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revision 182 by ph10, Wed Jun 13 15:09:54 2007 UTC revision 211 by ph10, Thu Aug 9 09:52:43 2007 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. Perl's regular expressions are described in its own documentation, and
13    regular expressions in general are covered in a number of books, some of which
14    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15    published by O'Reilly, covers regular expressions in great detail. This
16    description of PCRE's regular expressions is intended as reference material.
17  .P  .P
18  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
19  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this, you must
# Line 164  Thus \ecz becomes hex 1A, but \ec{ becom Line 168  Thus \ecz becomes hex 1A, but \ec{ becom
168  After \ex, 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
169  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
170  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
171  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
172  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
173  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
174  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
175  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
176    there is no terminating }, this form of escape is not recognized. Instead, the
177    initial \ex will be interpreted as a basic hexadecimal escape, with no
178    following digits, giving a character whose value is zero.
179  .P  .P
180  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
181  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 240  meanings Line 247  meanings
247  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
248  .rs  .rs
249  .sp  .sp
250  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
251  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
252  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
253  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
254  .\" </a>  .\" </a>
255  later,  later,
# Line 376  Inside a character class, \eR matches th Line 383  Inside a character class, \eR matches th
383  .rs  .rs
384  .sp  .sp
385  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
386  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
387  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
388    characters whose codepoints are less than 256, but they do work in this mode.
389    The extra escape sequences are:
390  .sp  .sp
391    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
392    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 529  The special property L& is also supporte Line 538  The special property L& is also supporte
538  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
539  a modifier or "other".  a modifier or "other".
540  .P  .P
541    The Cs (Surrogate) property applies only to characters in the range U+D800 to
542    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
543    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
544    (see the discussion of PCRE_NO_UTF8_CHECK in the
545    .\" HREF
546    \fBpcreapi\fP
547    .\"
548    page).
549    .P
550  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
551  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
552  properties with "Is".  properties with "Is".
# Line 553  atomic group Line 571  atomic group
571  (see below).  (see below).
572  .\"  .\"
573  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
574  preceding character.  preceding character. None of them have codepoints less than 256, so in
575    non-UTF-8 mode \eX matches any one character.
576  .P  .P
577  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
578  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 1287  previous example can be rewritten as Line 1306  previous example can be rewritten as
1306  .sp  .sp
1307    \ed++foo    \ed++foo
1308  .sp  .sp
1309    Note that a possessive quantifier can be used with an entire group, for
1310    example:
1311    .sp
1312      (abc|xyz){2,3}+
1313    .sp
1314  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1315  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
1316  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1361  subpattern is possible using named paren Line 1385  subpattern is possible using named paren
1385  .P  .P
1386  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1387  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence, which is a feature introduced in
1388  Perl 5.10. This escape must be followed by a positive or a negative number,  Perl 5.10. This escape must be followed by an unsigned number or a negative
1389  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1390  .sp  .sp
1391    (ring), \e1    (ring), \e1
1392    (ring), \eg1    (ring), \eg1
1393    (ring), \eg{1}    (ring), \eg{1}
1394  .sp  .sp
1395  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1396  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1397  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1398    example:
1399  .sp  .sp
1400    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1401  .sp  .sp
# Line 1953  description of the interface to the call Line 1978  description of the interface to the call
1978  documentation.  documentation.
1979  .  .
1980  .  .
1981    .SH "BACTRACKING CONTROL"
1982    .rs
1983    .sp
1984    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1985    are described in the Perl documentation as "experimental and subject to change
1986    or removal in a future version of Perl". It goes on to say: "Their usage in
1987    production code should be noted to avoid problems during upgrades." The same
1988    remarks apply to the PCRE features described in this section.
1989    .P
1990    Since these verbs are specifically related to backtracking, they can be used
1991    only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
1992    backtracking algorithm. They cause an error if encountered by
1993    \fBpcre_dfa_exec()\fP.
