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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE REGULAR EXPRESSION DETAILS  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions supported by PCRE are
8  described below. Regular expressions are also described in the Perl  described below. Regular expressions are also described in the Perl
9  documentation and in a number of other books, some of which have copious  documentation and in a number of books, some of which have copious examples.
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in great detail. This description of PCRE's regular
12  reference documentation.  expressions is intended as reference material.
13    .P
14  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
15  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
19  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
20  .\" </a>  .\" </a>
21  section on UTF-8 support  section on UTF-8 support
22  .\"  .\"
23  in the main  in the main
24  .\" HREF  .\" HREF
25  \fBpcre\fR  \fBpcre\fP
26  .\"  .\"
27  page.  page.
28    .P
29    The remainder of this document discusses the patterns that are supported by
30    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31    From release 6.0, PCRE offers a second matching function,
32    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33    Perl-compatible. The advantages and disadvantages of the alternative function,
34    and how it differs from the normal function, are discussed in the
35    .\" HREF
36    \fBpcrematching\fP
37    .\"
38    page.
39    .P
40  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
41  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
42  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
43    .sp
44    The quick brown fox    The quick brown fox
45    .sp
46  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
47  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
48  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
49  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
50    always possible. For characters with higher values, the concept of case is
51    supported if PCRE is compiled with Unicode property support, but not otherwise.
52    If you want to use caseless matching for characters 128 and above, you must
53    ensure that PCRE is compiled with Unicode property support as well as with
54    UTF-8 support.
55    .P
56    The power of regular expressions comes from the ability to include alternatives
57    and repetitions in the pattern. These are encoded in the pattern by the use of
58    \fImetacharacters\fP, which do not stand for themselves but instead are
59  interpreted in some special way.  interpreted in some special way.
60    .P
61  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
62  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
63  recognized in square brackets. Outside square brackets, the meta-characters are  recognized in square brackets. Outside square brackets, the metacharacters are
64  as follows:  as follows:
65    .sp
66    \\      general escape character with several uses    \e      general escape character with several uses
67    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
68    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
69    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 78  as follows:
78    +      1 or more quantifier    +      1 or more quantifier
79           also "possessive quantifier"           also "possessive quantifier"
80    {      start min/max quantifier    {      start min/max quantifier
81    .sp
82  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
83  a character class the only meta-characters are:  a character class the only metacharacters are:
84    .sp
85    \\      general escape character    \e      general escape character
86    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
87    -      indicates character range    -      indicates character range
88    .\" JOIN
89    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
90             syntax)             syntax)
91    ]      terminates the character class    ]      terminates the character class
92    .sp
93  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
94    .
95  .SH BACKSLASH  .SH BACKSLASH
96  .rs  .rs
97  .sp  .sp
98  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
99  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
100  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
101  outside character classes.  outside character classes.
102    .P
103  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
104  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
105  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
106  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
107  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
108    .P
109  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
110  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
111  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
112  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.
113  pattern.  .P
   
114  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
115  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
116  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
117  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
118    .sp
119    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
120    .sp
121    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
122      \eQabc$xyz\eE        abc$xyz        abc followed by the
123                                        contents of $xyz                                        contents of $xyz
124    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
125    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
126    .sp
127  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
128    .
129    .
130    .\" HTML <a name="digitsafterbackslash"></a>
131    .SS "Non-printing characters"
132    .rs
133    .sp
134  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
135  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
136  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
137  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is usually easier to
138  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
139  represents:  represents:
140    .sp
141    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
142    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
143    \\e        escape (hex 1B)    \ee        escape (hex 1B)
144    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
145    \\n        newline (hex 0A)    \en        newline (hex 0A)
146    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
147    \\t        tab (hex 09)    \et        tab (hex 09)
148    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
149    \\xhh      character with hex code hh    \exhh      character with hex code hh
150    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
151    .sp
152  The precise effect of \\cx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
153  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.
154  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
155  7B.  7B.
156    .P
157  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
158  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{
159  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
160  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
161  other than hexadecimal digits appear between \\x{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
162  terminating }, this form of escape is not recognized. Instead, the initial  and }, or if there is no terminating }, this form of escape is not recognized.
163  \\x will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
164  digits, giving a byte whose value is zero.  with no following digits, giving a character whose value is zero.
165    .P
166  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
167  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
168  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
169    .P
170  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
171  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
172  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
173  (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
174  character that follows is itself an octal digit.  follows is itself an octal digit.
175    .P
176  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.
177  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
178  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
179  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
180  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
181  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
182    .\" </a>
183    later,
184    .\"
185    following the discussion of
186    .\" HTML <a href="#subpattern">
187    .\" </a>
188    parenthesized subpatterns.
189    .\"
190    .P
191  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
192  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
193  digits following the backslash, and generates a single byte from the least  digits following the backslash, ane uses them to generate a data character. Any
194  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
195  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
196    to \e777 are permitted. For example:
197    \\040   is another way of writing a space  .sp
198    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
199    .\" JOIN
200      \e40    is the same, provided there are fewer than 40
201              previous capturing subpatterns              previous capturing subpatterns
202    \\7     is always a back reference    \e7     is always a back reference
203    \\11    might be a back reference, or another way of  .\" JOIN
204      \e11    might be a back reference, or another way of
205              writing a tab              writing a tab
206    \\011   is always a tab    \e011   is always a tab
207    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
208    \\113   might be a back reference, otherwise the  .\" JOIN
209      \e113   might be a back reference, otherwise the
210              character with octal code 113              character with octal code 113
211    \\377   might be a back reference, otherwise  .\" JOIN
212      \e377   might be a back reference, otherwise
213              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
214    \\81    is either a back reference, or a binary zero  .\" JOIN
215      \e81    is either a back reference, or a binary zero
216              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
217    .sp
218  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
219  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
220    .P
221  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
222  (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
223  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
224  backspace character (hex 08). Outside a character class it has a different  sequence \eX is interpreted as the character "X". Outside a character class,
225  meaning (see below).  these sequences have different meanings
226    .\" HTML <a href="#uniextseq">
227  The third use of backslash is for specifying generic character types:  .\" </a>
228    (see below).
