are given to subpatterns with the same number. However, you can give the same
1273    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1274    .
1275    .
1276  .SH REPETITION  .SH REPETITION
1277  .rs  .rs
1278  .sp  .sp
1279  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1280  items:  items:
1281    .sp
1282    a literal data character    a literal data character
1283    the . metacharacter    the dot metacharacter
1284    the \\C escape sequence    the \eC escape sequence
1285    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1286      the \eR escape sequence
1287      an escape such as \ed that matches a single character
1288    a character class    a character class
1289    a back reference (see next section)    a back reference (see next section)
1290    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1291      a recursive or "subroutine" call to a subpattern
1292    .sp
1293  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1294  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1295  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
1296  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1297    .sp
1298    z{2,4}    z{2,4}
1299    .sp
1300  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
1301  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
1302  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
1303  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1304    .sp
1305    [aeiou]{3,}    [aeiou]{3,}
1306    .sp
1307  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1308    .sp
1309    \\d{8}    \ed{8}
1310    .sp
1311  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
1312  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
1313  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
1314  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1315    .P
1316  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
1317  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
1318  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1319    support is available, \eX{3} matches three Unicode extended sequences, each of
1320    which may be several bytes long (and they may be of different lengths).
1321    .P
1322  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
1323  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1324    subpatterns that are referenced as
1325  For convenience (and historical compatibility) the three most common  .\" HTML <a href="#subpatternsassubroutines">
1326  quantifiers have single-character abbreviations:  .\" </a>
1327    subroutines
1328    .\"
1329    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1330    quantifier are omitted from the compiled pattern.
1331    .P
1332    For convenience, the three most common quantifiers have single-character
1333    abbreviations:
1334    .sp
1335    *    is equivalent to {0,}    *    is equivalent to {0,}
1336    +    is equivalent to {1,}    +    is equivalent to {1,}
1337    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1338    .sp
1339  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
1340  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:
1341    .sp
1342    (a?)*    (a?)*
1343    .sp
1344  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
1345  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
1346  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
1347  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1348    .P
1349  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
1350  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1351  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
1352  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 */
1353  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1354  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1355    .sp
1356    /\\*.*\\*/    /\e*.*\e*/
1357    .sp
1358  to the string  to the string
1359    .sp
1360    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1361    .sp
1362  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 .*
1363  item.  item.
1364    .P
1365  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
1366  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1367  pattern  pattern
1368    .sp
1369    /\\*.*?\\*/    /\e*.*?\e*/
1370    .sp
1371  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
1372  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1373  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
1374  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
1375    .sp
1376    \\d??\\d    \ed??\ed
1377    .sp
1378  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
1379  way the rest of the pattern matches.  way the rest of the pattern matches.
1380    .P
1381  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),
1382  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
1383  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
1384  default behaviour.  default behaviour.
1385    .P
1386  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1387  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
1388  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1389    .P
1390  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
1391  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
1392  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1393  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
1394  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
1395  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1396    .P
1397  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
1398  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1399  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1400    .P
1401  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1402  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1403  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
1404  succeed. Consider, for example:  succeeds. Consider, for example:
1405    .sp
1406    (.*)abc\\1    (.*)abc\e1
1407    .sp
1408  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
1409  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1410    .P
1411  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1412  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1413    .sp
1414    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1415    .sp
1416  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1417  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1418  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1419  example, after  example, after
1420    .sp
1421    /(a|(b))+/    /(a|(b))+/
1422    .sp
1423  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1424    .
1425  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1426    .\" HTML <a name="atomicgroup"></a>
1427    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1428  .rs  .rs
1429  .sp  .sp
1430  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1431  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
1432  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
1433  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
1434  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
1435  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1436    .P
1437  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
1438    .sp
1439    123456bar    123456bar
1440    .sp
1441  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
1442  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+
1443  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"
1444  (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
1445  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.
1446    .P
1447  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
1448  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
1449  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1450    .sp
1451    (?>\\d+)foo    (?>\ed+)foo
1452    .sp
1453  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
1454  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
1455  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1456  normal.  normal.
1457    .P
1458  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
1459  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1460  the current point in the subject string.  the current point in the subject string.
