/[pcre]/code/trunk/doc/pcrepattern.3

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-revision 172 by ph10,
Tue Jun  5 10:40:13 2007 UTC
+revision 208 by ph10,
Mon Aug  6 15:23:29 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 240 
 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. A named back reference
+ enclosed in braces, is an absolute or relative back reference. A named back
- can be coded as \eg{name}. Back references are discussed
+ reference can be coded as \eg{name}. Back references are discussed
  .\" HTML <a href="#backreferences">
  .\" </a>
  later,
-Line 262 
 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 277 
 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 295 
 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 336 
 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 513 
 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 958 
 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 1007 
 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 1212 
 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 1286 
 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 1898 
 Cambridge CB2 3QH, England.
+Line 1986 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 29 May 2007
+ Last updated: 06 August 2007
  Copyright (c) 1997-2007 University of Cambridge.
  .fi

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