229    \\d     any decimal digit  .\"
230    \\D     any character that is not a decimal digit  .
231    \\s     any whitespace character  .
232    \\S     any character that is not a whitespace character  .SS "Generic character types"
233    \\w     any "word" character  .rs
234    \\W     any "non-word" character  .sp
235    The third use of backslash is for specifying generic character types. The
236    following are always recognized:
237    .sp
238      \ed     any decimal digit
239      \eD     any character that is not a decimal digit
240      \es     any whitespace character
241      \eS     any character that is not a whitespace character
242      \ew     any "word" character
243      \eW     any "non-word" character
244    .sp
245  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
246  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.
247    .P
248  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
249  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
250    matching point is at the end of the subject string, all of them fail, since
251  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
252  This makes it different from the the POSIX "space" class. The \\s characters  .P
253  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).
254    This makes it different from the the POSIX "space" class. The \es characters
255  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
256  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
257  digits is controlled by PCRE's character tables, and may vary if locale-  does.)
258  specific matching is taking place (see  .P
259    A "word" character is an underscore or any character less than 256 that is a
260    letter or digit. The definition of letters and digits is controlled by PCRE's
261    low-valued character tables, and may vary if locale-specific matching is taking
262    place (see
263  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
264  .\" </a>  .\" </a>
265  "Locale support"  "Locale support"
266  .\"  .\"
267  in the  in the
268  .\" HREF  .\" HREF
269  \fBpcreapi\fR  \fBpcreapi\fP
270  .\"  .\"
271  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in the "fr_FR" (French) locale, some character codes
272  than 128 are used for accented letters, and these are matched by \\w.  greater than 128 are used for accented letters, and these are matched by \ew.
273    .P
274  These character type sequences can appear both inside and outside character  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
275  classes. They each match one character of the appropriate type. If the current  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
276  matching point is at the end of the subject string, all of them fail, since  character property support is available. The use of locales with Unicode is
277  there is no character to match.  discouraged.
278    .
279    .
280    .\" HTML <a name="uniextseq"></a>
281    .SS Unicode character properties
282    .rs
283    .sp
284    When PCRE is built with Unicode character property support, three additional
285    escape sequences to match character properties are available when UTF-8 mode
286    is selected. They are:
287    .sp
288      \ep{\fIxx\fP}   a character with the \fIxx\fP property
289      \eP{\fIxx\fP}   a character without the \fIxx\fP property
290      \eX       an extended Unicode sequence
291    .sp
292    The property names represented by \fIxx\fP above are limited to the Unicode
293    script names, the general category properties, and "Any", which matches any
294    character (including newline). Other properties such as "InMusicalSymbols" are
295    not currently supported by PCRE. Note that \eP{Any} does not match any
296    characters, so always causes a match failure.
297    .P
298    Sets of Unicode characters are defined as belonging to certain scripts. A
299    character from one of these sets can be matched using a script name. For
300    example:
301    .sp
302      \ep{Greek}
303      \eP{Han}
304    .sp
305    Those that are not part of an identified script are lumped together as
306    "Common". The current list of scripts is:
307    .P
308    Arabic,
309    Armenian,
310    Bengali,
311    Bopomofo,
312    Braille,
313    Buginese,
314    Buhid,
315    Canadian_Aboriginal,
316    Cherokee,
317    Common,
318    Coptic,
319    Cypriot,
320    Cyrillic,
321    Deseret,
322    Devanagari,
323    Ethiopic,
324    Georgian,
325    Glagolitic,
326    Gothic,
327    Greek,
328    Gujarati,
329    Gurmukhi,
330    Han,
331    Hangul,
332    Hanunoo,
333    Hebrew,
334    Hiragana,
335    Inherited,
336    Kannada,
337    Katakana,
338    Kharoshthi,
339    Khmer,
340    Lao,
341    Latin,
342    Limbu,
343    Linear_B,
344    Malayalam,
345    Mongolian,
346    Myanmar,
347    New_Tai_Lue,
348    Ogham,
349    Old_Italic,
350    Old_Persian,
351    Oriya,
352    Osmanya,
353    Runic,
354    Shavian,
355    Sinhala,
356    Syloti_Nagri,
357    Syriac,
358    Tagalog,
359    Tagbanwa,
360    Tai_Le,
361    Tamil,
362    Telugu,
363    Thaana,
364    Thai,
365    Tibetan,
366    Tifinagh,
367    Ugaritic,
368    Yi.
369    .P
370    Each character has exactly one general category property, specified by a
371    two-letter abbreviation. For compatibility with Perl, negation can be specified
372    by including a circumflex between the opening brace and the property name. For
373    example, \ep{^Lu} is the same as \eP{Lu}.
374    .P
375    If only one letter is specified with \ep or \eP, it includes all the general
376    category properties that start with that letter. In this case, in the absence
377    of negation, the curly brackets in the escape sequence are optional; these two
378    examples have the same effect:
379    .sp
380      \ep{L}
381      \epL
382    .sp
383    The following general category property codes are supported:
384    .sp
385      C     Other
386      Cc    Control
387      Cf    Format
388      Cn    Unassigned
389      Co    Private use
390      Cs    Surrogate
391    .sp
392      L     Letter
393      Ll    Lower case letter
394      Lm    Modifier letter
395      Lo    Other letter
396      Lt    Title case letter
397      Lu    Upper case letter
398    .sp
399      M     Mark
400      Mc    Spacing mark
401      Me    Enclosing mark
402      Mn    Non-spacing mark
403    .sp
404      N     Number
405      Nd    Decimal number
406      Nl    Letter number
407      No    Other number
408    .sp
409      P     Punctuation
410      Pc    Connector punctuation
411      Pd    Dash punctuation
412      Pe    Close punctuation
413      Pf    Final punctuation
414      Pi    Initial punctuation
415      Po    Other punctuation
416      Ps    Open punctuation
417    .sp
418      S     Symbol
419      Sc    Currency symbol
420      Sk    Modifier symbol
421      Sm    Mathematical symbol
422      So    Other symbol
423    .sp
424      Z     Separator
425      Zl    Line separator
426      Zp    Paragraph separator
427      Zs    Space separator
428    .sp
429    The special property L& is also supported: it matches a character that has
430    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
431    a modifier or "other".