1461    .P
1462  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1463  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
1464  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
1465  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,
1466  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1467    .P
1468  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1469  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1470  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
1471  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1472  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1473  previous example can be rewritten as  previous example can be rewritten as
1474    .sp
1475    \\d++bar    \ed++foo
1476    .sp
1477    Note that a possessive quantifier can be used with an entire group, for
1478    example:
1479    .sp
1480      (abc|xyz){2,3}+
1481    .sp
1482  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1483  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
1484  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
1485  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1486    difference; possessive quantifiers should be slightly faster.
1487  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1488  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1489    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1490    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1491    package, and PCRE copied it from there. It ultimately found its way into Perl
1492    at release 5.10.
1493    .P
1494    PCRE has an optimization that automatically "possessifies" certain simple
1495    pattern constructs. For example, the sequence A+B is treated as A++B because
1496    there is no point in backtracking into a sequence of A's when B must follow.
1497    .P
1498  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
1499  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
1500  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
1501  pattern  pattern
1502    .sp
1503    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1504    .sp
1505  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
1506  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1507  quickly. However, if it is applied to  quickly. However, if it is applied to
1508    .sp
1509    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1510    .sp
1511  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
1512  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
1513  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
1514  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
1515  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
1516  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
1517  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1518    an atomic group, like this:
1519    ((?>\\D+)|<\\d+>)*[!?]  .sp
1520      ((?>\eD+)|<\ed+>)*[!?]
1521    .sp
1522  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1523    .
1524  .SH BACK REFERENCES  .
1525    .\" HTML <a name="backreferences"></a>
1526    .SH "BACK REFERENCES"
1527  .rs  .rs
1528  .sp  .sp
1529  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
1530  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1531  (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
1532  previous capturing left parentheses.  previous capturing left parentheses.
1533    .P
1534  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
1535  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
1536  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
1537  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
1538  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
1539  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1540    in an earlier iteration.
1541    .P
1542    It is not possible to have a numerical "forward back reference" to a subpattern
1543    whose number is 10 or more using this syntax because a sequence such as \e50 is
1544    interpreted as a character defined in octal. See the subsection entitled
1545    "Non-printing characters"
1546    .\" HTML <a href="#digitsafterbackslash">
1547    .\" </a>
1548    above
1549    .\"
1550    for further details of the handling of digits following a backslash. There is
1551    no such problem when named parentheses are used. A back reference to any
1552    subpattern is possible using named parentheses (see below).
1553    .P
1554    Another way of avoiding the ambiguity inherent in the use of digits following a
1555    backslash is to use the \eg escape sequence, which is a feature introduced in
1556    Perl 5.10. This escape must be followed by an unsigned number or a negative
1557    number, optionally enclosed in braces. These examples are all identical:
1558    .sp
1559      (ring), \e1
1560      (ring), \eg1
1561      (ring), \eg{1}
1562    .sp
1563    An unsigned number specifies an absolute reference without the ambiguity that
1564    is present in the older syntax. It is also useful when literal digits follow
1565    the reference. A negative number is a relative reference. Consider this
1566    example:
1567    .sp
1568      (abc(def)ghi)\eg{-1}
1569    .sp
1570    The sequence \eg{-1} is a reference to the most recently started capturing
1571    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1572    would be equivalent to \e1. The use of relative references can be helpful in
1573    long patterns, and also in patterns that are created by joining together
1574    fragments that contain references within themselves.
1575    .P
1576  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1577  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1578  itself (see  itself (see
# Line 832  itself (see Line 1581  itself (see
1581  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1582  .\"  .\"
1583  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1584    .sp
1585    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1586    .sp
1587  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1588  "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
1589  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1590    .sp
1591    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1592    .sp
1593  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
1594  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1595    .P
1596  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1597  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1598    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1599    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1600    references, is also supported. We could rewrite the above example in any of
1601    the following ways:
1602    .sp
1603      (?<p1>(?i)rah)\es+\ek<p1>
1604      (?'p1'(?i)rah)\es+\ek{p1}
1605      (?P<p1>(?i)rah)\es+(?P=p1)
1606      (?<p1>(?i)rah)\es+\eg{p1}
1607    .sp
1608    A subpattern that is referenced by name may appear in the pattern before or
1609    after the reference.
