2263 lines
94 KiB
HTML
2263 lines
94 KiB
HTML
<html>
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<head>
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<title>pcrepattern specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrepattern man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
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<li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
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<li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
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<li><a name="TOC4" href="#SEC4">BACKSLASH</a>
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<li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
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<li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
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<li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
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<li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
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<li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
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<li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
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<li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
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<li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
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<li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
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<li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
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<li><a name="TOC15" href="#SEC15">REPETITION</a>
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<li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
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<li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
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<li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
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<li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
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<li><a name="TOC20" href="#SEC20">COMMENTS</a>
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<li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
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<li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
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<li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
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<li><a name="TOC24" href="#SEC24">CALLOUTS</a>
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<li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
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<li><a name="TOC26" href="#SEC26">SEE ALSO</a>
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<li><a name="TOC27" href="#SEC27">AUTHOR</a>
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<li><a name="TOC28" href="#SEC28">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
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<P>
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The syntax and semantics of the regular expressions that are supported by PCRE
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are described in detail below. There is a quick-reference syntax summary in the
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<a href="pcresyntax.html"><b>pcresyntax</b></a>
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page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
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also supports some alternative regular expression syntax (which does not
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conflict with the Perl syntax) in order to provide some compatibility with
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regular expressions in Python, .NET, and Oniguruma.
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</P>
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<P>
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Perl's regular expressions are described in its own documentation, and
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regular expressions in general are covered in a number of books, some of which
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have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
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published by O'Reilly, covers regular expressions in great detail. This
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description of PCRE's regular expressions is intended as reference material.
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</P>
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<P>
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The original operation of PCRE was on strings of one-byte characters. However,
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there is now also support for UTF-8 character strings. To use this, you must
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build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
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the PCRE_UTF8 option. There is also a special sequence that can be given at the
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start of a pattern:
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<pre>
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(*UTF8)
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</pre>
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Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
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option. This feature is not Perl-compatible. How setting UTF-8 mode affects
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pattern matching is mentioned in several places below. There is also a summary
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of UTF-8 features in the
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<a href="pcre.html#utf8support">section on UTF-8 support</a>
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in the main
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<a href="pcre.html"><b>pcre</b></a>
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page.
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</P>
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<P>
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The remainder of this document discusses the patterns that are supported by
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PCRE when its main matching function, <b>pcre_exec()</b>, is used.
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From release 6.0, PCRE offers a second matching function,
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<b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
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Perl-compatible. Some of the features discussed below are not available when
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<b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
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alternative function, and how it differs from the normal function, are
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discussed in the
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<a href="pcrematching.html"><b>pcrematching</b></a>
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page.
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</P>
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<br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
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<P>
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PCRE supports five different conventions for indicating line breaks in
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strings: a single CR (carriage return) character, a single LF (linefeed)
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character, the two-character sequence CRLF, any of the three preceding, or any
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Unicode newline sequence. The
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<a href="pcreapi.html"><b>pcreapi</b></a>
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page has
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<a href="pcreapi.html#newlines">further discussion</a>
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about newlines, and shows how to set the newline convention in the
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<i>options</i> arguments for the compiling and matching functions.
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</P>
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<P>
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It is also possible to specify a newline convention by starting a pattern
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string with one of the following five sequences:
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<pre>
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(*CR) carriage return
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(*LF) linefeed
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(*CRLF) carriage return, followed by linefeed
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(*ANYCRLF) any of the three above
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(*ANY) all Unicode newline sequences
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</pre>
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These override the default and the options given to <b>pcre_compile()</b>. For
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example, on a Unix system where LF is the default newline sequence, the pattern
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<pre>
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(*CR)a.b
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</pre>
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changes the convention to CR. That pattern matches "a\nb" because LF is no
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longer a newline. Note that these special settings, which are not
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Perl-compatible, are recognized only at the very start of a pattern, and that
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they must be in upper case. If more than one of them is present, the last one
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is used.
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</P>
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<P>
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The newline convention does not affect what the \R escape sequence matches. By
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default, this is any Unicode newline sequence, for Perl compatibility. However,
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this can be changed; see the description of \R in the section entitled
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<a href="#newlineseq">"Newline sequences"</a>
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below. A change of \R setting can be combined with a change of newline
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convention.
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</P>
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<br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
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<P>
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A regular expression is a pattern that is matched against a subject string from
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left to right. Most characters stand for themselves in a pattern, and match the
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corresponding characters in the subject. As a trivial example, the pattern
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<pre>
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The quick brown fox
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</pre>
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matches a portion of a subject string that is identical to itself. When
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caseless matching is specified (the PCRE_CASELESS option), letters are matched
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independently of case. In UTF-8 mode, PCRE always understands the concept of
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case for characters whose values are less than 128, so caseless matching is
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always possible. For characters with higher values, the concept of case is
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supported if PCRE is compiled with Unicode property support, but not otherwise.
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If you want to use caseless matching for characters 128 and above, you must
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ensure that PCRE is compiled with Unicode property support as well as with
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UTF-8 support.
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</P>
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<P>
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The power of regular expressions comes from the ability to include alternatives
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and repetitions in the pattern. These are encoded in the pattern by the use of
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<i>metacharacters</i>, which do not stand for themselves but instead are
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interpreted in some special way.
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</P>
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<P>
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There are two different sets of metacharacters: those that are recognized
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anywhere in the pattern except within square brackets, and those that are
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recognized within square brackets. Outside square brackets, the metacharacters
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are as follows:
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<pre>
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\ general escape character with several uses
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^ assert start of string (or line, in multiline mode)
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$ assert end of string (or line, in multiline mode)
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. match any character except newline (by default)
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[ start character class definition
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| start of alternative branch
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( start subpattern
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) end subpattern
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? extends the meaning of (
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also 0 or 1 quantifier
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also quantifier minimizer
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* 0 or more quantifier
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+ 1 or more quantifier
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also "possessive quantifier"
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{ start min/max quantifier
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</pre>
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Part of a pattern that is in square brackets is called a "character class". In
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a character class the only metacharacters are:
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<pre>
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\ general escape character
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^ negate the class, but only if the first character
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- indicates character range
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[ POSIX character class (only if followed by POSIX syntax)
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] terminates the character class
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</pre>
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The following sections describe the use of each of the metacharacters.
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</P>
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<br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
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<P>
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The backslash character has several uses. Firstly, if it is followed by a
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non-alphanumeric character, it takes away any special meaning that character
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may have. This use of backslash as an escape character applies both inside and
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outside character classes.
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</P>
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<P>
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For example, if you want to match a * character, you write \* in the pattern.
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This escaping action applies whether or not the following character would
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otherwise be interpreted as a metacharacter, so it is always safe to precede a
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non-alphanumeric with backslash to specify that it stands for itself. In
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particular, if you want to match a backslash, you write \\.
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</P>
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<P>
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If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
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pattern (other than in a character class) and characters between a # outside
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a character class and the next newline are ignored. An escaping backslash can
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be used to include a whitespace or # character as part of the pattern.
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</P>
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<P>
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If you want to remove the special meaning from a sequence of characters, you
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can do so by putting them between \Q and \E. This is different from Perl in
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that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
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Perl, $ and @ cause variable interpolation. Note the following examples:
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<pre>
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Pattern PCRE matches Perl matches
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\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
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\Qabc\$xyz\E abc\$xyz abc\$xyz
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\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
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</pre>
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The \Q...\E sequence is recognized both inside and outside character classes.
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<a name="digitsafterbackslash"></a></P>
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<br><b>
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Non-printing characters
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</b><br>
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<P>
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A second use of backslash provides a way of encoding non-printing characters
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in patterns in a visible manner. There is no restriction on the appearance of
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non-printing characters, apart from the binary zero that terminates a pattern,
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but when a pattern is being prepared by text editing, it is usually easier to
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use one of the following escape sequences than the binary character it
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represents:
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<pre>
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\a alarm, that is, the BEL character (hex 07)
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\cx "control-x", where x is any character
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\e escape (hex 1B)
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\f formfeed (hex 0C)
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\n linefeed (hex 0A)
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\r carriage return (hex 0D)
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\t tab (hex 09)
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\ddd character with octal code ddd, or backreference
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\xhh character with hex code hh
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\x{hhh..} character with hex code hhh..
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</pre>
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The precise effect of \cx is as follows: if x is a lower case letter, it
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is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
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Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
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7B.
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</P>
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<P>
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After \x, from zero to two hexadecimal digits are read (letters can be in
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upper or lower case). Any number of hexadecimal digits may appear between \x{
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and }, but the value of the character code must be less than 256 in non-UTF-8
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mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
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hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
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point, which is 10FFFF.