432    .P
433    The long synonyms for these properties that Perl supports (such as \ep{Letter})
434    are not supported by PCRE, nor is it permitted to prefix any of these
435    properties with "Is".
436    .P
437    No character that is in the Unicode table has the Cn (unassigned) property.
438    Instead, this property is assumed for any code point that is not in the
439    Unicode table.
440    .P
441    Specifying caseless matching does not affect these escape sequences. For
442    example, \ep{Lu} always matches only upper case letters.
443    .P
444    The \eX escape matches any number of Unicode characters that form an extended
445    Unicode sequence. \eX is equivalent to
446    .sp
447      (?>\ePM\epM*)
448    .sp
449    That is, it matches a character without the "mark" property, followed by zero
450    or more characters with the "mark" property, and treats the sequence as an
451    atomic group
452    .\" HTML <a href="#atomicgroup">
453    .\" </a>
454    (see below).
455    .\"
456    Characters with the "mark" property are typically accents that affect the
457    preceding character.
458    .P
459    Matching characters by Unicode property is not fast, because PCRE has to search
460    a structure that contains data for over fifteen thousand characters. That is
461    why the traditional escape sequences such as \ed and \ew do not use Unicode
462    properties in PCRE.
463    .
464    .
465    .\" HTML <a name="smallassertions"></a>
466    .SS "Simple assertions"
467    .rs
468    .sp
469  The fourth use of backslash is for certain simple assertions. An assertion  The fourth use of backslash is for certain simple assertions. An assertion
470  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,
471  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
472  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
473  assertions are  .\" HTML <a href="#bigassertions">
474    .\" </a>
475    \\b     matches at a word boundary  below.
476    \\B     matches when not at a word boundary  .\"
477    \\A     matches at start of subject  The backslashed assertions are:
478    \\Z     matches at end of subject or before newline at end  .sp
479    \\z     matches at end of subject    \eb     matches at a word boundary
480    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
481      \eA     matches at start of subject
482  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at end of subject or before newline at end
483      \ez     matches at end of subject
484      \eG     matches at first matching position in subject
485    .sp
486    These assertions may not appear in character classes (but note that \eb has a
487  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
488    .P
489  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
490  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
491  \\w and the other matches \\W), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
492  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
493    .P
494  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
495  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
496  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
497  multiline mode.  independent of multiline mode. These three assertions are not affected by the
498    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
499  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
500  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
501  matching is to start at a point other than the beginning of the subject, \\A  at a point other than the beginning of the subject, \eA can never match. The
502  can never match. The difference between \\Z and \\z is that \\Z matches before  difference between \eZ and \ez is that \eZ matches before a newline at the end
503  a newline that is the last character of the string as well as at the end of the  of the string as well as at the very end, whereas \ez matches only at the end.
504  string, whereas \\z matches only at the end.  .P
505    The \eG assertion is true only when the current matching position is at the
506  The \\G assertion is true only when the current matching position is at the  start point of the match, as specified by the \fIstartoffset\fP argument of
507  start point of the match, as specified by the \fIstartoffset\fR argument of  \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
508  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
 non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  
509  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
510  implementation where \\G can be useful.  implementation where \eG can be useful.
511    .P
512  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
513  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
514  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
515  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
516  reproduce this behaviour.  reproduce this behaviour.
517    .P
518  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
519  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
520  regular expression.  regular expression.
521    .
522  .SH CIRCUMFLEX AND DOLLAR  .
523    .SH "CIRCUMFLEX AND DOLLAR"
524  .rs  .rs
525  .sp  .sp
526  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
527  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
528  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
529  \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
530  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
531  meaning (see below).  meaning
532    .\" HTML <a href="#characterclass">
533    .\" </a>
534    (see below).
535    .\"
536    .P
537  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
538  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
539  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 541  possible alternatives start with a circu
541  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
542  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
543  to be anchored.)  to be anchored.)
544    .P
545  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
546  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
547  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
548  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
549  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
550  Dollar has no special meaning in a character class.  character class.
551    .P
552  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
553  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
554  does not affect the \\Z assertion.  does not affect the \eZ assertion.
555    .P
556  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
557  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
558  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
559  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
560  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
561  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
562  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
563  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
564  \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
565  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
566    patterns that are anchored in single line mode because all branches start with
567  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
568    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
569    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
570    .P
571    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
572  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
573  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
574    .
575  .SH FULL STOP (PERIOD, DOT)  .
576    .SH "FULL STOP (PERIOD, DOT)"
577  .rs  .rs
578  .sp  .sp
579  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
580  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
581  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. When
582  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  a line ending is defined as a single character (CR or LF), dot never matches
583  dots match newlines as well. The handling of dot is entirely independent of the  that character; when the two-character sequence CRLF is used, dot does not
584  handling of circumflex and dollar, the only relationship being that they both  match CR if it is immediately followed by LF, but otherwise it matches all
585  involve newline characters. Dot has no special meaning in a character class.  characters (including isolated CRs and LFs).
586    .P
587  .SH MATCHING A SINGLE BYTE  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
588    option is set, a dot matches any one character, without exception. If newline
589    is defined as the two-character sequence CRLF, it takes two dots to match it.
590    .P
591    The handling of dot is entirely independent of the handling of circumflex and
592    dollar, the only relationship being that they both involve newlines. Dot has no
593    special meaning in a character class.
594    .
595    .
596    .SH "MATCHING A SINGLE BYTE"
597  .rs  .rs
598  .sp  .sp
599  Outside a character class, the escape sequence \\C matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
600  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The
601  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  feature is provided in Perl in order to match individual bytes in UTF-8 mode.
