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-revision 167 by ph10,
Wed May  9 15:53:54 2007 UTC
+revision 210 by ph10,
Wed Aug  8 14:24:50 2007 UTC
 Line 4 
 PCRE - Perl-compatible regular expressio
  .SH "PCRE REGULAR EXPRESSION DETAILS"
  .rs
  .sp
- 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
- described below. Regular expressions are also described in the Perl
+ are described in detail below. There is a quick-reference syntax summary in the
- documentation and in a number of books, some of which have copious examples.
+ .\" HREF
- Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
+ \fBpcresyntax\fP
- regular expressions in great detail. This description of PCRE's regular
+ .\"
- expressions is intended as reference material.
+ page. Perl's regular expressions are described in its own documentation, and
+ regular expressions in general are covered in a number of books, some of which
+ have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
+ published by O'Reilly, covers regular expressions in great detail. This
+ description of PCRE's regular expressions is intended as reference material.
  .P
  The original operation of PCRE was on strings of one-byte characters. However,
  there is now also support for UTF-8 character strings. To use this, you must
-Line 30 
 The remainder of this document discusses
+Line 34 
 The remainder of this document discusses
  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
  From release 6.0, PCRE offers a second matching function,
  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
- Perl-compatible. The advantages and disadvantages of the alternative function,
+ Perl-compatible. Some of the features discussed below are not available when
- and how it differs from the normal function, are discussed in the
+ \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
+ alternative function, and how it differs from the normal function, are
+ discussed in the
  .\" HREF
  \fBpcrematching\fP
  .\"
-Line 238 
 meanings
+Line 244 
 meanings
  .SS "Absolute and relative back references"
  .rs
  .sp
- The sequence \eg followed by a positive or negative number, optionally enclosed
+ The sequence \eg followed by an unsigned or a negative number, optionally
- in braces, is an absolute or relative back reference. Back references are
+ enclosed in braces, is an absolute or relative back reference. A named back
- discussed
+ reference can be coded as \eg{name}. Back references are discussed
  .\" HTML <a href="#backreferences">
  .\" </a>
  later,
-Line 260 
 following are always recognized:
+Line 266 
 following are always recognized:
  .sp
    \ed     any decimal digit
    \eD     any character that is not a decimal digit
+   \eh     any horizontal whitespace character
+   \eH     any character that is not a horizontal whitespace character
    \es     any whitespace character
    \eS     any character that is not a whitespace character
+   \ev     any vertical whitespace character
+   \eV     any character that is not a vertical whitespace character
    \ew     any "word" character
    \eW     any "non-word" character
  .sp
-Line 275 
 there is no character to match.
+Line 285 
 there is no character to match.
  .P
  For compatibility with Perl, \es does not match the VT character (code 11).
  This makes it different from the the POSIX "space" class. The \es characters
- are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
+ are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
  included in a Perl script, \es may match the VT character. In PCRE, it never
- does.)
+ does.
+ .P
+ In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
+ \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
+ character property support is available. These sequences retain their original
+ meanings from before UTF-8 support was available, mainly for efficiency
+ reasons.
+ .P
+ The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
+ other sequences, these do match certain high-valued codepoints in UTF-8 mode.
+ The horizontal space characters are:
+ .sp
+   U+0009     Horizontal tab
+   U+0020     Space
+   U+00A0     Non-break space
+   U+1680     Ogham space mark
+   U+180E     Mongolian vowel separator
+   U+2000     En quad
+   U+2001     Em quad
+   U+2002     En space
+   U+2003     Em space
+   U+2004     Three-per-em space
+   U+2005     Four-per-em space
+   U+2006     Six-per-em space
+   U+2007     Figure space
+   U+2008     Punctuation space
+   U+2009     Thin space
+   U+200A     Hair space
+   U+202F     Narrow no-break space
+   U+205F     Medium mathematical space
+   U+3000     Ideographic space
+ .sp
+ The vertical space characters are:
+ .sp
+   U+000A     Linefeed
+   U+000B     Vertical tab
+   U+000C     Formfeed
+   U+000D     Carriage return
+   U+0085     Next line
+   U+2028     Line separator
+   U+2029     Paragraph separator
  .P
  A "word" character is an underscore or any character less than 256 that is a
  letter or digit. The definition of letters and digits is controlled by PCRE's
-Line 293 
 in the
+Line 343 
 in the
  .\"
  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
  or "french" in Windows, some character codes greater than 128 are used for
- accented letters, and these are matched by \ew.
+ accented letters, and these are matched by \ew. The use of locales with Unicode
- .P
+ is discouraged.
- In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
- \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
- character property support is available. The use of locales with Unicode is
- discouraged.
  .