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</P>
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<P>
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If characters other than hexadecimal digits appear between \x{ and }, or if
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there is no terminating }, this form of escape is not recognized. Instead, the
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initial \x will be interpreted as a basic hexadecimal escape, with no
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following digits, giving a character whose value is zero.
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</P>
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<P>
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Characters whose value is less than 256 can be defined by either of the two
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syntaxes for \x. There is no difference in the way they are handled. For
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example, \xdc is exactly the same as \x{dc}.
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</P>
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<P>
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After \0 up to two further octal digits are read. If there are fewer than two
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digits, just those that are present are used. Thus the sequence \0\x\07
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specifies two binary zeros followed by a BEL character (code value 7). Make
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sure you supply two digits after the initial zero if the pattern character that
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follows is itself an octal digit.
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</P>
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<P>
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The handling of a backslash followed by a digit other than 0 is complicated.
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Outside a character class, PCRE reads it and any following digits as a decimal
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number. If the number is less than 10, or if there have been at least that many
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previous capturing left parentheses in the expression, the entire sequence is
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taken as a <i>back reference</i>. A description of how this works is given
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<a href="#backreferences">later,</a>
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following the discussion of
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<a href="#subpattern">parenthesized subpatterns.</a>
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</P>
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<P>
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Inside a character class, or if the decimal number is greater than 9 and there
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have not been that many capturing subpatterns, PCRE re-reads up to three octal
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digits following the backslash, and uses them to generate a data character. Any
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subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
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character specified in octal must be less than \400. In UTF-8 mode, values up
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to \777 are permitted. For example:
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<pre>
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\040 is another way of writing a space
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\40 is the same, provided there are fewer than 40 previous capturing subpatterns
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\7 is always a back reference
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\11 might be a back reference, or another way of writing a tab
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\011 is always a tab
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\0113 is a tab followed by the character "3"
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\113 might be a back reference, otherwise the character with octal code 113
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\377 might be a back reference, otherwise the byte consisting entirely of 1 bits
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\81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
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</pre>
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Note that octal values of 100 or greater must not be introduced by a leading
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zero, because no more than three octal digits are ever read.
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</P>
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<P>
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All the sequences that define a single character value can be used both inside
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and outside character classes. In addition, inside a character class, the
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sequence \b is interpreted as the backspace character (hex 08), and the
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sequences \R and \X are interpreted as the characters "R" and "X",
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respectively. Outside a character class, these sequences have different
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meanings
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<a href="#uniextseq">(see below).</a>
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</P>
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<br><b>
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Absolute and relative back references
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</b><br>
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<P>
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The sequence \g followed by an unsigned or a negative number, optionally
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enclosed in braces, is an absolute or relative back reference. A named back
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reference can be coded as \g{name}. Back references are discussed
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<a href="#backreferences">later,</a>
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following the discussion of
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<a href="#subpattern">parenthesized subpatterns.</a>
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</P>
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<br><b>
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Absolute and relative subroutine calls
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</b><br>
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<P>
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For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
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a number enclosed either in angle brackets or single quotes, is an alternative
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syntax for referencing a subpattern as a "subroutine". Details are discussed
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<a href="#onigurumasubroutines">later.</a>
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Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
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synonymous. The former is a back reference; the latter is a subroutine call.
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</P>
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<br><b>
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Generic character types
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</b><br>
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<P>
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Another use of backslash is for specifying generic character types. The
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following are always recognized:
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<pre>
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\d any decimal digit
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\D any character that is not a decimal digit
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\h any horizontal whitespace character
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\H any character that is not a horizontal whitespace character
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\s any whitespace character
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\S any character that is not a whitespace character
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\v any vertical whitespace character
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\V any character that is not a vertical whitespace character
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\w any "word" character
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\W any "non-word" character
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</pre>
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Each pair of escape sequences partitions the complete set of characters into
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two disjoint sets. Any given character matches one, and only one, of each pair.
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</P>
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<P>
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These character type sequences can appear both inside and outside character
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classes. They each match one character of the appropriate type. If the current
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matching point is at the end of the subject string, all of them fail, since
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there is no character to match.
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</P>
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<P>
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For compatibility with Perl, \s does not match the VT character (code 11).
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This makes it different from the the POSIX "space" class. The \s characters
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are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
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included in a Perl script, \s may match the VT character. In PCRE, it never
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does.
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</P>
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<P>
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In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
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\w, and always match \D, \S, and \W. This is true even when Unicode
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character property support is available. These sequences retain their original
|
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meanings from before UTF-8 support was available, mainly for efficiency
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reasons. Note that this also affects \b, because it is defined in terms of \w
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and \W.
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</P>
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<P>
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The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
|
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other sequences, these do match certain high-valued codepoints in UTF-8 mode.
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The horizontal space characters are:
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<pre>
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U+0009 Horizontal tab
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U+0020 Space
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U+00A0 Non-break space
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U+1680 Ogham space mark
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U+180E Mongolian vowel separator
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U+2000 En quad
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U+2001 Em quad
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U+2002 En space
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U+2003 Em space
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U+2004 Three-per-em space
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U+2005 Four-per-em space
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U+2006 Six-per-em space
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U+2007 Figure space
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U+2008 Punctuation space
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U+2009 Thin space
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U+200A Hair space
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U+202F Narrow no-break space
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U+205F Medium mathematical space
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U+3000 Ideographic space
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</pre>
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The vertical space characters are:
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<pre>
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U+000A Linefeed
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U+000B Vertical tab
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U+000C Formfeed
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U+000D Carriage return
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U+0085 Next line
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U+2028 Line separator
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U+2029 Paragraph separator
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</PRE>
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</P>
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<P>
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A "word" character is an underscore or any character less than 256 that is a
|
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letter or digit. The definition of letters and digits is controlled by PCRE's
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|
low-valued character tables, and may vary if locale-specific matching is taking
|
|
place (see
|
|
<a href="pcreapi.html#localesupport">"Locale support"</a>
|
|
in the
|
|
<a href="pcreapi.html"><b>pcreapi</b></a>
|
|
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 \w. The use of locales with Unicode
|
|
is discouraged.
|
|
<a name="newlineseq"></a></P>
|
|
<br><b>
|
|
Newline sequences
|
|
</b><br>
|
|
<P>
|
|
Outside a character class, by default, the escape sequence \R matches any
|
|
Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
|
|
equivalent to the following:
|
|
<pre>
|
|
(?>\r\n|\n|\x0b|\f|\r|\x85)
|
|
</pre>
|
|
This is an example of an "atomic group", details of which are given
|
|
<a href="#atomicgroup">below.</a>
|
|
This particular group matches either the two-character sequence CR followed by
|
|
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
|
|
U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
|
|
line, U+0085). The two-character sequence is treated as a single unit that
|
|
cannot be split.
|
|
</P>
|
|
<P>
|
|
In UTF-8 mode, two additional characters whose codepoints are greater than 255
|
|
are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
|
|
Unicode character property support is not needed for these characters to be
|
|
recognized.
|
|
</P>
|
|
<P>
|
|
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
|
|
complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
|
|
either at compile time or when the pattern is matched. (BSR is an abbrevation
|
|
for "backslash R".) This can be made the default when PCRE is built; if this is
|
|
the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
|
|
It is also possible to specify these settings by starting a pattern string with
|
|
one of the following sequences:
|
|
<pre>
|
|
(*BSR_ANYCRLF) CR, LF, or CRLF only
|
|
(*BSR_UNICODE) any Unicode newline sequence
|
|
</pre>
|
|
These override the default and the options given to <b>pcre_compile()</b>, but
|
|
they can be overridden by options given to <b>pcre_exec()</b>. Note that these
|
|
special settings, which are not Perl-compatible, are recognized only at the
|
|
very start of a pattern, and that they must be in upper case. If more than one
|
|
of them is present, the last one is used. They can be combined with a change of
|
|
newline convention, for example, a pattern can start with:
|
|
<pre>
|
|
(*ANY)(*BSR_ANYCRLF)
|
|
</pre>
|
|
Inside a character class, \R matches the letter "R".
|
|
<a name="uniextseq"></a></P>
|
|
<br><b>
|
|
Unicode character properties
|
|
</b><br>
|
|
<P>
|
|
When PCRE is built with Unicode character property support, three additional
|
|
escape sequences that match characters with specific properties are available.
|
|
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:
|
|
<pre>
|
|
\p{<i>xx</i>} a character with the <i>xx</i> property
|
|
\P{<i>xx</i>} a character without the <i>xx</i> property
|
|
\X an extended Unicode sequence
|
|
</pre>
|
|
The property names represented by <i>xx</i> above are limited to the Unicode
|
|
script names, the general category properties, and "Any", which matches any
|
|
character (including newline). Other properties such as "InMusicalSymbols" are
|
|
not currently supported by PCRE. Note that \P{Any} does not match any
|
|
characters, so always causes a match failure.