1610    .P
1611  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
1612  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1613  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1614    .sp
1615    (a|(bc))\\2    (a|(bc))\e2
1616    .sp
1617  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
1618  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
1619  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1620  with a digit character, some delimiter must be used to terminate the back  .P
1621  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1622  Otherwise an empty comment can be used.  following a backslash are taken as part of a potential back reference number.
1623    If the pattern continues with a digit character, some delimiter must be used to
1624    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1625    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1626    .\" HTML <a href="#comments">
1627    .\" </a>
1628    "Comments"
1629    .\"
1630    below) can be used.
1631    .P
1632  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
1633  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.
1634  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1635  example, the pattern  example, the pattern
1636    .sp
1637    (a|b\\1)+    (a|b\e1)+
1638    .sp
1639  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
1640  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1641  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
1642  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
1643  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
1644  minimum of zero.  minimum of zero.
1645    .
1646    .
1647    .\" HTML <a name="bigassertions"></a>
1648  .SH ASSERTIONS  .SH ASSERTIONS
1649  .rs  .rs
1650  .sp  .sp
1651  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
1652  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1653  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
1654    .\" HTML <a href="#smallassertions">
1655    .\" </a>
1656    above.
1657    .\"
1658    .P
1659  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1660  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
1661  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1662    except that it does not cause the current matching position to be changed.
1663  An assertion subpattern is matched in the normal way, except that it does not  .P
1664  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1665  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
1666    of assertion contains capturing subpatterns within it, these are counted for
1667    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1668    However, substring capturing is carried out only for positive assertions,
1669    because it does not make sense for negative assertions.
1670    .
1671    .
1672    .SS "Lookahead assertions"
1673    .rs
1674    .sp
1675    Lookahead assertions start with (?= for positive assertions and (?! for
1676    negative assertions. For example,
1677    .sp
1678      \ew+(?=;)
1679    .sp
1680  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
1681  the match, and  the match, and
1682    .sp
1683    foo(?!bar)    foo(?!bar)
1684    .sp
1685  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
1686  apparently similar pattern  apparently similar pattern
1687    .sp
1688    (?!foo)bar    (?!foo)bar
1689    .sp
1690  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
1691  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1692  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1693  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1694    .P
1695  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
1696  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
1697  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.
1698    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1699    synonym for (?!).
1700    .
1701    .
1702    .\" HTML <a name="lookbehind"></a>
1703    .SS "Lookbehind assertions"
1704    .rs
1705    .sp
1706  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1707  negative assertions. For example,  negative assertions. For example,
1708    .sp
1709    (?<!foo)bar    (?<!foo)bar
1710    .sp
1711  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
1712  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
1713  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
1714  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1715    .sp
1716    (?<=bullock|donkey)    (?<=bullock|donkey)
1717    .sp
1718  is permitted, but  is permitted, but
1719    .sp
1720    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1721    .sp
1722  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1723  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
1724  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
1725  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1726    .sp
1727    (?<=ab(c|de))    (?<=ab(c|de))
1728    .sp
1729  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
1730  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
1731    branches:
1732    .sp
1733    (?<=abc|abde)    (?<=abc|abde)
1734    .sp
1735    In some cases, the Perl 5.10 escape sequence \eK
1736    .\" HTML <a href="#resetmatchstart">
1737    .\" </a>
1738    (see above)
1739    .\"
1740    can be used instead of a lookbehind assertion to get round the fixed-length
1741    restriction.
1742    .P
1743  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1744  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
1745  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1746  match is deemed to fail.  assertion fails.
1747    .P
1748  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)
1749  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1750  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1751    different numbers of bytes, are also not permitted.
1752  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1753  efficient matching at the end of the subject string. Consider a simple pattern  .\" HTML <a href="#subpatternsassubroutines">
1754  such as  .\" </a>
1755    "Subroutine"
1756    .\"
1757    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1758    as the subpattern matches a fixed-length string.
1759    .\" HTML <a href="#recursion">
1760    .\" </a>
1761    Recursion,
1762    .\"
1763    however, is not supported.