  .
  .SS "Newline sequences"
  .rs
  .sp
  Outside a character class, the escape sequence \eR matches any Unicode newline
- sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
+ sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
  the following:
  .sp
    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
-Line 334 
 Inside a character class, \eR matches th
+Line 380 
 Inside a character class, \eR matches th
  .rs
  .sp
  When PCRE is built with Unicode character property support, three additional
- escape sequences to match character properties are available when UTF-8 mode
+ escape sequences that match characters with specific properties are available.
- is selected. They are:
+ When not in UTF-8 mode, these sequences are of course limited to testing
+ characters whose codepoints are less than 256, but they do work in this mode.
+ The extra escape sequences are:
  .sp
    \ep{\fIxx\fP}   a character with the \fIxx\fP property
    \eP{\fIxx\fP}   a character without the \fIxx\fP property
-Line 511 
 atomic group
+Line 559 
 atomic group
  (see below).
  .\"
  Characters with the "mark" property are typically accents that affect the
- preceding character.
+ preceding character. None of them have codepoints less than 256, so in
+ non-UTF-8 mode \eX matches any one character.
  .P
  Matching characters by Unicode property is not fast, because PCRE has to search
  a structure that contains data for over fifteen thousand characters. That is
-Line 519 
 why the traditional escape sequences suc
+Line 568 
 why the traditional escape sequences suc
  properties in PCRE.
  .
  .
+ .\" HTML <a name="resetmatchstart"></a>
+ .SS "Resetting the match start"
+ .rs
+ .sp
+ The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
+ matched characters not to be included in the final matched sequence. For
+ example, the pattern:
+ .sp
+   foo\eKbar
+ .sp
+ matches "foobar", but reports that it has matched "bar". This feature is
+ similar to a lookbehind assertion
+ .\" HTML <a href="#lookbehind">
+ .\" </a>
+ (described below).
+ .\"
+ However, in this case, the part of the subject before the real match does not
+ have to be of fixed length, as lookbehind assertions do. The use of \eK does
+ not interfere with the setting of
+ .\" HTML <a href="#subpattern">
+ .\" </a>
+ captured substrings.
+ .\"
+ For example, when the pattern
+ .sp
+   (foo)\eKbar
+ .sp
+ matches "foobar", the first substring is still set to "foo".
+ .
+ .
  .\" HTML <a name="smallassertions"></a>
  .SS "Simple assertions"
  .rs
-Line 926 
 is reached, an option setting in one bra
+Line 1005 
 is reached, an option setting in one bra
  the above patterns match "SUNDAY" as well as "Saturday".
  .
  .
+ .SH "DUPLICATE SUBPATTERN NUMBERS"
+ .rs
+ .sp
+ Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
+ the same numbers for its capturing parentheses. Such a subpattern starts with
+ (?| and is itself a non-capturing subpattern. For example, consider this
+ pattern:
+ .sp
+   (?|(Sat)ur|(Sun))day
+ .sp
+ Because the two alternatives are inside a (?| group, both sets of capturing
+ parentheses are numbered one. Thus, when the pattern matches, you can look
+ at captured substring number one, whichever alternative matched. This construct
+ is useful when you want to capture part, but not all, of one of a number of
+ alternatives. Inside a (?| group, parentheses are numbered as usual, but the
+ number is reset at the start of each branch. The numbers of any capturing
+ buffers that follow the subpattern start after the highest number used in any
+ branch. The following example is taken from the Perl documentation.
+ The numbers underneath show in which buffer the captured content will be
+ stored.
+ .sp
+   # before  ---------------branch-reset----------- after
+   / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
+   # 1            2         2  3        2     3     4
+ .sp
+ A backreference or a recursive call to a numbered subpattern always refers to
+ the first one in the pattern with the given number.
+ .P
+ An alternative approach to using this "branch reset" feature is to use
+ duplicate named subpatterns, as described in the next section.
+ .
+ .
  .SH "NAMED SUBPATTERNS"
  .rs
  .sp
-Line 975 
 abbreviation. This pattern (ignoring the
+Line 1086 
 abbreviation. This pattern (ignoring the
    (?<DN>Sat)(?:urday)?
  .sp
  There are five capturing substrings, but only one is ever set after a match.
+ (An alternative way of solving this problem is to use a "branch reset"
+ subpattern, as described in the previous section.)