|
|
</P>
|
|
<P>
|
|
Sets of Unicode characters are defined as belonging to certain scripts. A
|
|
character from one of these sets can be matched using a script name. For
|
|
example:
|
|
<pre>
|
|
\p{Greek}
|
|
\P{Han}
|
|
</pre>
|
|
Those that are not part of an identified script are lumped together as
|
|
"Common". The current list of scripts is:
|
|
</P>
|
|
<P>
|
|
Arabic,
|
|
Armenian,
|
|
Balinese,
|
|
Bengali,
|
|
Bopomofo,
|
|
Braille,
|
|
Buginese,
|
|
Buhid,
|
|
Canadian_Aboriginal,
|
|
Cherokee,
|
|
Common,
|
|
Coptic,
|
|
Cuneiform,
|
|
Cypriot,
|
|
Cyrillic,
|
|
Deseret,
|
|
Devanagari,
|
|
Ethiopic,
|
|
Georgian,
|
|
Glagolitic,
|
|
Gothic,
|
|
Greek,
|
|
Gujarati,
|
|
Gurmukhi,
|
|
Han,
|
|
Hangul,
|
|
Hanunoo,
|
|
Hebrew,
|
|
Hiragana,
|
|
Inherited,
|
|
Kannada,
|
|
Katakana,
|
|
Kharoshthi,
|
|
Khmer,
|
|
Lao,
|
|
Latin,
|
|
Limbu,
|
|
Linear_B,
|
|
Malayalam,
|
|
Mongolian,
|
|
Myanmar,
|
|
New_Tai_Lue,
|
|
Nko,
|
|
Ogham,
|
|
Old_Italic,
|
|
Old_Persian,
|
|
Oriya,
|
|
Osmanya,
|
|
Phags_Pa,
|
|
Phoenician,
|
|
Runic,
|
|
Shavian,
|
|
Sinhala,
|
|
Syloti_Nagri,
|
|
Syriac,
|
|
Tagalog,
|
|
Tagbanwa,
|
|
Tai_Le,
|
|
Tamil,
|
|
Telugu,
|
|
Thaana,
|
|
Thai,
|
|
Tibetan,
|
|
Tifinagh,
|
|
Ugaritic,
|
|
Yi.
|
|
</P>
|
|
<P>
|
|
Each character has exactly one general category property, specified by a
|
|
two-letter abbreviation. For compatibility with Perl, negation can be specified
|
|
by including a circumflex between the opening brace and the property name. For
|
|
example, \p{^Lu} is the same as \P{Lu}.
|
|
</P>
|
|
<P>
|
|
If only one letter is specified with \p or \P, it includes all the general
|
|
category properties that start with that letter. In this case, in the absence
|
|
of negation, the curly brackets in the escape sequence are optional; these two
|
|
examples have the same effect:
|
|
<pre>
|
|
\p{L}
|
|
\pL
|
|
</pre>
|
|
The following general category property codes are supported:
|
|
<pre>
|
|
C Other
|
|
Cc Control
|
|
Cf Format
|
|
Cn Unassigned
|
|
Co Private use
|
|
Cs Surrogate
|
|
|
|
L Letter
|
|
Ll Lower case letter
|
|
Lm Modifier letter
|
|
Lo Other letter
|
|
Lt Title case letter
|
|
Lu Upper case letter
|
|
|
|
M Mark
|
|
Mc Spacing mark
|
|
Me Enclosing mark
|
|
Mn Non-spacing mark
|
|
|
|
N Number
|
|
Nd Decimal number
|
|
Nl Letter number
|
|
No Other number
|
|
|
|
P Punctuation
|
|
Pc Connector punctuation
|
|
Pd Dash punctuation
|
|
Pe Close punctuation
|
|
Pf Final punctuation
|
|
Pi Initial punctuation
|
|
Po Other punctuation
|
|
Ps Open punctuation
|
|
|
|
S Symbol
|
|
Sc Currency symbol
|
|
Sk Modifier symbol
|
|
Sm Mathematical symbol
|
|
So Other symbol
|
|
|
|
Z Separator
|
|
Zl Line separator
|
|
Zp Paragraph separator
|
|
Zs Space separator
|
|
</pre>
|
|
The special property L& is also supported: it matches a character that has
|
|
the Lu, Ll, or Lt property, in other words, a letter that is not classified as
|
|
a modifier or "other".
|
|
</P>
|
|
<P>
|
|
The Cs (Surrogate) property applies only to characters in the range U+D800 to
|
|
U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
|
|
cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
|
|
(see the discussion of PCRE_NO_UTF8_CHECK in the
|
|
<a href="pcreapi.html"><b>pcreapi</b></a>
|
|
page).
|
|
</P>
|
|
<P>
|
|
The long synonyms for these properties that Perl supports (such as \p{Letter})
|
|
are not supported by PCRE, nor is it permitted to prefix any of these
|
|
properties with "Is".
|
|
</P>
|
|
<P>
|
|
No character that is in the Unicode table has the Cn (unassigned) property.
|
|
Instead, this property is assumed for any code point that is not in the
|
|
Unicode table.
|
|
</P>
|
|
<P>
|
|
Specifying caseless matching does not affect these escape sequences. For
|
|
example, \p{Lu} always matches only upper case letters.
|
|
</P>
|
|
<P>
|
|
The \X escape matches any number of Unicode characters that form an extended
|
|
Unicode sequence. \X is equivalent to
|
|
<pre>
|
|
(?>\PM\pM*)
|
|
</pre>
|
|
That is, it matches a character without the "mark" property, followed by zero
|
|
or more characters with the "mark" property, and treats the sequence as an
|
|
atomic group
|
|
<a href="#atomicgroup">(see below).</a>
|
|
Characters with the "mark" property are typically accents that affect the
|
|
preceding character. None of them have codepoints less than 256, so in
|
|
non-UTF-8 mode \X matches any one character.
|
|
</P>
|
|
<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
|
|
why the traditional escape sequences such as \d and \w do not use Unicode
|
|
properties in PCRE.
|
|
<a name="resetmatchstart"></a></P>
|
|
<br><b>
|
|
Resetting the match start
|
|
</b><br>
|
|
<P>
|
|
The escape sequence \K, 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:
|
|
<pre>
|
|
foo\Kbar
|
|
</pre>
|
|
matches "foobar", but reports that it has matched "bar". This feature is
|
|
similar to a lookbehind assertion
|
|
<a href="#lookbehind">(described below).</a>
|
|
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 \K does
|
|
not interfere with the setting of
|
|
<a href="#subpattern">captured substrings.</a>
|
|
For example, when the pattern
|
|
<pre>
|
|
(foo)\Kbar
|
|
</pre>
|
|
matches "foobar", the first substring is still set to "foo".
|
|
<a name="smallassertions"></a></P>
|
|
<br><b>
|
|
Simple assertions
|
|
</b><br>
|
|
<P>
|
|
The final use of backslash is for certain simple assertions. An assertion
|
|
specifies a condition that has to be met at a particular point in a match,
|
|
without consuming any characters from the subject string. The use of
|
|
subpatterns for more complicated assertions is described
|
|
<a href="#bigassertions">below.</a>
|
|
The backslashed assertions are:
|
|
<pre>
|
|
\b matches at a word boundary
|
|
\B matches when not at a word boundary
|
|
\A matches at the start of the subject
|
|
\Z matches at the end of the subject
|
|
also matches before a newline at the end of the subject
|
|
\z matches only at the end of the subject
|
|
\G matches at the first matching position in the subject
|
|
</pre>
|
|
These assertions may not appear in character classes (but note that \b has a
|
|
different meaning, namely the backspace character, inside a character class).
|
|
</P>
|
|
<P>
|
|
A word boundary is a position in the subject string where the current character
|
|
and the previous character do not both match \w or \W (i.e. one matches
|
|
\w and the other matches \W), or the start or end of the string if the
|
|
first or last character matches \w, respectively.
|
|
</P>
|
|
<P>
|
|
The \A, \Z, and \z assertions differ from the traditional circumflex and
|
|
dollar (described in the next section) in that they only ever match at the very
|
|
start and end of the subject string, whatever options are set. Thus, they are
|
|
independent of multiline mode. These three assertions are not affected by the
|
|
PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
|
|
circumflex and dollar metacharacters. However, if the <i>startoffset</i>
|
|
argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
|
|
at a point other than the beginning of the subject, \A can never match. The
|
|
difference between \Z and \z is that \Z matches before a newline at the end
|
|
of the string as well as at the very end, whereas \z matches only at the end.