602  Because it breaks up UTF-8 characters into individual bytes, what remains in  Because it breaks up UTF-8 characters into individual bytes, what remains in
603  the string may be a malformed UTF-8 string. For this reason it is best avoided.  the string may be a malformed UTF-8 string. For this reason, the \eC escape
604    sequence is best avoided.
605  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  .P
606  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  PCRE does not allow \eC to appear in lookbehind assertions
607    .\" HTML <a href="#lookbehind">
608  .SH SQUARE BRACKETS  .\" </a>
609    (described below),
610    .\"
611    because in UTF-8 mode this would make it impossible to calculate the length of
612    the lookbehind.
613    .
614    .
615    .\" HTML <a name="characterclass"></a>
616    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
617  .rs  .rs
618  .sp  .sp
619  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
# Line 348  square bracket. A closing square bracket Line 621  square bracket. A closing square bracket
621  closing square bracket is required as a member of the class, it should be the  closing square bracket is required as a member of the class, it should be the
622  first data character in the class (after an initial circumflex, if present) or  first data character in the class (after an initial circumflex, if present) or
623  escaped with a backslash.  escaped with a backslash.
624    .P
625  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
626  character may occupy more than one byte. A matched character must be in the set  character may occupy more than one byte. A matched character must be in the set
627  of characters defined by the class, unless the first character in the class  of characters defined by the class, unless the first character in the class
# Line 356  definition is a circumflex, in which cas Line 629  definition is a circumflex, in which cas
629  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
630  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
631  backslash.  backslash.
632    .P
633  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
634  [^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
635  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
636  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
637  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
638  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
639    string.
640    .P
641  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
642  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.
643    .P
644  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
645  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
646  "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
647  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
648  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
649    always possible. For characters with higher values, the concept of case is
650  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.
651  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching for characters 128 and above, you must
652  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
653    UTF-8 support.
654    .P
655    Characters that might indicate line breaks (CR and LF) are never treated in any
656    special way when matching character classes, whatever line-ending sequence is
657    in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is
658    used. A class such as [^a] always matches one of these characters.
659    .P
660  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
661  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,
662  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
663  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
664  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.
665    .P
666  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
667  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
668  ("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
669  "-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
670  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
671  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
672  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
673    .P
674  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
675  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
676  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
677  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
678    .P
679  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
680  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
681  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
682  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E
683    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
684  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
685  character class, and add the characters that they match to the class. For  property support.
686  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
687    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
688    in a character class, and add the characters that they match to the class. For
689    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
690  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
691  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
692  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
693    .P
694  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
695  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
696  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
697    introducing a POSIX class name - see the next section), and the terminating
698  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
699    does no harm.
700    .
701    .
702    .SH "POSIX CHARACTER CLASSES"
703  .rs  .rs
704  .sp  .sp
705  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
706  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
707  this notation. For example,  this notation. For example,
708    .sp
709    [01[:alpha:]%]    [01[:alpha:]%]
710    .sp
711  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
712  are  are
713    .sp
714    alnum    letters and digits    alnum    letters and digits
715    alpha    letters    alpha    letters
716    ascii    character codes 0 - 127    ascii    character codes 0 - 127
717    blank    space or tab only    blank    space or tab only
718    cntrl    control characters    cntrl    control characters
719    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
720    graph    printing characters, excluding space    graph    printing characters, excluding space
721    lower    lower case letters    lower    lower case letters
722    print    printing characters, including space    print    printing characters, including space
723    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
724    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
725    upper    upper case letters    upper    upper case letters
726    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
727    xdigit   hexadecimal digits    xdigit   hexadecimal digits
728    .sp
729  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
730  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
731  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
732  compatibility).  compatibility).
733    .P
734  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
735  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
736  after the colon. For example,  after the colon. For example,
737    .sp
738    [12[:^digit:]]    [12[:^digit:]]
739    .sp
740  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
741  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
742  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
743    .P
744  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
745  the POSIX character classes.  the POSIX character classes.
746    .
747  .SH VERTICAL BAR  .
748    .SH "VERTICAL BAR"
749  .rs  .rs
750  .sp  .sp
751  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
752  the pattern  the pattern
753    .sp
754    gilbert|sullivan    gilbert|sullivan
755    .sp
756  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
757  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
758  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
759  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
760  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
761  pattern as well as the alternative in the subpattern.  .\" </a>
762    (defined below),
763  .SH INTERNAL OPTION SETTING  .\"
764    "succeeds" means matching the rest of the main pattern as well as the
765    alternative in the subpattern.
766    .
767    .
768    .SH "INTERNAL OPTION SETTING"
769  .rs  .rs
770  .sp  .sp
771  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
772  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
773  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
774    .sp
775    i  for PCRE_CASELESS    i  for PCRE_CASELESS
776    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
777    s  for PCRE_DOTALL    s  for PCRE_DOTALL
778    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
779    .sp
780  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
781  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
782  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
783  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
784  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
785  unset.  unset.
786    .P
787  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
788  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
789  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
790  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
791  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
792    .P
793  An option change within a subpattern affects only that part of the current  An option change within a subpattern affects only that part of the current
794  pattern that follows it, so  pattern that follows it, so
795    .sp
796    (a(?i)b)c    (a(?i)b)c
797    .sp
798  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).
799  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
800  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
801  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
802    .sp
803    (a(?i)b|c)    (a(?i)b|c)
804    .sp
805  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
806  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
807  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
808  behaviour otherwise.  behaviour otherwise.
809    .P
810  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
811  same way as the Perl-compatible options by using the characters U and X  changed in the same way as the Perl-compatible options by using the characters
812  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
813  earlier in the pattern than any of the additional features it turns on, even  .
814  when it is at top level. It is best put at the start.  .
815    .\" HTML <a name="subpattern"></a>
816  .SH SUBPATTERNS  .SH SUBPATTERNS
817  .rs  .rs
818  .sp  .sp
819  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
820  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
821    .sp
822  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
823    .sp
824    cat(aract|erpillar|)    cat(aract|erpillar|)
825    .sp
826  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
827  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or the empty string.