+ .P
  The convenience function for extracting the data by name returns the substring
  for the first (and in this example, the only) subpattern of that name that
  matched. This saves searching to find which numbered subpattern it was. If you
-Line 1180 
 previous example can be rewritten as
+Line 1294 
 previous example can be rewritten as
  .sp
    \ed++foo
  .sp
+ Note that a possessive quantifier can be used with an entire group, for
+ example:
+ .sp
+   (abc|xyz){2,3}+
+ .sp
  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
  option is ignored. They are a convenient notation for the simpler forms of
  atomic group. However, there is no difference in the meaning of a possessive
-Line 1254 
 subpattern is possible using named paren
+Line 1373 
 subpattern is possible using named paren
  .P
  Another way of avoiding the ambiguity inherent in the use of digits following a
  backslash is to use the \eg escape sequence, which is a feature introduced in
- 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
- optionally enclosed in braces. These examples are all identical:
+ number, optionally enclosed in braces. These examples are all identical:
  .sp
    (ring), \e1
    (ring), \eg1
    (ring), \eg{1}
  .sp
- A positive number specifies an absolute reference without the ambiguity that is
+ An unsigned number specifies an absolute reference without the ambiguity that
- 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
- reference. A negative number is a relative reference. Consider this example:
+ the reference. A negative number is a relative reference. Consider this
+ example:
  .sp
    (abc(def)ghi)\eg{-1}
  .sp
-Line 1293 
 back reference, the case of letters is r
+Line 1413 
 back reference, the case of letters is r
  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
  capturing subpattern is matched caselessly.
  .P
- Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'
+ There are several different ways of writing back references to named
- or the Python syntax (?P=name). We could rewrite the above example in either of
+ subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
+ \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
+ back reference syntax, in which \eg can be used for both numeric and named
+ references, is also supported. We could rewrite the above example in any of
  the following ways:
  .sp
    (?<p1>(?i)rah)\es+\ek<p1>
+   (?'p1'(?i)rah)\es+\ek{p1}
    (?P<p1>(?i)rah)\es+(?P=p1)
+   (?<p1>(?i)rah)\es+\eg{p1}
  .sp
  A subpattern that is referenced by name may appear in the pattern before or
  after the reference.
-Line 1421 
 lengths, but it is acceptable if rewritt
+Line 1546 
 lengths, but it is acceptable if rewritt
  .sp
    (?<=abc|abde)
  .sp
+ In some cases, the Perl 5.10 escape sequence \eK
+ .\" HTML <a href="#resetmatchstart">
+ .\" </a>
+ (see above)
+ .\"
+ can be used instead of a lookbehind assertion; this is not restricted to a
+ fixed-length.
+ .P
  The implementation of lookbehind assertions is, for each alternative, to
  temporarily move the current position back by the fixed length and then try to
  match. If there are insufficient characters before the current position, the
-Line 1515 
 recursion, a pseudo-condition called DEF
+Line 1648 
 recursion, a pseudo-condition called DEF
  .sp
  If the text between the parentheses consists of a sequence of digits, the
  condition is true if the capturing subpattern of that number has previously
  matched. An alternative notation is to precede the digits with a plus or minus
  sign. In this case, the subpattern number is relative rather than absolute.
  The most recently opened parentheses can be referenced by (?(-1), the next most
  recent by (?(-2), and so on. In looping constructs it can also make sense to
  refer to subsequent groups with constructs such as (?(+2).
  .P
-Line 1537 
 parenthesis is required. Otherwise, sinc
+Line 1670 
 parenthesis is required. Otherwise, sinc
  subpattern matches nothing. In other words, this pattern matches a sequence of
  non-parentheses, optionally enclosed in parentheses.
  .P
  If you were embedding this pattern in a larger one, you could use a relative
  reference:
  .sp
    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
-Line 1685 
 pattern, so instead you could use this:
+Line 1818 
 pattern, so instead you could use this:
    ( \e( ( (?>[^()]+) | (?1) )* \e) )
  .sp
  We have put the pattern into parentheses, and caused the recursion to refer to
  them instead of the whole pattern.
  .P
  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
  is made easier by the use of relative references. (A Perl 5.10 feature.)
-Line 1706 
 could rewrite the above example as follo
+Line 1839 
 could rewrite the above example as follo
    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
  .sp
  If there is more than one subpattern with the same name, the earliest one is
  used.
  .P
  This particular example pattern that we have been looking at contains nested
  unlimited repeats, and so the use of atomic grouping for matching strings of
-Line 1768 
 relative, as in these examples:
+Line 1901 
 relative, as in these examples:
  .sp
    (...(absolute)...)...(?2)...
    (...(relative)...)...(?-1)...
    (...(?+1)...(relative)...
  .sp
  An earlier example pointed out that the pattern
  .sp
-Line 1833 
 description of the interface to the call
+Line 1966 
 description of the interface to the call
  documentation.
  .
  .