|
|
</P>
|
|
<P>
|
|
The \G assertion is true only when the current matching position is at the
|
|
start point of the match, as specified by the <i>startoffset</i> argument of
|
|
<b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
|
|
non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
|
|
arguments, you can mimic Perl's /g option, and it is in this kind of
|
|
implementation where \G can be useful.
|
|
</P>
|
|
<P>
|
|
Note, however, that PCRE's interpretation of \G, as the start of the current
|
|
match, is subtly different from Perl's, which defines it as the end of the
|
|
previous match. In Perl, these can be different when the previously matched
|
|
string was empty. Because PCRE does just one match at a time, it cannot
|
|
reproduce this behaviour.
|
|
</P>
|
|
<P>
|
|
If all the alternatives of a pattern begin with \G, the expression is anchored
|
|
to the starting match position, and the "anchored" flag is set in the compiled
|
|
regular expression.
|
|
</P>
|
|
<br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
|
|
<P>
|
|
Outside a character class, in the default matching mode, the circumflex
|
|
character is an assertion that is true only if the current matching point is
|
|
at the start of the subject string. If the <i>startoffset</i> argument of
|
|
<b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
|
|
option is unset. Inside a character class, circumflex has an entirely different
|
|
meaning
|
|
<a href="#characterclass">(see below).</a>
|
|
</P>
|
|
<P>
|
|
Circumflex need not be the first character of the pattern if a number of
|
|
alternatives are involved, but it should be the first thing in each alternative
|
|
in which it appears if the pattern is ever to match that branch. If all
|
|
possible alternatives start with a circumflex, that is, if the pattern is
|
|
constrained to match only at the start of the subject, it is said to be an
|
|
"anchored" pattern. (There are also other constructs that can cause a pattern
|
|
to be anchored.)
|
|
</P>
|
|
<P>
|
|
A dollar character is an assertion that is true only if the current matching
|
|
point is at the end of the subject string, or immediately before a newline
|
|
at the end of the string (by default). Dollar need not be the last character of
|
|
the pattern if a number of alternatives are involved, but it should be the last
|
|
item in any branch in which it appears. Dollar has no special meaning in a
|
|
character class.
|
|
</P>
|
|
<P>
|
|
The meaning of dollar can be changed so that it matches only at the very end of
|
|
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
|
|
does not affect the \Z assertion.
|
|
</P>
|
|
<P>
|
|
The meanings of the circumflex and dollar characters are changed if the
|
|
PCRE_MULTILINE option is set. When this is the case, a circumflex matches
|
|
immediately after internal newlines as well as at the start of the subject
|
|
string. It does not match after a newline that ends the string. A dollar
|
|
matches before any newlines in the string, as well as at the very end, when
|
|
PCRE_MULTILINE is set. When newline is specified as the two-character
|
|
sequence CRLF, isolated CR and LF characters do not indicate newlines.
|
|
</P>
|
|
<P>
|
|
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
|
|
\n represents a newline) in multiline mode, but not otherwise. Consequently,
|
|
patterns that are anchored in single line mode because all branches start with
|
|
^ are not anchored in multiline mode, and a match for circumflex is possible
|
|
when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
|
|
PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
|
|
</P>
|
|
<P>
|
|
Note that the sequences \A, \Z, and \z can be used to match the start and
|
|
end of the subject in both modes, and if all branches of a pattern start with
|
|
\A it is always anchored, whether or not PCRE_MULTILINE is set.
|
|
</P>
|
|
<br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
|
|
<P>
|
|
Outside a character class, a dot in the pattern matches any one character in
|
|
the subject string except (by default) a character that signifies the end of a
|
|
line. In UTF-8 mode, the matched character may be more than one byte long.
|
|
</P>
|
|
<P>
|
|
When a line ending is defined as a single character, dot never matches that
|
|
character; when the two-character sequence CRLF is used, dot does not match CR
|
|
if it is immediately followed by LF, but otherwise it matches all characters
|
|
(including isolated CRs and LFs). When any Unicode line endings are being
|
|
recognized, dot does not match CR or LF or any of the other line ending
|
|
characters.
|
|
</P>
|
|
<P>
|
|
The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
|
|
option is set, a dot matches any one character, without exception. If the
|
|
two-character sequence CRLF is present in the subject string, it takes two dots
|
|
to match it.
|
|
</P>
|
|
<P>
|
|
The handling of dot is entirely independent of the handling of circumflex and
|
|
dollar, the only relationship being that they both involve newlines. Dot has no
|
|
special meaning in a character class.
|
|
</P>
|
|
<br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
|
|
<P>
|
|
Outside a character class, the escape sequence \C matches any one byte, both
|
|
in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
|
|
characters. The feature is provided in Perl in order to match individual bytes
|
|
in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
|
|
what remains in the string may be a malformed UTF-8 string. For this reason,
|
|
the \C escape sequence is best avoided.
|
|
</P>
|
|
<P>
|
|
PCRE does not allow \C to appear in lookbehind assertions
|
|
<a href="#lookbehind">(described below),</a>
|
|
because in UTF-8 mode this would make it impossible to calculate the length of
|
|
the lookbehind.
|
|
<a name="characterclass"></a></P>
|
|
<br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
|
|
<P>
|
|
An opening square bracket introduces a character class, terminated by a closing
|
|
square bracket. A closing square bracket on its own is not special. If a
|
|
closing square bracket is required as a member of the class, it should be the
|
|
first data character in the class (after an initial circumflex, if present) or
|
|
escaped with a backslash.
|
|
</P>
|
|
<P>
|
|
A character class matches a single character in the subject. In UTF-8 mode, the
|
|
character may occupy more than one byte. A matched character must be in the set
|
|
of characters defined by the class, unless the first character in the class
|
|
definition is a circumflex, in which case the subject character must not be in
|
|
the set defined by the class. If a circumflex is actually required as a member
|
|
of the class, ensure it is not the first character, or escape it with a
|
|
backslash.
|
|
</P>
|
|
<P>
|
|
For example, the character class [aeiou] matches any lower case vowel, while
|
|
[^aeiou] matches any character that is not a lower case vowel. Note that a
|
|
circumflex is just a convenient notation for specifying the characters that
|
|
are in the class by enumerating those that are not. A class that starts with a
|
|
circumflex is not an assertion: it still consumes a character from the subject
|
|
string, and therefore it fails if the current pointer is at the end of the
|
|
string.
|
|
</P>
|
|
<P>
|
|
In UTF-8 mode, characters with values greater than 255 can be included in a
|
|
class as a literal string of bytes, or by using the \x{ escaping mechanism.
|
|
</P>
|
|
<P>
|
|
When caseless matching is set, any letters in a class represent both their
|
|
upper case and lower case versions, so for example, a caseless [aeiou] matches
|
|
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
|
|
caseful version would. In UTF-8 mode, PCRE always understands the concept of
|
|
case for characters whose values are less than 128, so caseless matching is
|
|
always possible. For characters with higher values, the concept of case is
|
|
supported if PCRE is compiled with Unicode property support, but not otherwise.
|
|
If you want to use caseless matching for characters 128 and above, you must
|
|
ensure that PCRE is compiled with Unicode property support as well as with
|
|
UTF-8 support.
|
|
</P>
|
|
<P>
|
|
Characters that might indicate line breaks are never treated in any special way
|
|
when matching character classes, whatever line-ending sequence is in use, and
|
|
whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
|
|
such as [^a] always matches one of these characters.
|
|
</P>
|
|
<P>
|
|
The minus (hyphen) character can be used to specify a range of characters in a
|
|
character class. For example, [d-m] matches any letter between d and m,
|
|
inclusive. If a minus character is required in a class, it must be escaped with
|
|
a backslash or appear in a position where it cannot be interpreted as
|
|
indicating a range, typically as the first or last character in the class.
|
|
</P>
|
|
<P>
|
|
It is not possible to have the literal character "]" as the end character of a
|
|
range. A pattern such as [W-]46] is interpreted as a class of two characters
|
|
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
|
|
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
|
|
the end of range, so [W-\]46] is interpreted as a class containing a range
|
|
followed by two other characters. The octal or hexadecimal representation of
|
|
"]" can also be used to end a range.
|
|
</P>
|
|
<P>
|
|
Ranges operate in the collating sequence of character values. They can also be
|
|
used for characters specified numerically, for example [\000-\037]. In UTF-8
|
|
mode, ranges can include characters whose values are greater than 255, for
|
|
example [\x{100}-\x{2ff}].