1994    .P
1995    The new verbs make use of what was previously invalid syntax: an opening
1996    parenthesis followed by an asterisk. In Perl, they are generally of the form
1997    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
1998    form is just (*VERB). Any number of these verbs may occur in a pattern. There
1999    are two kinds:
2000    .
2001    .SS "Verbs that act immediately"
2002    .rs
2003    .sp
2004    The following verbs act as soon as they are encountered:
2005    .sp
2006       (*ACCEPT)
2007    .sp
2008    This verb causes the match to end successfully, skipping the remainder of the
2009    pattern. When inside a recursion, only the innermost pattern is ended
2010    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2011    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2012    captured. For example:
2013    .sp
2014      A(A|B(*ACCEPT)|C)D
2015    .sp
2016    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2017    captured.
2018    .sp
2019      (*FAIL) or (*F)
2020    .sp
2021    This verb causes the match to fail, forcing backtracking to occur. It is
2022    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2023    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2024    Perl features that are not present in PCRE. The nearest equivalent is the
2025    callout feature, as for example in this pattern:
2026    .sp
2027      a+(?C)(*FAIL)
2028    .sp
2029    A match with the string "aaaa" always fails, but the callout is taken before
2030    each backtrack happens (in this example, 10 times).
2031    .
2032    .SS "Verbs that act after backtracking"
2033    .rs
2034    .sp
2035    The following verbs do nothing when they are encountered. Matching continues
2036    with what follows, but if there is no subsequent match, a failure is forced.
2037    The verbs differ in exactly what kind of failure occurs.
2038    .sp
2039      (*COMMIT)
2040    .sp
2041    This verb causes the whole match to fail outright if the rest of the pattern
2042    does not match. Even if the pattern is unanchored, no further attempts to find
2043    a match by advancing the start point take place. Once (*COMMIT) has been
2044    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2045    starting point, or not at all. For example:
2046    .sp
2047      a+(*COMMIT)b
2048    .sp
2049    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2050    dynamic anchor, or "I've started, so I must finish."
2051    .sp
2052      (*PRUNE)
2053    .sp
2054    This verb causes the match to fail at the current position if the rest of the
2055    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2056    advance to the next starting character then happens. Backtracking can occur as
2057    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2058    if there is no match to the right, backtracking cannot cross (*PRUNE).
2059    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2060    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2061    be expressed in any other way.
2062    .sp
2063      (*SKIP)
2064    .sp
2065    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2066    "bumpalong" advance is not to the next character, but to the position in the
2067    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2068    was matched leading up to it cannot be part of a successful match. Consider:
2069    .sp
2070      a+(*SKIP)b
2071    .sp
2072    If the subject is "aaaac...", after the first match attempt fails (starting at
2073    the first character in the string), the starting point skips on to start the
2074    next attempt at "c". Note that a possessive quantifer does not have the same
2075    effect in this example; although it would suppress backtracking during the
2076    first match attempt, the second attempt would start at the second character
2077    instead of skipping on to "c".
2078    .sp
2079      (*THEN)
2080    .sp
2081    This verb causes a skip to the next alternation if the rest of the pattern does
2082    not match. That is, it cancels pending backtracking, but only within the
2083    current alternation. Its name comes from the observation that it can be used
2084    for a pattern-based if-then-else block:
2085    .sp
2086      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2087    .sp
2088    If the COND1 pattern matches, FOO is tried (and possibly further items after
2089    the end of the group if FOO succeeds); on failure the matcher skips to the
2090    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2091    is used outside of any alternation, it acts exactly like (*PRUNE).
2092    .
2093    .
2094  .SH "SEE ALSO"  .SH "SEE ALSO"
2095  .rs  .rs
2096  .sp  .sp
# Line 1973  Cambridge CB2 3QH, England. Line 2111  Cambridge CB2 3QH, England.
2111  .rs  .rs
2112  .sp  .sp
2113  .nf  .nf
2114  Last updated: 13 June 2007  Last updated: 09 August 2007
2115  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
2116  .fi  .fi

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