1764    .P
1765    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1766    specify efficient matching of fixed-length strings at the end of subject
1767    strings. Consider a simple pattern such as
1768    .sp
1769    abcd$    abcd$
1770    .sp
1771  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
1772  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
1773  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
1774    .sp
1775    ^.*abcd$    ^.*abcd$
1776    .sp
1777  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
1778  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,
1779  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"
1780  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,
1781  if the pattern is written as  if the pattern is written as
1782    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1783    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1784    .sp
1785  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
1786  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
1787  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1788  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1789    .
1790    .
1791    .SS "Using multiple assertions"
1792    .rs
1793    .sp
1794  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1795    .sp
1796    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1797    .sp
1798  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
1799  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1800  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
1801  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".
1802  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
1803  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
1804  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1805    .sp
1806    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1807    .sp
1808  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1809  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
1810  preceding three characters are not "999".  preceding three characters are not "999".
1811    .P
1812  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1813    .sp
1814    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1815    .sp
1816  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
1817  preceded by "foo", while  preceded by "foo", while
1818    .sp
1819    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1820    .sp
1821  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
1822  characters that are not "999".  characters that are not "999".
1823    .
1824  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1825  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1826  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  
1827  .rs  .rs
1828  .sp  .sp
1829  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1830  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1831  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1832  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1833    .sp
1834    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1835    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1836    .sp
1837  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1838  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
1839  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1840    .P
1841  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1842  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1843  subpattern of that number has previously matched. The number must be greater  .
1844  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1845  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1846  it into three parts for ease of discussion:  .sp
1847    If the text between the parentheses consists of a sequence of digits, the
1848    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if a capturing subpattern of that number has previously
1849    matched. If there is more than one capturing subpattern with the same number
1850    (see the earlier
1851    .\"
1852    .\" HTML <a href="#recursion">
1853    .\" </a>
1854    section about duplicate subpattern numbers),
1855    .\"
1856    the condition is true if any of them have been set. An alternative notation is
1857    to precede the digits with a plus or minus sign. In this case, the subpattern
1858    number is relative rather than absolute. The most recently opened parentheses
1859    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1860    looping constructs it can also make sense to refer to subsequent groups with
1861    constructs such as (?(+2).
1862    .P
1863    Consider the following pattern, which contains non-significant white space to
1864    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1865    three parts for ease of discussion:
1866    .sp
1867      ( \e( )?    [^()]+    (?(1) \e) )
1868    .sp
1869  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1870  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
1871  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 1875  the condition is true, and so the yes-pa
1875  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1876  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1877  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1878    .P
1879  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
1880  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1881  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1882      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1883  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1884    This makes the fragment independent of the parentheses in the larger pattern.
1885    .
1886    .SS "Checking for a used subpattern by name"
1887    .rs
1888    .sp
1889    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1890    subpattern by name. For compatibility with earlier versions of PCRE, which had
1891    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1892    there is a possible ambiguity with this syntax, because subpattern names may
1893    consist entirely of digits. PCRE looks first for a named subpattern; if it
1894    cannot find one and the name consists entirely of digits, PCRE looks for a
1895    subpattern of that number, which must be greater than zero. Using subpattern
1896    names that consist entirely of digits is not recommended.
1897    .P
1898    Rewriting the above example to use a named subpattern gives this:
1899    .sp
1900      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1901    .sp
1902    If the name used in a condition of this kind is a duplicate, the test is
1903    applied to all subpatterns of the same name, and is true if any one of them has
1904    matched.
1905    .
1906    .SS "Checking for pattern recursion"
1907    .rs
1908    .sp
1909    If the condition is the string (R), and there is no subpattern with the name R,
1910    the condition is true if a recursive call to the whole pattern or any
1911    subpattern has been made. If digits or a name preceded by ampersand follow the
1912    letter R, for example:
1913    .sp
1914      (?(R3)...) or (?(R&name)...)
1915    .sp
1916    the condition is true if the most recent recursion is into a subpattern whose
1917    number or name is given. This condition does not check the entire recursion
1918    stack. If the name used in a condition of this kind is a duplicate, the test is
1919    applied to all subpatterns of the same name, and is true if any one of them is
1920    the most recent recursion.