828    .sp
829  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
830  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
831  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
832  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
833  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
834    .P
835  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
836    .sp
837    the ((red|white) (king|queen))    the ((red|white) (king|queen))
838    .sp
839  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,
840  2, and 3, respectively.  2, and 3, respectively.
841    .P
842  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
843  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
844  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
845  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
846  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
847  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
848    .sp
849    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
850    .sp
851  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
852  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535, and the maximum depth
853  of nesting of all subpatterns, both capturing and non-capturing, is 200.  of nesting of all subpatterns, both capturing and non-capturing, is 200.
854    .P
855  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
856  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
857  the ":". Thus the two patterns  the ":". Thus the two patterns
858    .sp
859    (?i:saturday|sunday)    (?i:saturday|sunday)
860    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
861    .sp
862  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
863  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
864  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
865  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
866    .
867  .SH NAMED SUBPATTERNS  .
868    .SH "NAMED SUBPATTERNS"
869  .rs  .rs
870  .sp  .sp
871  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
872  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
873  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
874  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns, something that Perl does
875  not provide. The Python syntax (?P<name>...) is used. Names consist of  not provide. The Python syntax (?P<name>...) is used. References to capturing
876  alphanumeric characters and underscores, and must be unique within a pattern.  parentheses from other parts of the pattern, such as
877    .\" HTML <a href="#backreferences">
878  Named capturing parentheses are still allocated numbers as well as names. The  .\" </a>
879  PCRE API provides function calls for extracting the name-to-number translation  backreferences,
880  table from a compiled pattern. For further details see the  .\"
881    .\" HTML <a href="#recursion">
882    .\" </a>
883    recursion,
884    .\"
885    and
886    .\" HTML <a href="#conditions">
887    .\" </a>
888    conditions,
889    .\"
890    can be made by name as well as by number.
891    .P
892    Names consist of up to 32 alphanumeric characters and underscores. Named
893    capturing parentheses are still allocated numbers as well as names. The PCRE
894    API provides function calls for extracting the name-to-number translation table
895    from a compiled pattern. There is also a convenience function for extracting a
896    captured substring by name.
897    .P
898    By default, a name must be unique within a pattern, but it is possible to relax
899    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
900    be useful for patterns where only one instance of the named parentheses can
901    match. Suppose you want to match the name of a weekday, either as a 3-letter
902    abbreviation or as the full name, and in both cases you want to extract the
903    abbreviation. This pattern (ignoring the line breaks) does the job:
904    .sp
905      (?P<DN>Mon|Fri|Sun)(?:day)?|
906      (?P<DN>Tue)(?:sday)?|
907      (?P<DN>Wed)(?:nesday)?|
908      (?P<DN>Thu)(?:rsday)?|
909      (?P<DN>Sat)(?:urday)?
910    .sp
911    There are five capturing substrings, but only one is ever set after a match.
912    The convenience function for extracting the data by name returns the substring
913    for the first, and in this example, the only, subpattern of that name that
914    matched. This saves searching to find which numbered subpattern it was. If you
915    make a reference to a non-unique named subpattern from elsewhere in the
916    pattern, the one that corresponds to the lowest number is used. For further
917    details of the interfaces for handling named subpatterns, see the
918  .\" HREF  .\" HREF
919  \fBpcreapi\fR  \fBpcreapi\fP
920  .\"  .\"
921  documentation.  documentation.
922    .
923    .
924  .SH REPETITION  .SH REPETITION
925  .rs  .rs
926  .sp  .sp
927  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
928  items:  items:
929    .sp
930    a literal data character    a literal data character
931    the . metacharacter    the . metacharacter
932    the \\C escape sequence    the \eC escape sequence
933    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
934      an escape such as \ed that matches a single character
935    a character class    a character class
936    a back reference (see next section)    a back reference (see next section)
937    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
938    .sp
939  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
940  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
941  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
942  be less than or equal to the second. For example:  be less than or equal to the second. For example:
943    .sp
944    z{2,4}    z{2,4}
945    .sp
946  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
947  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
948  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
949  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
950    .sp
951    [aeiou]{3,}    [aeiou]{3,}
952    .sp
953  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
954    .sp
955    \\d{8}    \ed{8}
956    .sp
957  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
958  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
959  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
960  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
961    .P
962  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
963  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
964  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
965    support is available, \eX{3} matches three Unicode extended sequences, each of
966    which may be several bytes long (and they may be of different lengths).
967    .P
968  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
969  previous item and the quantifier were not present.  previous item and the quantifier were not present.
970    .P
971  For convenience (and historical compatibility) the three most common  For convenience (and historical compatibility) the three most common
972  quantifiers have single-character abbreviations:  quantifiers have single-character abbreviations:
973    .sp
974    *    is equivalent to {0,}    *    is equivalent to {0,}
975    +    is equivalent to {1,}    +    is equivalent to {1,}
976    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
977    .sp
978  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
979  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:
980    .sp
981    (a?)*    (a?)*
982    .sp
983  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
984  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
985  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
986  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
987    .P
988  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
989  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
990  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
991  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 */
992  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
993  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
994    .sp
995    /\\*.*\\*/    /\e*.*\e*/
996    .sp
997  to the string  to the string
998    .sp
999    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1000    .sp
1001  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 .*
1002  item.  item.
1003    .P
1004  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
1005  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1006  pattern  pattern
1007    .sp
1008    /\\*.*?\\*/    /\e*.*?\e*/
1009    .sp
1010  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
1011  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1012  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
1013  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
1014    .sp
1015    \\d??\\d    \ed??\ed
1016    .sp
1017  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
1018  way the rest of the pattern matches.  way the rest of the pattern matches.
1019    .P
1020  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
1021  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
1022  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
1023  default behaviour.  default behaviour.