+ .SH "BACTRACKING CONTROL"
+ .rs
+ .sp
+ Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
+ are described in the Perl documentation as "experimental and subject to change
+ or removal in a future version of Perl". It goes on to say: "Their usage in
+ production code should be noted to avoid problems during upgrades." The same
+ remarks apply to the PCRE features described in this section.
+ .P
+ Since these verbs are specifically related to backtracking, they can be used
+ only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
+ backtracking algorithm. They cause an error if encountered by
+ \fBpcre_dfa_exec()\fP.
+ .P
+ The new verbs make use of what was previously invalid syntax: an opening
+ parenthesis followed by an asterisk. In Perl, they are generally of the form
+ (*VERB:ARG) but PCRE does not support the use of arguments, so its general
+ form is just (*VERB). Any number of these verbs may occur in a pattern. There
+ are two kinds:
+ .
+ .SS "Verbs that act immediately"
+ .rs
+ .sp
+ The following verbs act as soon as they are encountered:
+ .sp
+    (*ACCEPT)
+ .sp
+ This verb causes the match to end successfully, skipping the remainder of the
+ pattern. When inside a recursion, only the innermost pattern is ended
+ immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
+ capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
+ captured. For example:
+ .sp
+   A(A|B(*ACCEPT)|C)D
+ .sp
+ This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
+ captured.
+ .sp
+   (*FAIL) or (*F)
+ .sp
+ This verb causes the match to fail, forcing backtracking to occur. It is
+ equivalent to (?!) but easier to read. The Perl documentation notes that it is
+ probably useful only when combined with (?{}) or (??{}). Those are, of course,
+ Perl features that are not present in PCRE. The nearest equivalent is the
+ callout feature, as for example in this pattern:
+ .sp
+   a+(?C)(*FAIL)
+ .sp
+ A match with the string "aaaa" always fails, but the callout is taken before
+ each backtrack happens (in this example, 10 times).
+ .
+ .SS "Verbs that act after backtracking"
+ .rs
+ .sp
+ The following verbs do nothing when they are encountered. Matching continues
+ with what follows, but if there is no subsequent match, a failure is forced.
+ The verbs differ in exactly what kind of failure occurs.
+ .sp
+   (*COMMIT)
+ .sp
+ This verb causes the whole match to fail outright if the rest of the pattern
+ does not match. Even if the pattern is unanchored, no further attempts to find
+ a match by advancing the start point take place. Once (*COMMIT) has been
+ passed, \fBpcre_exec()\fP is committed to finding a match at the current
+ starting point, or not at all. For example:
+ .sp
+   a+(*COMMIT)b
+ .sp
+ This matches "xxaab" but not "aacaab". It can be thought of as a kind of
+ dynamic anchor, or "I've started, so I must finish."
+ .sp
+   (*PRUNE)
+ .sp
+ This verb causes the match to fail at the current position if the rest of the
+ pattern does not match. If the pattern is unanchored, the normal "bumpalong"
+ advance to the next starting character then happens. Backtracking can occur as
+ usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
+ if there is no match to the right, backtracking cannot cross (*PRUNE).
+ In simple cases, the use of (*PRUNE) is just an alternative to an atomic
+ group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
+ be expressed in any other way.
+ .sp
+   (*SKIP)
+ .sp
+ This verb is like (*PRUNE), except that if the pattern is unanchored, the
+ "bumpalong" advance is not to the next character, but to the position in the
+ subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
+ was matched leading up to it cannot be part of a successful match. Consider:
+ .sp
+   a+(*SKIP)b
+ .sp
+ If the subject is "aaaac...", after the first match attempt fails (starting at
+ the first character in the string), the starting point skips on to start the
+ next attempt at "c". Note that a possessive quantifer does not have the same
+ effect in this example; although it would suppress backtracking during the
+ first match attempt, the second attempt would start at the second character
+ instead of skipping on to "c".
+ .sp
+   (*THEN)
+ .sp
+ This verb causes a skip to the next alternation if the rest of the pattern does
+ not match. That is, it cancels pending backtracking, but only within the
+ current alternation. Its name comes from the observation that it can be used
+ for a pattern-based if-then-else block:
+ .sp
+   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
+ .sp
+ If the COND1 pattern matches, FOO is tried (and possibly further items after
+ the end of the group if FOO succeeds); on failure the matcher skips to the
+ second alternative and tries COND2, without backtracking into COND1. If (*THEN)
+ is used outside of any alternation, it acts exactly like (*PRUNE).
+ .
+ .
  .SH "SEE ALSO"
  .rs
  .sp
-Line 1853 
 Cambridge CB2 3QH, England.
+Line 2099 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 09 May 2007
+ Last updated: 08 August 2007
  Copyright (c) 1997-2007 University of Cambridge.
  .fi

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