1921    .P
1922    At "top level", all these recursion test conditions are false.
1923    .\" HTML <a href="#recursion">
1924    .\" </a>
1925    The syntax for recursive patterns
1926    .\"
1927    is described below.
1928    .
1929    .SS "Defining subpatterns for use by reference only"
1930    .rs
1931    .sp
1932    If the condition is the string (DEFINE), and there is no subpattern with the
1933    name DEFINE, the condition is always false. In this case, there may be only one
1934    alternative in the subpattern. It is always skipped if control reaches this
1935    point in the pattern; the idea of DEFINE is that it can be used to define
1936    "subroutines" that can be referenced from elsewhere. (The use of
1937    .\" HTML <a href="#subpatternsassubroutines">
1938    .\" </a>
1939    "subroutines"
1940    .\"
1941    is described below.) For example, a pattern to match an IPv4 address could be
1942    written like this (ignore whitespace and line breaks):
1943    .sp
1944      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1945      \eb (?&byte) (\e.(?&byte)){3} \eb
1946    .sp
1947    The first part of the pattern is a DEFINE group inside which a another group
1948    named "byte" is defined. This matches an individual component of an IPv4
1949    address (a number less than 256). When matching takes place, this part of the
1950    pattern is skipped because DEFINE acts like a false condition. The rest of the
1951    pattern uses references to the named group to match the four dot-separated
1952    components of an IPv4 address, insisting on a word boundary at each end.
1953    .
1954    .SS "Assertion conditions"
1955    .rs
1956    .sp
1957    If the condition is not in any of the above formats, it must be an assertion.
1958  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1959  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1960  alternatives on the second line:  alternatives on the second line:
1961    .sp
1962    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1963    \\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} )
1964    .sp
1965  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1966  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
1967  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
1968  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1969  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
1970  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.
1971    .
1972    .
1973    .\" HTML <a name="comments"></a>
1974  .SH COMMENTS  .SH COMMENTS
1975  .rs  .rs
1976  .sp  .sp
1977  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
1978  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1979  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.
1980    .P
1981  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1982  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1983  character in the pattern.  next newline in the pattern.
1984    .
1985  .SH RECURSIVE PATTERNS  .
1986    .\" HTML <a name="recursion"></a>
1987    .SH "RECURSIVE PATTERNS"
1988  .rs  .rs
1989  .sp  .sp
1990  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1991  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1992  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
1993  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1994  experimental facility that allows regular expressions to recurse (amongst other  .P
1995  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
1996  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
1997  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1998    pattern using code interpolation to solve the parentheses problem can be
1999    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
2000    .sp
2001      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2002    .sp
2003  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
2004  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2005  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2006  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2007    supports special syntax for recursion of the entire pattern, and also for
2008  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2009  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.
2010  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2011  "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
2012  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2013    provided that it occurs inside that subpattern. (If not, it is a
2014  For example, this PCRE pattern solves the nested parentheses problem (assume  .\" HTML <a href="#subpatternsassubroutines">
2015  the PCRE_EXTENDED option is set so that white space is ignored):  .\" </a>
2016    "subroutine"
2017    \\( ( (?>[^()]+) | (?R) )* \\)  .\"
2018    call, which is described in the next section.) The special item (?R) or (?0) is
2019    a recursive call of the entire regular expression.
2020    .P
2021    This PCRE pattern solves the nested parentheses problem (assume the
2022    PCRE_EXTENDED option is set so that white space is ignored):
2023    .sp
2024      \e( ( [^()]++ | (?R) )* \e)
2025    .sp
2026  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2027  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
2028  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2029  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2030    to avoid backtracking into sequences of non-parentheses.
2031    .P
2032  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
2033  pattern, so instead you could use this:  pattern, so instead you could use this:
2034    .sp
2035    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( [^()]++ | (?1) )* \e) )
2036    .sp
2037  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
2038  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2039  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2040  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
2041  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).
2042  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
2043    most recently opened parentheses preceding the recursion. In other words, a
2044    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
2045    it is encountered.