1024    .P
1025  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1026  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
1027  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1028    .P
1029  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
1030  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
1031  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1032  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
1033  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
1034  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1035    .P
1036  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
1037  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1038  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1039    .P
1040  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1041  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1042  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail, and a later one
1043  succeed. Consider, for example:  succeed. Consider, for example:
1044    .sp
1045    (.*)abc\\1    (.*)abc\e1
1046    .sp
1047  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
1048  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1049    .P
1050  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1051  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1052    .sp
1053    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1054    .sp
1055  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1056  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1057  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1058  example, after  example, after
1059    .sp
1060    /(a|(b))+/    /(a|(b))+/
1061    .sp
1062  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1063    .
1064  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1065    .\" HTML <a name="atomicgroup"></a>
1066    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1067  .rs  .rs
1068  .sp  .sp
1069  With both maximizing and minimizing repetition, failure of what follows  With both maximizing and minimizing repetition, failure of what follows
# Line 735  number of repeats allows the rest of the Line 1072  number of repeats allows the rest of the
1072  useful to prevent this, either to change the nature of the match, or to cause  useful to prevent this, either to change the nature of the match, or to cause
1073  it fail earlier than it otherwise might, when the author of the pattern knows  it fail earlier than it otherwise might, when the author of the pattern knows
1074  there is no point in carrying on.  there is no point in carrying on.
1075    .P
1076  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
1077    .sp
1078    123456bar    123456bar
1079    .sp
1080  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
1081  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+
1082  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"
1083  (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
1084  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.
1085    .P
1086  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher would give up
1087  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
1088  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1089    .sp
1090    (?>\\d+)foo    (?>\ed+)foo
1091    .sp
1092  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
1093  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
1094  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1095  normal.  normal.
1096    .P
1097  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
1098  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1099  the current point in the subject string.  the current point in the subject string.
1100    .P
1101  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1102  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
1103  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
1104  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,
1105  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1106    .P
1107  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1108  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1109  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
1110  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1111  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1112  previous example can be rewritten as  previous example can be rewritten as
1113    .sp
1114    \\d++bar    \ed++foo
1115    .sp
1116  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1117  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
1118  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning or processing of a
1119  possessive quantifier and the equivalent atomic group.  possessive quantifier and the equivalent atomic group.
1120    .P
1121  The possessive quantifier syntax is an extension to the Perl syntax. It  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey
1122  originates in Sun's Java package.  Friedl originated the idea (and the name) in the first edition of his book.
1123    Mike McCloskey liked it, so implemented it when he built Sun's Java package,
1124    and PCRE copied it from there.
1125    .P
1126  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
1127  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
1128  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
1129  pattern  pattern
1130    .sp
1131    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1132    .sp
1133  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
1134  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1135  quickly. However, if it is applied to  quickly. However, if it is applied to
1136    .sp
1137    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1138    .sp
1139  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
1140  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
1141  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
1142  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
1143  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
1144  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
1145  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1146    an atomic group, like this:
1147    ((?>\\D+)|<\\d+>)*[!?]  .sp
1148      ((?>\eD+)|<\ed+>)*[!?]
1149    .sp
1150  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1151    .
1152  .SH BACK REFERENCES  .
1153    .\" HTML <a name="backreferences"></a>
1154    .SH "BACK REFERENCES"
1155  .rs  .rs
1156  .sp  .sp
1157  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
1158  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1159  (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
1160  previous capturing left parentheses.  previous capturing left parentheses.
1161    .P
1162  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
1163  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
1164  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
1165  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
1166  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
1167  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1168    in an earlier iteration.
1169    .P
1170    It is not possible to have a numerical "forward back reference" to subpattern
1171    whose number is 10 or more. However, a back reference to any subpattern is
1172    possible using named parentheses (see below). See also the subsection entitled
1173    "Non-printing characters"
1174    .\" HTML <a href="#digitsafterbackslash">
1175    .\" </a>
1176    above
1177    .\"
1178    for further details of the handling of digits following a backslash.
1179    .P
1180  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1181  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1182  itself (see  itself (see
# Line 832  itself (see Line 1185  itself (see
1185  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1186  .\"  .\"
1187  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1188    .sp
1189    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1190    .sp
1191  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1192  "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
1193  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1194    .sp
1195    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1196    .sp
1197  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
1198  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1199    .P
1200  Back references to named subpatterns use the Python syntax (?P=name). We could  Back references to named subpatterns use the Python syntax (?P=name). We could
1201  rewrite the above example as follows:  rewrite the above example as follows:
1202    .sp
1203    (?<p1>(?i)rah)\\s+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1204    .sp
1205    A subpattern that is referenced by name may appear in the pattern before or
1206    after the reference.
1207    .P
1208  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
1209  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1210  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1211    .sp
1212    (a|(bc))\\2    (a|(bc))\e2
1213    .sp
1214  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". Because there may be
1215  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1216  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1217  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1218  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1219  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1220    .\" HTML <a href="#comments">
1221    .\" </a>
1222    "Comments"
1223    .\"
1224    below) can be used.
1225    .P
1226  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
1227  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.
1228  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1229  example, the pattern  example, the pattern
1230    .sp
1231    (a|b\\1)+    (a|b\e1)+
1232    .sp
1233  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
1234  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1235  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
1236  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
1237  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
1238  minimum of zero.  minimum of zero.
1239    .
1240    .
1241    .\" HTML <a name="bigassertions"></a>
1242  .SH ASSERTIONS  .SH ASSERTIONS
1243  .rs  .rs
1244  .sp  .sp
1245  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
1246  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1247  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
1248    .\" HTML <a href="#smallassertions">
1249    .\" </a>
1250    above.
1251    .\"
1252    .P
1253  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1254  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
1255  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1256    except that it does not cause the current matching position to be changed.
1257  An assertion subpattern is matched in the normal way, except that it does not  .P
1258  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1259  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
1260    of assertion contains capturing subpatterns within it, these are counted for
1261    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1262    However, substring capturing is carried out only for positive assertions,
1263    because it does not make sense for negative assertions.
1264    .
1265    .