|
|
</P>
|
|
<P>
|
|
If a range that includes letters is used when caseless matching is set, it
|
|
matches the letters in either case. For example, [W-c] is equivalent to
|
|
[][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
|
|
tables for a French locale are in use, [\xc8-\xcb] matches accented E
|
|
characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
|
|
characters with values greater than 128 only when it is compiled with Unicode
|
|
property support.
|
|
</P>
|
|
<P>
|
|
The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
|
|
in a character class, and add the characters that they match to the class. For
|
|
example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
|
|
conveniently be used with the upper case character types to specify a more
|
|
restricted set of characters than the matching lower case type. For example,
|
|
the class [^\W_] matches any letter or digit, but not underscore.
|
|
</P>
|
|
<P>
|
|
The only metacharacters that are recognized in character classes are backslash,
|
|
hyphen (only where it can be interpreted as specifying a range), circumflex
|
|
(only at the start), opening square bracket (only when it can be interpreted as
|
|
introducing a POSIX class name - see the next section), and the terminating
|
|
closing square bracket. However, escaping other non-alphanumeric characters
|
|
does no harm.
|
|
</P>
|
|
<br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
|
|
<P>
|
|
Perl supports the POSIX notation for character classes. This uses names
|
|
enclosed by [: and :] within the enclosing square brackets. PCRE also supports
|
|
this notation. For example,
|
|
<pre>
|
|
[01[:alpha:]%]
|
|
</pre>
|
|
matches "0", "1", any alphabetic character, or "%". The supported class names
|
|
are
|
|
<pre>
|
|
alnum letters and digits
|
|
alpha letters
|
|
ascii character codes 0 - 127
|
|
blank space or tab only
|
|
cntrl control characters
|
|
digit decimal digits (same as \d)
|
|
graph printing characters, excluding space
|
|
lower lower case letters
|
|
print printing characters, including space
|
|
punct printing characters, excluding letters and digits
|
|
space white space (not quite the same as \s)
|
|
upper upper case letters
|
|
word "word" characters (same as \w)
|
|
xdigit hexadecimal digits
|
|
</pre>
|
|
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
|
|
space (32). Notice that this list includes the VT character (code 11). This
|
|
makes "space" different to \s, which does not include VT (for Perl
|
|
compatibility).
|
|
</P>
|
|
<P>
|
|
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
|
|
5.8. Another Perl extension is negation, which is indicated by a ^ character
|
|
after the colon. For example,
|
|
<pre>
|
|
[12[:^digit:]]
|
|
</pre>
|
|
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
|
|
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
|
|
supported, and an error is given if they are encountered.
|
|
</P>
|
|
<P>
|
|
In UTF-8 mode, characters with values greater than 128 do not match any of
|
|
the POSIX character classes.
|
|
</P>
|
|
<br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
|
|
<P>
|
|
Vertical bar characters are used to separate alternative patterns. For example,
|
|
the pattern
|
|
<pre>
|
|
gilbert|sullivan
|
|
</pre>
|
|
matches either "gilbert" or "sullivan". Any number of alternatives may appear,
|
|
and an empty alternative is permitted (matching the empty string). The matching
|
|
process tries each alternative in turn, from left to right, and the first one
|
|
that succeeds is used. If the alternatives are within a subpattern
|
|
<a href="#subpattern">(defined below),</a>
|
|
"succeeds" means matching the rest of the main pattern as well as the
|
|
alternative in the subpattern.
|
|
</P>
|
|
<br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
|
|
<P>
|
|
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
|
|
PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
|
|
the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
|
|
The option letters are
|
|
<pre>
|
|
i for PCRE_CASELESS
|
|
m for PCRE_MULTILINE
|
|
s for PCRE_DOTALL
|
|
x for PCRE_EXTENDED
|
|
</pre>
|
|
For example, (?im) sets caseless, multiline matching. It is also possible to
|
|
unset these options by preceding the letter with a hyphen, and a combined
|
|
setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
|
|
PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
|
|
permitted. If a letter appears both before and after the hyphen, the option is
|
|
unset.
|
|
</P>
|
|
<P>
|
|
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
|
|
changed in the same way as the Perl-compatible options by using the characters
|
|
J, U and X respectively.
|
|
</P>
|
|
<P>
|
|
When one of these option changes occurs at top level (that is, not inside
|
|
subpattern parentheses), the change applies to the remainder of the pattern
|
|
that follows. If the change is placed right at the start of a pattern, PCRE
|
|
extracts it into the global options (and it will therefore show up in data
|
|
extracted by the <b>pcre_fullinfo()</b> function).
|
|
</P>
|
|
<P>
|
|
An option change within a subpattern (see below for a description of
|
|
subpatterns) affects only that part of the current pattern that follows it, so
|
|
<pre>
|
|
(a(?i)b)c
|
|
</pre>
|
|
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
|
|
By this means, options can be made to have different settings in different
|
|
parts of the pattern. Any changes made in one alternative do carry on
|
|
into subsequent branches within the same subpattern. For example,
|
|
<pre>
|
|
(a(?i)b|c)
|
|
</pre>
|
|
matches "ab", "aB", "c", and "C", even though when matching "C" the first
|
|
branch is abandoned before the option setting. This is because the effects of
|
|
option settings happen at compile time. There would be some very weird
|
|
behaviour otherwise.
|
|
</P>
|
|
<P>
|
|
<b>Note:</b> There are other PCRE-specific options that can be set by the
|
|
application when the compile or match functions are called. In some cases the
|
|
pattern can contain special leading sequences such as (*CRLF) to override what
|
|
the application has set or what has been defaulted. Details are given in the
|
|
section entitled
|
|
<a href="#newlineseq">"Newline sequences"</a>
|
|
above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
|
|
mode; this is equivalent to setting the PCRE_UTF8 option.
|
|
<a name="subpattern"></a></P>
|
|
<br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
|
|
<P>
|
|
Subpatterns are delimited by parentheses (round brackets), which can be nested.
|
|
Turning part of a pattern into a subpattern does two things:
|
|
<br>
|
|
<br>
|
|
1. It localizes a set of alternatives. For example, the pattern
|
|
<pre>
|
|
cat(aract|erpillar|)
|
|
</pre>
|
|
matches one of the words "cat", "cataract", or "caterpillar". Without the
|
|
parentheses, it would match "cataract", "erpillar" or an empty string.
|
|
<br>
|
|
<br>
|
|
2. It sets up the subpattern as a capturing subpattern. This means that, when
|
|
the whole pattern matches, that portion of the subject string that matched the
|
|
subpattern is passed back to the caller via the <i>ovector</i> argument of
|
|
<b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
|
|
from 1) to obtain numbers for the capturing subpatterns.
|
|
</P>
|
|
<P>
|
|
For example, if the string "the red king" is matched against the pattern
|
|
<pre>
|
|
the ((red|white) (king|queen))
|
|
</pre>
|
|
the captured substrings are "red king", "red", and "king", and are numbered 1,
|
|
2, and 3, respectively.
|
|
</P>
|
|
<P>
|
|
The fact that plain parentheses fulfil two functions is not always helpful.
|
|
There are often times when a grouping subpattern is required without a
|
|
capturing requirement. If an opening parenthesis is followed by a question mark
|
|
and a colon, the subpattern does not do any capturing, and is not counted when
|
|
computing the number of any subsequent capturing subpatterns. For example, if
|
|
the string "the white queen" is matched against the pattern
|
|
<pre>
|
|
the ((?:red|white) (king|queen))
|
|
</pre>
|
|
the captured substrings are "white queen" and "queen", and are numbered 1 and
|
|
2. The maximum number of capturing subpatterns is 65535.
|
|
</P>
|
|
<P>
|
|
As a convenient shorthand, if any option settings are required at the start of
|
|
a non-capturing subpattern, the option letters may appear between the "?" and
|
|
the ":". Thus the two patterns
|
|
<pre>
|
|
(?i:saturday|sunday)
|
|
(?:(?i)saturday|sunday)
|
|
</pre>
|
|
match exactly the same set of strings. Because alternative branches are tried
|
|
from left to right, and options are not reset until the end of the subpattern
|
|
is reached, an option setting in one branch does affect subsequent branches, so
|
|
the above patterns match "SUNDAY" as well as "Saturday".
|
|
</P>
|
|
<br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
(?|(Sat)ur|(Sun))day
|
|
</pre>
|
|
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.
|
|
<pre>
|
|
# before ---------------branch-reset----------- after
|
|
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
|
|
# 1 2 2 3 2 3 4
|
|
</pre>
|
|
A backreference or a recursive call to a numbered subpattern always refers to
|
|
the first one in the pattern with the given number.