2046  This particular example pattern contains nested unlimited repeats, and so the  .P
2047  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2048  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
2049  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2050    .\" HTML <a href="#subpatternsassubroutines">
2051    .\" </a>
2052    "subroutine"
2053    .\"
2054    calls, as described in the next section.
2055    .P
2056    An alternative approach is to use named parentheses instead. The Perl syntax
2057    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2058    could rewrite the above example as follows:
2059    .sp
2060      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2061    .sp
2062    If there is more than one subpattern with the same name, the earliest one is
2063    used.
2064    .P
2065    This particular example pattern that we have been looking at contains nested
2066    unlimited repeats, and so the use of a possessive quantifier for matching
2067    strings of non-parentheses is important when applying the pattern to strings
2068    that do not match. For example, when this pattern is applied to
2069    .sp
2070    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2071    .sp
2072  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,
2073  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
2074  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
2075  before failure can be reported.  before failure can be reported.
2076    .P
2077  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
2078  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
2079  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  
2080  .\" HREF  .\" HREF
2081  \fBpcrecallout\fR  \fBpcrecallout\fP
2082  .\"  .\"
2083  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2084    .sp
2085    (ab(cd)ef)    (ab(cd)ef)
2086    .sp
2087  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
2088  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
2089    matched at the top level, its final value is unset, even if it is (temporarily)
2090    \\( ( ( (?>[^()]+) | (?R) )* ) \\)  set at a deeper level.
2091       ^                        ^  .P
2092       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2093    obtain extra memory to store data during a recursion, which it does by using
2094  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
2095  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2096  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.  
   
2097  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.
2098  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
2099  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2100  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
2101    .sp
2102    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
2103    .sp
2104  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
2105  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2106  is the actual recursive call.  is the actual recursive call.
2107    .
2108    .
2109    .\" HTML <a name="recursiondifference"></a>
2110    .SS "Recursion difference from Perl"
2111    .rs
2112    .sp
2113    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2114    treated as an atomic group. That is, once it has matched some of the subject
2115    string, it is never re-entered, even if it contains untried alternatives and
2116    there is a subsequent matching failure. This can be illustrated by the
2117    following pattern, which purports to match a palindromic string that contains
2118    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2119    .sp
2120      ^(.|(.)(?1)\e2)$
2121    .sp
2122    The idea is that it either matches a single character, or two identical
2123    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2124    it does not if the pattern is longer than three characters. Consider the
2125    subject string "abcba":
2126    .P
2127    At the top level, the first character is matched, but as it is not at the end
2128    of the string, the first alternative fails; the second alternative is taken
2129    and the recursion kicks in. The recursive call to subpattern 1 successfully
2130    matches the next character ("b"). (Note that the beginning and end of line
2131    tests are not part of the recursion).
2132    .P
2133    Back at the top level, the next character ("c") is compared with what
2134    subpattern 2 matched, which was "a". This fails. Because the recursion is
2135    treated as an atomic group, there are now no backtracking points, and so the
2136    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2137    try the second alternative.) However, if the pattern is written with the
2138    alternatives in the other order, things are different:
2139    .sp
2140      ^((.)(?1)\e2|.)$
2141    .sp
2142    This time, the recursing alternative is tried first, and continues to recurse
2143    until it runs out of characters, at which point the recursion fails. But this
2144    time we do have another alternative to try at the higher level. That is the big
2145    difference: in the previous case the remaining alternative is at a deeper
2146    recursion level, which PCRE cannot use.
2147    .P
2148    To change the pattern so that matches all palindromic strings, not just those
2149    with an odd number of characters, it is tempting to change the pattern to this:
2150    .sp
2151      ^((.)(?1)\e2|.?)$
2152    .sp
2153    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2154    deeper recursion has matched a single character, it cannot be entered again in
2155    order to match an empty string. The solution is to separate the two cases, and
2156    write out the odd and even cases as alternatives at the higher level:
2157    .sp
2158      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2159    .sp
2160    If you want to match typical palindromic phrases, the pattern has to ignore all
2161    non-word characters, which can be done like this:
2162    .sp
2163      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2164    .sp
2165    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2166    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2167    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2168    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2169    more) to match typical phrases, and Perl takes so long that you think it has
2170    gone into a loop.