1266    .SS "Lookahead assertions"
1267    .rs
1268    .sp
1269    Lookahead assertions start with (?= for positive assertions and (?! for
1270    negative assertions. For example,
1271    .sp
1272      \ew+(?=;)
1273    .sp
1274  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
1275  the match, and  the match, and
1276    .sp
1277    foo(?!bar)    foo(?!bar)
1278    .sp
1279  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
1280  apparently similar pattern  apparently similar pattern
1281    .sp
1282    (?!foo)bar    (?!foo)bar
1283    .sp
1284  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
1285  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1286  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1287  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1288    .P
1289  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
1290  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
1291  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.
1292    .
1293    .
1294    .\" HTML <a name="lookbehind"></a>
1295    .SS "Lookbehind assertions"
1296    .rs
1297    .sp
1298  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1299  negative assertions. For example,  negative assertions. For example,
1300    .sp
1301    (?<!foo)bar    (?<!foo)bar
1302    .sp
1303  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
1304  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
1305  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
1306  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1307    .sp
1308    (?<=bullock|donkey)    (?<=bullock|donkey)
1309    .sp
1310  is permitted, but  is permitted, but
1311    .sp
1312    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1313    .sp
1314  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1315  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
1316  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1317  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1318    .sp
1319    (?<=ab(c|de))    (?<=ab(c|de))
1320    .sp
1321  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
1322  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
1323    .sp
1324    (?<=abc|abde)    (?<=abc|abde)
1325    .sp
1326  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1327  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed width and then try to
1328  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1329  match is deemed to fail.  match is deemed to fail.
1330    .P
1331  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)
1332  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1333  the length of the lookbehind.  the length of the lookbehind. The \eX escape, which can match different numbers
1334    of bytes, is also not permitted.
1335    .P
1336  Atomic groups can be used in conjunction with lookbehind assertions to specify  Atomic groups can be used in conjunction with lookbehind assertions to specify
1337  efficient matching at the end of the subject string. Consider a simple pattern  efficient matching at the end of the subject string. Consider a simple pattern
1338  such as  such as
1339    .sp
1340    abcd$    abcd$
1341    .sp
1342  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
1343  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
1344  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
1345    .sp
1346    ^.*abcd$    ^.*abcd$
1347    .sp
1348  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
1349  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,
1350  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"
1351  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,
1352  if the pattern is written as  if the pattern is written as
1353    .sp
1354    ^(?>.*)(?<=abcd)    ^(?>.*)(?<=abcd)
1355    .sp
1356  or, equivalently,  or, equivalently, using the possessive quantifier syntax,
1357    .sp
1358    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1359    .sp
1360  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
1361  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
1362  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1363  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1364    .
1365    .
1366    .SS "Using multiple assertions"
1367    .rs
1368    .sp
1369  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1370    .sp
1371    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1372    .sp
1373  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
1374  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1375  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
1376  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".
1377  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
1378  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
1379  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1380    .sp
1381    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1382    .sp
1383  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1384  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
1385  preceding three characters are not "999".  preceding three characters are not "999".
1386    .P
1387  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1388    .sp
1389    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1390    .sp
1391  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
1392  preceded by "foo", while  preceded by "foo", while
1393    .sp
1394    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1395    .sp
1396  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
1397  characters that are not "999".  characters that are not "999".
1398    .
1399  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1400  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1401  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  
1402  .rs  .rs
1403  .sp  .sp
1404  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1405  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1406  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1407  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1408    .sp
1409    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1410    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1411    .sp
1412  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1413  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
1414  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1415    .P
1416  There are three kinds of condition. If the text between the parentheses  There are three kinds of condition. If the text between the parentheses
1417  consists of a sequence of digits, the condition is satisfied if the capturing  consists of a sequence of digits, or a sequence of alphanumeric characters and
1418  subpattern of that number has previously matched. The number must be greater  underscores, the condition is satisfied if the capturing subpattern of that
1419  than zero. Consider the following pattern, which contains non-significant white  number or name has previously matched. There is a possible ambiguity here,
1420  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  because subpattern names may consist entirely of digits. PCRE looks first for a
1421  it into three parts for ease of discussion:  named subpattern; if it cannot find one and the text consists entirely of
1422    digits, it looks for a subpattern of that number, which must be greater than
1423    ( \\( )?    [^()]+    (?(1) \\) )  zero. Using subpattern names that consist entirely of digits is not
1424    recommended.
1425    .P
1426    Consider the following pattern, which contains non-significant white space to
1427    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1428    three parts for ease of discussion:
1429    .sp
1430      ( \e( )?    [^()]+    (?(1) \e) )
1431    .sp
1432  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1433  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
1434  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 1046  or not. If they did, that is, if subject Line 1437  or not. If they did, that is, if subject
1437  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
1438  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1439  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1440  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a
1441    named subpattern gives this:
1442  If the condition is the string (R), it is satisfied if a recursive call to the  .sp
1443  pattern or subpattern has been made. At "top level", the condition is false.    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )
1444  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1445    If the condition is the string (R), and there is no subpattern with the name R,
1446    the condition is satisfied if a recursive call to the pattern or subpattern has
1447    been made. At "top level", the condition is false. This is a PCRE extension.
1448    Recursive patterns are described in the next section.
1449    .P
1450  If the condition is not a sequence of digits or (R), it must be an assertion.  If the condition is not a sequence of digits or (R), it must be an assertion.
1451  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1452  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1453  alternatives on the second line:  alternatives on the second line:
1454    .sp
1455    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1456    \\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} )
1457    .sp
1458  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1459  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
1460  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
1461  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1462  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
1463  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.
1464    .
1465    .
1466    .\" HTML <a name="comments"></a>
1467  .SH COMMENTS  .SH COMMENTS
1468  .rs  .rs
1469  .sp  .sp
1470  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
1471  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1472  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.
1473    .P
1474  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1475  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1476  character in the pattern.  next newline in the pattern.
1477    .
1478  .SH RECURSIVE PATTERNS  .