|
|
</P>
|
|
<P>
|
|
An alternative approach to using this "branch reset" feature is to use
|
|
duplicate named subpatterns, as described in the next section.
|
|
</P>
|
|
<br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
|
|
<P>
|
|
Identifying capturing parentheses by number is simple, but it can be very hard
|
|
to keep track of the numbers in complicated regular expressions. Furthermore,
|
|
if an expression is modified, the numbers may change. To help with this
|
|
difficulty, PCRE supports the naming of subpatterns. This feature was not
|
|
added to Perl until release 5.10. Python had the feature earlier, and PCRE
|
|
introduced it at release 4.0, using the Python syntax. PCRE now supports both
|
|
the Perl and the Python syntax.
|
|
</P>
|
|
<P>
|
|
In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
|
|
(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
|
|
parentheses from other parts of the pattern, such as
|
|
<a href="#backreferences">backreferences,</a>
|
|
<a href="#recursion">recursion,</a>
|
|
and
|
|
<a href="#conditions">conditions,</a>
|
|
can be made by name as well as by number.
|
|
</P>
|
|
<P>
|
|
Names consist of up to 32 alphanumeric characters and underscores. Named
|
|
capturing parentheses are still allocated numbers as well as names, exactly as
|
|
if the names were not present. The PCRE API provides function calls for
|
|
extracting the name-to-number translation table from a compiled pattern. There
|
|
is also a convenience function for extracting a captured substring by name.
|
|
</P>
|
|
<P>
|
|
By default, a name must be unique within a pattern, but it is possible to relax
|
|
this constraint by setting the PCRE_DUPNAMES option at compile time. This can
|
|
be useful for patterns where only one instance of the named parentheses can
|
|
match. Suppose you want to match the name of a weekday, either as a 3-letter
|
|
abbreviation or as the full name, and in both cases you want to extract the
|
|
abbreviation. This pattern (ignoring the line breaks) does the job:
|
|
<pre>
|
|
(?<DN>Mon|Fri|Sun)(?:day)?|
|
|
(?<DN>Tue)(?:sday)?|
|
|
(?<DN>Wed)(?:nesday)?|
|
|
(?<DN>Thu)(?:rsday)?|
|
|
(?<DN>Sat)(?:urday)?
|
|
</pre>
|
|
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>
|
|
<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
|
|
make a reference to a non-unique named subpattern from elsewhere in the
|
|
pattern, the one that corresponds to the lowest number is used. For further
|
|
details of the interfaces for handling named subpatterns, see the
|
|
<a href="pcreapi.html"><b>pcreapi</b></a>
|
|
documentation.
|
|
</P>
|
|
<P>
|
|
<b>Warning:</b> You cannot use different names to distinguish between two
|
|
subpatterns with the same number (see the previous section) because PCRE uses
|
|
only the numbers when matching.
|
|
</P>
|
|
<br><a name="SEC15" href="#TOC1">REPETITION</a><br>
|
|
<P>
|
|
Repetition is specified by quantifiers, which can follow any of the following
|
|
items:
|
|
<pre>
|
|
a literal data character
|
|
the dot metacharacter
|
|
the \C escape sequence
|
|
the \X escape sequence (in UTF-8 mode with Unicode properties)
|
|
the \R escape sequence
|
|
an escape such as \d that matches a single character
|
|
a character class
|
|
a back reference (see next section)
|
|
a parenthesized subpattern (unless it is an assertion)
|
|
</pre>
|
|
The general repetition quantifier specifies a minimum and maximum number of
|
|
permitted matches, by giving the two numbers in curly brackets (braces),
|
|
separated by a comma. The numbers must be less than 65536, and the first must
|
|
be less than or equal to the second. For example:
|
|
<pre>
|
|
z{2,4}
|
|
</pre>
|
|
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
|
|
character. If the second number is omitted, but the comma is present, there is
|
|
no upper limit; if the second number and the comma are both omitted, the
|
|
quantifier specifies an exact number of required matches. Thus
|
|
<pre>
|
|
[aeiou]{3,}
|
|
</pre>
|
|
matches at least 3 successive vowels, but may match many more, while
|
|
<pre>
|
|
\d{8}
|
|
</pre>
|
|
matches exactly 8 digits. An opening curly bracket that appears in a position
|
|
where a quantifier is not allowed, or one that does not match the syntax of a
|
|
quantifier, is taken as a literal character. For example, {,6} is not a
|
|
quantifier, but a literal string of four characters.
|
|
</P>
|
|
<P>
|
|
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
|
|
bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
|
|
which is represented by a two-byte sequence. Similarly, when Unicode property
|
|
support is available, \X{3} matches three Unicode extended sequences, each of
|
|
which may be several bytes long (and they may be of different lengths).
|
|
</P>
|
|
<P>
|
|
The quantifier {0} is permitted, causing the expression to behave as if the
|
|
previous item and the quantifier were not present. This may be useful for
|
|
subpatterns that are referenced as
|
|
<a href="#subpatternsassubroutines">subroutines</a>
|
|
from elsewhere in the pattern. Items other than subpatterns that have a {0}
|
|
quantifier are omitted from the compiled pattern.
|
|
</P>
|
|
<P>
|
|
For convenience, the three most common quantifiers have single-character
|
|
abbreviations:
|
|
<pre>
|
|
* is equivalent to {0,}
|
|
+ is equivalent to {1,}
|
|
? is equivalent to {0,1}
|
|
</pre>
|
|
It is possible to construct infinite loops by following a subpattern that can
|
|
match no characters with a quantifier that has no upper limit, for example:
|
|
<pre>
|
|
(a?)*
|
|
</pre>
|
|
Earlier versions of Perl and PCRE used to give an error at compile time for
|
|
such patterns. However, because there are cases where this can be useful, such
|
|
patterns are now accepted, but if any repetition of the subpattern does in fact
|
|
match no characters, the loop is forcibly broken.
|
|
</P>
|
|
<P>
|
|
By default, the quantifiers are "greedy", that is, they match as much as
|
|
possible (up to the maximum number of permitted times), without causing the
|
|
rest of the pattern to fail. The classic example of where this gives problems
|
|
is in trying to match comments in C programs. These appear between /* and */
|
|
and within the comment, individual * and / characters may appear. An attempt to
|
|
match C comments by applying the pattern
|
|
<pre>
|
|
/\*.*\*/
|
|
</pre>
|
|
to the string
|
|
<pre>
|
|
/* first comment */ not comment /* second comment */
|
|
</pre>
|
|
fails, because it matches the entire string owing to the greediness of the .*
|
|
item.
|
|
</P>
|
|
<P>
|
|
However, if a quantifier is followed by a question mark, it ceases to be
|
|
greedy, and instead matches the minimum number of times possible, so the
|
|
pattern
|
|
<pre>
|
|
/\*.*?\*/
|
|
</pre>
|
|
does the right thing with the C comments. The meaning of the various
|
|
quantifiers is not otherwise changed, just the preferred number of matches.
|
|
Do not confuse this use of question mark with its use as a quantifier in its
|
|
own right. Because it has two uses, it can sometimes appear doubled, as in
|
|
<pre>
|
|
\d??\d
|
|
</pre>
|
|
which matches one digit by preference, but can match two if that is the only
|
|
way the rest of the pattern matches.
|
|
</P>
|
|
<P>
|
|
If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
|
|
the quantifiers are not greedy by default, but individual ones can be made
|
|
greedy by following them with a question mark. In other words, it inverts the
|
|
default behaviour.
|
|
</P>
|
|
<P>
|
|
When a parenthesized subpattern is quantified with a minimum repeat count that
|
|
is greater than 1 or with a limited maximum, more memory is required for the
|
|
compiled pattern, in proportion to the size of the minimum or maximum.
|
|
</P>
|
|
<P>
|
|
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
|
|
to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
|
|
implicitly anchored, because whatever follows will be tried against every
|
|
character position in the subject string, so there is no point in retrying the
|
|
overall match at any position after the first. PCRE normally treats such a
|
|
pattern as though it were preceded by \A.
|
|
</P>
|
|
<P>
|
|
In cases where it is known that the subject string contains no newlines, it is
|
|
worth setting PCRE_DOTALL in order to obtain this optimization, or
|
|
alternatively using ^ to indicate anchoring explicitly.
|
|
</P>
|
|
<P>
|
|
However, there is one situation where the optimization cannot be used. When .*
|
|
is inside capturing parentheses that are the subject of a backreference
|
|
elsewhere in the pattern, a match at the start may fail where a later one
|
|
succeeds. Consider, for example:
|
|
<pre>
|
|
(.*)abc\1
|
|
</pre>
|
|
If the subject is "xyz123abc123" the match point is the fourth character. For
|
|
this reason, such a pattern is not implicitly anchored.