2171    .P
2172    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2173    string does not start with a palindrome that is shorter than the entire string.
2174    For example, although "abcba" is correctly matched, if the subject is "ababa",
2175    PCRE finds the palindrome "aba" at the start, then fails at top level because
2176    the end of the string does not follow. Once again, it cannot jump back into the
2177    recursion to try other alternatives, so the entire match fails.
2178    .
2179    .
2180  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2181  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
2182  .rs  .rs
2183  .sp  .sp
2184  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
2185  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
2186  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2187  pattern  before or after the reference. A numbered reference can be absolute or
2188    relative, as in these examples:
2189    (sens|respons)e and \\1ibility  .sp
2190      (...(absolute)...)...(?2)...
2191      (...(relative)...)...(?-1)...
2192      (...(?+1)...(relative)...
2193    .sp
2194    An earlier example pointed out that the pattern
2195    .sp
2196      (sens|respons)e and \e1ibility
2197    .sp
2198  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
2199  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
2200    .sp
2201    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2202    .sp
2203  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
2204  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2205  refer.  .P
2206    Like recursive subpatterns, a subroutine call is always treated as an atomic
2207    group. That is, once it has matched some of the subject string, it is never
2208    re-entered, even if it contains untried alternatives and there is a subsequent
2209    matching failure. Any capturing parentheses that are set during the subroutine
2210    call revert to their previous values afterwards.
2211    .P
2212    When a subpattern is used as a subroutine, processing options such as
2213    case-independence are fixed when the subpattern is defined. They cannot be
2214    changed for different calls. For example, consider this pattern:
2215    .sp
2216      (abc)(?i:(?-1))
2217    .sp
2218    It matches "abcabc". It does not match "abcABC" because the change of
2219    processing option does not affect the called subpattern.
2220    .
2221    .
2222    .\" HTML <a name="onigurumasubroutines"></a>
2223    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2224    .rs
2225    .sp
2226    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2227    a number enclosed either in angle brackets or single quotes, is an alternative
2228    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2229    are two of the examples used above, rewritten using this syntax:
2230    .sp
2231      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2232      (sens|respons)e and \eg'1'ibility
2233    .sp
2234    PCRE supports an extension to Oniguruma: if a number is preceded by a
2235    plus or a minus sign it is taken as a relative reference. For example:
2236    .sp
2237      (abc)(?i:\eg<-1>)
2238    .sp
2239    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2240    synonymous. The former is a back reference; the latter is a subroutine call.
2241    .
2242    .
2243  .SH CALLOUTS  .SH CALLOUTS
2244  .rs  .rs
2245  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 2247  Perl has a feature whereby using the seq
2247  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
2248  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
2249  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
2250    .P
2251  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
2252  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
2253  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.
2254  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2255    .P
2256  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2257  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
2258  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.
2259  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2260    .sp
2261    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2262    .sp
2263  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
2264    automatically installed before each item in the pattern. They are all numbered
2265    255.
2266    .P
2267    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2268  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
2269  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
2270  \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
2271  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
2272  function is given in the  description of the interface to the callout function is given in the
2273  .\" HREF  .\" HREF
2274  \fBpcrecallout\fR  \fBpcrecallout\fP
2275  .\"  .\"
2276  documentation.  documentation.
2277    .
2278  .in 0  .
2279  Last updated: 03 February 2003  .SH "BACKTRACKING CONTROL"
2280  .br  .rs
2281  Copyright (c) 1997-2003 University of Cambridge.  .sp
2282    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2283    are described in the Perl documentation as "experimental and subject to change
2284    or removal in a future version of Perl". It goes on to say: "Their usage in
2285    production code should be noted to avoid problems during upgrades." The same
2286    remarks apply to the PCRE features described in this section.
2287    .P
2288    Since these verbs are specifically related to backtracking, most of them can be
2289    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2290    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2291    failing negative assertion, they cause an error if encountered by
2292    \fBpcre_dfa_exec()\fP.
2293    .P
2294    If any of these verbs are used in an assertion or subroutine subpattern
2295    (including recursive subpatterns), their effect is confined to that subpattern;
2296    it does not extend to the surrounding pattern. Note that such subpatterns are
2297    processed as anchored at the point where they are tested.