1479    .\" HTML <a name="recursion"></a>
1480    .SH "RECURSIVE PATTERNS"
1481  .rs  .rs
1482  .sp  .sp
1483  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1484  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1485  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
1486  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth. Perl provides a facility
1487  experimental facility that allows regular expressions to recurse (amongst other  that allows regular expressions to recurse (amongst other things). It does this
1488  things). It does this by interpolating Perl code in the expression at run time,  by interpolating Perl code in the expression at run time, and the code can
1489  and the code can refer to the expression itself. A Perl pattern to solve the  refer to the expression itself. A Perl pattern to solve the parentheses problem
1490  parentheses problem can be created like this:  can be created like this:
1491    .sp
1492    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1493    .sp
1494  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
1495  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears. Obviously, PCRE cannot support
1496  the interpolation of Perl code. Instead, it supports some special syntax for  the interpolation of Perl code. Instead, it supports some special syntax for
1497  recursion of the entire pattern, and also for individual subpattern recursion.  recursion of the entire pattern, and also for individual subpattern recursion.
1498    .P
1499  The special item that consists of (? followed by a number greater than zero and  The special item that consists of (? followed by a number greater than zero and
1500  a closing parenthesis is a recursive call of the subpattern of the given  a closing parenthesis is a recursive call of the subpattern of the given
1501  number, provided that it occurs inside that subpattern. (If not, it is a  number, provided that it occurs inside that subpattern. (If not, it is a
1502  "subroutine" call, which is described in the next section.) The special item  "subroutine" call, which is described in the next section.) The special item
1503  (?R) is a recursive call of the entire regular expression.  (?R) is a recursive call of the entire regular expression.
1504    .P
1505  For example, this PCRE pattern solves the nested parentheses problem (assume  A recursive subpattern call is always treated as an atomic group. That is, once
1506  the PCRE_EXTENDED option is set so that white space is ignored):  it has matched some of the subject string, it is never re-entered, even if
1507    it contains untried alternatives and there is a subsequent matching failure.
1508    \\( ( (?>[^()]+) | (?R) )* \\)  .P
1509    This PCRE pattern solves the nested parentheses problem (assume the
1510    PCRE_EXTENDED option is set so that white space is ignored):
1511    .sp
1512      \e( ( (?>[^()]+) | (?R) )* \e)
1513    .sp
1514  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1515  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
1516  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1517  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1518    .P
1519  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
1520  pattern, so instead you could use this:  pattern, so instead you could use this:
1521    .sp
1522    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1523    .sp
1524  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
1525  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern. In a larger pattern, keeping track of
1526  parenthesis numbers can be tricky. It may be more convenient to use named  parenthesis numbers can be tricky. It may be more convenient to use named
1527  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  parentheses instead. For this, PCRE uses (?P>name), which is an extension to
1528  the Python syntax that PCRE uses for named parentheses (Perl does not provide  the Python syntax that PCRE uses for named parentheses (Perl does not provide
1529  named parentheses). We could rewrite the above example as follows:  named parentheses). We could rewrite the above example as follows:
1530    .sp
1531    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
1532    .sp
1533  This particular example pattern contains nested unlimited repeats, and so the  This particular example pattern contains nested unlimited repeats, and so the
1534  use of atomic grouping for matching strings of non-parentheses is important  use of atomic grouping for matching strings of non-parentheses is important
1535  when applying the pattern to strings that do not match. For example, when this  when applying the pattern to strings that do not match. For example, when this
1536  pattern is applied to  pattern is applied to
1537    .sp
1538    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1539    .sp
1540  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1541  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
1542  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
1543  before failure can be reported.  before failure can be reported.
1544    .P
1545  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1546  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1547  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1548  below and the  the next section and the
1549  .\" HREF  .\" HREF
1550  \fBpcrecallout\fR  \fBpcrecallout\fP
1551  .\"  .\"
1552  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1553    .sp
1554    (ab(cd)ef)    (ab(cd)ef)
1555    .sp
1556  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1557  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1558    .sp
1559    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1560       ^                        ^       ^                        ^
1561       ^                        ^       ^                        ^
1562    .sp
1563  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1564  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1565  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
1566  using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1567  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1568    .P
1569  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.
1570  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1571  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1572  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1573    .sp
1574    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1575    .sp
1576  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
1577  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1578  is the actual recursive call.  is the actual recursive call.
1579    .
1580    .
1581  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1582  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1583  .rs  .rs
1584  .sp  .sp
1585  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
1586  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
1587  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. An earlier example pointed out that the
1588  pattern  pattern
1589    .sp
1590    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1591    .sp
1592  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1593  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1594    .sp
1595    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1596    .sp
1597  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
1598  strings. Such references must, however, follow the subpattern to which they  strings. Such references, if given numerically, must follow the subpattern to
1599  refer.  which they refer. However, named references can refer to later subpatterns.
1600    .P
1601    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1602    group. That is, once it has matched some of the subject string, it is never
1603    re-entered, even if it contains untried alternatives and there is a subsequent
1604    matching failure.
1605    .
1606    .
1607  .SH CALLOUTS  .SH CALLOUTS
1608  .rs  .rs
1609  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1611  Perl has a feature whereby using the seq
1611  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
1612  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1613  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1614    .P
1615  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
1616  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
1617  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.
1618  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1619    .P
1620  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1621  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
1622  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.
1623  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1624    .sp
1625    (?C1)\dabc(?C2)def    (?C1)\dabc(?C2)def
1626    .sp
1627  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
1628    automatically installed before each item in the pattern. They are all numbered
1629    255.
1630    .P
1631    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1632  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
1633  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
1634  \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
1635  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1636  function is given in the  description of the interface to the callout function is given in the
1637  .\" HREF  .\" HREF
1638  \fBpcrecallout\fR  \fBpcrecallout\fP
1639  .\"  .\"
1640  documentation.  documentation.
1641    .P
1642  .in 0  .in 0
1643  Last updated: 03 February 2003  Last updated: 06 June 2006
1644  .br  .br
1645  Copyright (c) 1997-2003 University of Cambridge.  Copyright (c) 1997-2006 University of Cambridge.

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