|
|
</P>
|
|
<P>
|
|
When a capturing subpattern is repeated, the value captured is the substring
|
|
that matched the final iteration. For example, after
|
|
<pre>
|
|
(tweedle[dume]{3}\s*)+
|
|
</pre>
|
|
has matched "tweedledum tweedledee" the value of the captured substring is
|
|
"tweedledee". However, if there are nested capturing subpatterns, the
|
|
corresponding captured values may have been set in previous iterations. For
|
|
example, after
|
|
<pre>
|
|
/(a|(b))+/
|
|
</pre>
|
|
matches "aba" the value of the second captured substring is "b".
|
|
<a name="atomicgroup"></a></P>
|
|
<br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
|
|
<P>
|
|
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
|
|
repetition, failure of what follows normally causes the repeated item to be
|
|
re-evaluated to see if a different number of repeats allows the rest of the
|
|
pattern to match. Sometimes it is useful to prevent this, either to change the
|
|
nature of the match, or to cause it fail earlier than it otherwise might, when
|
|
the author of the pattern knows there is no point in carrying on.
|
|
</P>
|
|
<P>
|
|
Consider, for example, the pattern \d+foo when applied to the subject line
|
|
<pre>
|
|
123456bar
|
|
</pre>
|
|
After matching all 6 digits and then failing to match "foo", the normal
|
|
action of the matcher is to try again with only 5 digits matching the \d+
|
|
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
|
|
(a term taken from Jeffrey Friedl's book) provides the means for specifying
|
|
that once a subpattern has matched, it is not to be re-evaluated in this way.
|
|
</P>
|
|
<P>
|
|
If we use atomic grouping for the previous example, the matcher gives up
|
|
immediately on failing to match "foo" the first time. The notation is a kind of
|
|
special parenthesis, starting with (?> as in this example:
|
|
<pre>
|
|
(?>\d+)foo
|
|
</pre>
|
|
This kind of parenthesis "locks up" the part of the pattern it contains once
|
|
it has matched, and a failure further into the pattern is prevented from
|
|
backtracking into it. Backtracking past it to previous items, however, works as
|
|
normal.
|
|
</P>
|
|
<P>
|
|
An alternative description is that a subpattern of this type matches the string
|
|
of characters that an identical standalone pattern would match, if anchored at
|
|
the current point in the subject string.
|
|
</P>
|
|
<P>
|
|
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
|
|
the above example can be thought of as a maximizing repeat that must swallow
|
|
everything it can. So, while both \d+ and \d+? are prepared to adjust the
|
|
number of digits they match in order to make the rest of the pattern match,
|
|
(?>\d+) can only match an entire sequence of digits.
|
|
</P>
|
|
<P>
|
|
Atomic groups in general can of course contain arbitrarily complicated
|
|
subpatterns, and can be nested. However, when the subpattern for an atomic
|
|
group is just a single repeated item, as in the example above, a simpler
|
|
notation, called a "possessive quantifier" can be used. This consists of an
|
|
additional + character following a quantifier. Using this notation, the
|
|
previous example can be rewritten as
|
|
<pre>
|
|
\d++foo
|
|
</pre>
|
|
Note that a possessive quantifier can be used with an entire group, for
|
|
example:
|
|
<pre>
|
|
(abc|xyz){2,3}+
|
|
</pre>
|
|
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
|
|
quantifier and the equivalent atomic group, though there may be a performance
|
|
difference; possessive quantifiers should be slightly faster.
|
|
</P>
|
|
<P>
|
|
The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
|
|
Jeffrey Friedl originated the idea (and the name) in the first edition of his
|
|
book. Mike McCloskey liked it, so implemented it when he built Sun's Java
|
|
package, and PCRE copied it from there. It ultimately found its way into Perl
|
|
at release 5.10.
|
|
</P>
|
|
<P>
|
|
PCRE has an optimization that automatically "possessifies" certain simple
|
|
pattern constructs. For example, the sequence A+B is treated as A++B because
|
|
there is no point in backtracking into a sequence of A's when B must follow.
|
|
</P>
|
|
<P>
|
|
When a pattern contains an unlimited repeat inside a subpattern that can itself
|
|
be repeated an unlimited number of times, the use of an atomic group is the
|
|
only way to avoid some failing matches taking a very long time indeed. The
|
|
pattern
|
|
<pre>
|
|
(\D+|<\d+>)*[!?]
|
|
</pre>
|
|
matches an unlimited number of substrings that either consist of non-digits, or
|
|
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
|
|
quickly. However, if it is applied to
|
|
<pre>
|
|
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
|
|
</pre>
|
|
it takes a long time before reporting failure. This is because the string can
|
|
be divided between the internal \D+ repeat and the external * repeat in a
|
|
large number of ways, and all have to be tried. (The example uses [!?] rather
|
|
than a single character at the end, because both PCRE and Perl have an
|
|
optimization that allows for fast failure when a single character is used. They
|
|
remember the last single character that is required for a match, and fail early
|
|
if it is not present in the string.) If the pattern is changed so that it uses
|
|
an atomic group, like this:
|
|
<pre>
|
|
((?>\D+)|<\d+>)*[!?]
|
|
</pre>
|
|
sequences of non-digits cannot be broken, and failure happens quickly.
|
|
<a name="backreferences"></a></P>
|
|
<br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
|
|
<P>
|
|
Outside a character class, a backslash followed by a digit greater than 0 (and
|
|
possibly further digits) is a back reference to a capturing subpattern earlier
|
|
(that is, to its left) in the pattern, provided there have been that many
|
|
previous capturing left parentheses.
|
|
</P>
|
|
<P>
|
|
However, if the decimal number following the backslash is less than 10, it is
|
|
always taken as a back reference, and causes an error only if there are not
|
|
that many capturing left parentheses in the entire pattern. In other words, the
|
|
parentheses that are referenced need not be to the left of the reference for
|
|
numbers less than 10. A "forward back reference" of this type can make sense
|
|
when a repetition is involved and the subpattern to the right has participated
|
|
in an earlier iteration.
|
|
</P>
|
|
<P>
|
|
It is not possible to have a numerical "forward back reference" to a subpattern
|
|
whose number is 10 or more using this syntax because a sequence such as \50 is
|
|
interpreted as a character defined in octal. See the subsection entitled
|
|
"Non-printing characters"
|
|
<a href="#digitsafterbackslash">above</a>
|
|
for further details of the handling of digits following a backslash. There is
|
|
no such problem when named parentheses are used. A back reference to any
|
|
subpattern is possible using named parentheses (see below).
|
|
</P>
|
|
<P>
|
|
Another way of avoiding the ambiguity inherent in the use of digits following a
|
|
backslash is to use the \g escape sequence, which is a feature introduced in
|
|
Perl 5.10. This escape must be followed by an unsigned number or a negative
|
|
number, optionally enclosed in braces. These examples are all identical:
|
|
<pre>
|
|
(ring), \1
|
|
(ring), \g1
|
|
(ring), \g{1}
|
|
</pre>
|
|
An unsigned number specifies an absolute reference without the ambiguity that
|
|
is present in the older syntax. It is also useful when literal digits follow
|
|
the reference. A negative number is a relative reference. Consider this
|
|
example:
|
|
<pre>
|
|
(abc(def)ghi)\g{-1}
|
|
</pre>
|
|
The sequence \g{-1} is a reference to the most recently started capturing
|
|
subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
|
|
would be equivalent to \1. The use of relative references can be helpful in
|
|
long patterns, and also in patterns that are created by joining together
|
|
fragments that contain references within themselves.
|
|
</P>
|
|
<P>
|
|
A back reference matches whatever actually matched the capturing subpattern in
|
|
the current subject string, rather than anything matching the subpattern
|
|
itself (see
|
|
<a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
|
|
below for a way of doing that). So the pattern
|
|
<pre>
|
|
(sens|respons)e and \1ibility
|
|
</pre>
|
|
matches "sense and sensibility" and "response and responsibility", but not
|
|
"sense and responsibility". If caseful matching is in force at the time of the
|
|
back reference, the case of letters is relevant. For example,
|
|
<pre>
|
|
((?i)rah)\s+\1
|
|
</pre>
|
|
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
|
|
capturing subpattern is matched caselessly.