2298    .P
2299    The new verbs make use of what was previously invalid syntax: an opening
2300    parenthesis followed by an asterisk. In Perl, they are generally of the form
2301    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2302    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2303    are two kinds:
2304    .
2305    .SS "Verbs that act immediately"
2306    .rs
2307    .sp
2308    The following verbs act as soon as they are encountered:
2309    .sp
2310       (*ACCEPT)
2311    .sp
2312    This verb causes the match to end successfully, skipping the remainder of the
2313    pattern. When inside a recursion, only the innermost pattern is ended
2314    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2315    captured. (This feature was added to PCRE at release 8.00.) For example:
2316    .sp
2317      A((?:A|B(*ACCEPT)|C)D)
2318    .sp
2319    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2320    the outer parentheses.
2321    .sp
2322      (*FAIL) or (*F)
2323    .sp
2324    This verb causes the match to fail, forcing backtracking to occur. It is
2325    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2326    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2327    Perl features that are not present in PCRE. The nearest equivalent is the
2328    callout feature, as for example in this pattern:
2329    .sp
2330      a+(?C)(*FAIL)
2331    .sp
2332    A match with the string "aaaa" always fails, but the callout is taken before
2333    each backtrack happens (in this example, 10 times).
2334    .
2335    .SS "Verbs that act after backtracking"
2336    .rs
2337    .sp
2338    The following verbs do nothing when they are encountered. Matching continues
2339    with what follows, but if there is no subsequent match, a failure is forced.
2340    The verbs differ in exactly what kind of failure occurs.
2341    .sp
2342      (*COMMIT)
2343    .sp
2344    This verb causes the whole match to fail outright if the rest of the pattern
2345    does not match. Even if the pattern is unanchored, no further attempts to find
2346    a match by advancing the starting point take place. Once (*COMMIT) has been
2347    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2348    starting point, or not at all. For example:
2349    .sp
2350      a+(*COMMIT)b
2351    .sp
2352    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2353    dynamic anchor, or "I've started, so I must finish."
2354    .sp
2355      (*PRUNE)
2356    .sp
2357    This verb causes the match to fail at the current position if the rest of the
2358    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2359    advance to the next starting character then happens. Backtracking can occur as
2360    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2361    if there is no match to the right, backtracking cannot cross (*PRUNE).
2362    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2363    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2364    be expressed in any other way.
2365    .sp
2366      (*SKIP)
2367    .sp
2368    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2369    "bumpalong" advance is not to the next character, but to the position in the
2370    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2371    was matched leading up to it cannot be part of a successful match. Consider:
2372    .sp
2373      a+(*SKIP)b
2374    .sp
2375    If the subject is "aaaac...", after the first match attempt fails (starting at
2376    the first character in the string), the starting point skips on to start the
2377    next attempt at "c". Note that a possessive quantifer does not have the same
2378    effect as this example; although it would suppress backtracking during the
2379    first match attempt, the second attempt would start at the second character
2380    instead of skipping on to "c".
2381    .sp
2382      (*THEN)
2383    .sp
2384    This verb causes a skip to the next alternation if the rest of the pattern does
2385    not match. That is, it cancels pending backtracking, but only within the
2386    current alternation. Its name comes from the observation that it can be used
2387    for a pattern-based if-then-else block:
2388    .sp
2389      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2390    .sp
2391    If the COND1 pattern matches, FOO is tried (and possibly further items after
2392    the end of the group if FOO succeeds); on failure the matcher skips to the
2393    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2394    is used outside of any alternation, it acts exactly like (*PRUNE).
2395    .
2396    .
2397    .SH "SEE ALSO"
2398    .rs
2399    .sp
2400    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2401    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2402    .
2403    .
2404    .SH AUTHOR
2405    .rs
2406    .sp
2407    .nf
2408    Philip Hazel
2409    University Computing Service
2410    Cambridge CB2 3QH, England.
2411    .fi
2412    .
2413    .
2414    .SH REVISION
2415    .rs
2416    .sp
2417    .nf
2418    Last updated: 18 October 2009
2419    Copyright (c) 1997-2009 University of Cambridge.
2420    .fi

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