|
|
</P>
|
|
<P>
|
|
There are several different ways of writing back references to named
|
|
subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
|
|
\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
|
|
back reference syntax, in which \g can be used for both numeric and named
|
|
references, is also supported. We could rewrite the above example in any of
|
|
the following ways:
|
|
<pre>
|
|
(?<p1>(?i)rah)\s+\k<p1>
|
|
(?'p1'(?i)rah)\s+\k{p1}
|
|
(?P<p1>(?i)rah)\s+(?P=p1)
|
|
(?<p1>(?i)rah)\s+\g{p1}
|
|
</pre>
|
|
A subpattern that is referenced by name may appear in the pattern before or
|
|
after the reference.
|
|
</P>
|
|
<P>
|
|
There may be more than one back reference to the same subpattern. If a
|
|
subpattern has not actually been used in a particular match, any back
|
|
references to it always fail. For example, the pattern
|
|
<pre>
|
|
(a|(bc))\2
|
|
</pre>
|
|
always fails if it starts to match "a" rather than "bc". Because there may be
|
|
many capturing parentheses in a pattern, all digits following the backslash are
|
|
taken as part of a potential back reference number. If the pattern continues
|
|
with a digit character, some delimiter must be used to terminate the back
|
|
reference. If the PCRE_EXTENDED option is set, this can be whitespace.
|
|
Otherwise an empty comment (see
|
|
<a href="#comments">"Comments"</a>
|
|
below) can be used.
|
|
</P>
|
|
<P>
|
|
A back reference that occurs inside the parentheses to which it refers fails
|
|
when the subpattern is first used, so, for example, (a\1) never matches.
|
|
However, such references can be useful inside repeated subpatterns. For
|
|
example, the pattern
|
|
<pre>
|
|
(a|b\1)+
|
|
</pre>
|
|
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
|
|
the subpattern, the back reference matches the character string corresponding
|
|
to the previous iteration. In order for this to work, the pattern must be such
|
|
that the first iteration does not need to match the back reference. This can be
|
|
done using alternation, as in the example above, or by a quantifier with a
|
|
minimum of zero.
|
|
<a name="bigassertions"></a></P>
|
|
<br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
|
|
<P>
|
|
An assertion is a test on the characters following or preceding the current
|
|
matching point that does not actually consume any characters. The simple
|
|
assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
|
|
<a href="#smallassertions">above.</a>
|
|
</P>
|
|
<P>
|
|
More complicated assertions are coded as subpatterns. There are two kinds:
|
|
those that look ahead of the current position in the subject string, and those
|
|
that look behind it. An assertion subpattern is matched in the normal way,
|
|
except that it does not cause the current matching position to be changed.
|
|
</P>
|
|
<P>
|
|
Assertion subpatterns are not capturing subpatterns, and may not be repeated,
|
|
because it makes no sense to assert the same thing several times. If any kind
|
|
of assertion contains capturing subpatterns within it, these are counted for
|
|
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.
|
|
</P>
|
|
<br><b>
|
|
Lookahead assertions
|
|
</b><br>
|
|
<P>
|
|
Lookahead assertions start with (?= for positive assertions and (?! for
|
|
negative assertions. For example,
|
|
<pre>
|
|
\w+(?=;)
|
|
</pre>
|
|
matches a word followed by a semicolon, but does not include the semicolon in
|
|
the match, and
|
|
<pre>
|
|
foo(?!bar)
|
|
</pre>
|
|
matches any occurrence of "foo" that is not followed by "bar". Note that the
|
|
apparently similar pattern
|
|
<pre>
|
|
(?!foo)bar
|
|
</pre>
|
|
does not find an occurrence of "bar" that is preceded by something other than
|
|
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
|
|
(?!foo) is always true when the next three characters are "bar". A
|
|
lookbehind assertion is needed to achieve the other effect.
|
|
</P>
|
|
<P>
|
|
If you want to force a matching failure at some point in a pattern, the most
|
|
convenient way to do it is with (?!) because an empty string always matches, so
|
|
an assertion that requires there not to be an empty string must always fail.
|
|
<a name="lookbehind"></a></P>
|
|
<br><b>
|
|
Lookbehind assertions
|
|
</b><br>
|
|
<P>
|
|
Lookbehind assertions start with (?<= for positive assertions and (?<! for
|
|
negative assertions. For example,
|
|
<pre>
|
|
(?<!foo)bar
|
|
</pre>
|
|
does find an occurrence of "bar" that is not preceded by "foo". The contents of
|
|
a lookbehind assertion are restricted such that all the strings it matches must
|
|
have a fixed length. However, if there are several top-level alternatives, they
|
|
do not all have to have the same fixed length. Thus
|
|
<pre>
|
|
(?<=bullock|donkey)
|
|
</pre>
|
|
is permitted, but
|
|
<pre>
|
|
(?<!dogs?|cats?)
|
|
</pre>
|
|
causes an error at compile time. Branches that match different length strings
|
|
are permitted only at the top level of a lookbehind assertion. This is an
|
|
extension compared with Perl (at least for 5.8), which requires all branches to
|
|
match the same length of string. An assertion such as
|
|
<pre>
|
|
(?<=ab(c|de))
|
|
</pre>
|
|
is not permitted, because its single top-level branch can match two different
|
|
lengths, but it is acceptable if rewritten to use two top-level branches:
|
|
<pre>
|
|
(?<=abc|abde)
|
|
</pre>
|
|
In some cases, the Perl 5.10 escape sequence \K
|
|
<a href="#resetmatchstart">(see above)</a>
|
|
can be used instead of a lookbehind assertion; this is not restricted to a
|
|
fixed-length.
|
|
</P>
|
|
<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
|
|
assertion fails.
|
|
</P>
|
|
<P>
|
|
PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
|
|
to appear in lookbehind assertions, because it makes it impossible to calculate
|
|
the length of the lookbehind. The \X and \R escapes, which can match
|
|
different numbers of bytes, are also not permitted.
|
|
</P>
|
|
<P>
|
|
Possessive quantifiers can be used in conjunction with lookbehind assertions to
|
|
specify efficient matching at the end of the subject string. Consider a simple
|
|
pattern such as
|
|
<pre>
|
|
abcd$
|
|
</pre>
|
|
when applied to a long string that does not match. Because matching proceeds
|
|
from left to right, PCRE will look for each "a" in the subject and then see if
|
|
what follows matches the rest of the pattern. If the pattern is specified as
|
|
<pre>
|
|
^.*abcd$
|
|
</pre>
|
|
the initial .* matches the entire string at first, but when this fails (because
|
|
there is no following "a"), it backtracks to match all but the last character,
|
|
then all but the last two characters, and so on. Once again the search for "a"
|
|
covers the entire string, from right to left, so we are no better off. However,
|
|
if the pattern is written as
|
|
<pre>
|
|
^.*+(?<=abcd)
|
|
</pre>
|
|
there can be no backtracking for the .*+ item; it can match only the entire
|
|
string. The subsequent lookbehind assertion does a single test on the last four
|
|
characters. If it fails, the match fails immediately. For long strings, this
|
|
approach makes a significant difference to the processing time.
|
|
</P>
|
|
<br><b>
|
|
Using multiple assertions
|
|
</b><br>
|
|
<P>
|
|
Several assertions (of any sort) may occur in succession. For example,
|
|
<pre>
|
|
(?<=\d{3})(?<!999)foo
|
|
</pre>
|
|
matches "foo" preceded by three digits that are not "999". Notice that each of
|
|
the assertions is applied independently at the same point in the subject
|
|
string. First there is a check that the previous three characters are all
|
|
digits, and then there is a check that the same three characters are not "999".
|
|
This pattern does <i>not</i> match "foo" preceded by six characters, the first
|
|
of which are digits and the last three of which are not "999". For example, it
|
|
doesn't match "123abcfoo". A pattern to do that is
|
|
<pre>
|
|
(?<=\d{3}...)(?<!999)foo
|
|
</pre>
|
|
This time the first assertion looks at the preceding six characters, checking
|
|
that the first three are digits, and then the second assertion checks that the
|
|
preceding three characters are not "999".
|
|
</P>
|
|
<P>
|
|
Assertions can be nested in any combination. For example,
|
|
<pre>
|
|
(?<=(?<!foo)bar)baz
|
|
</pre>
|
|
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
|
|
preceded by "foo", while
|
|
<pre>
|
|
(?<=\d{3}(?!999)...)foo
|
|
</pre>
|
|
is another pattern that matches "foo" preceded by three digits and any three
|
|
characters that are not "999".
|
|
<a name="conditions"></a></P>
|
|
<br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
|
|
<P>
|
|
It is possible to cause the matching process to obey a subpattern
|
|
conditionally or to choose between two alternative subpatterns, depending on
|
|
the result of an assertion, or whether a previous capturing subpattern matched
|
|
or not. The two possible forms of conditional subpattern are
|
|
<pre>
|
|
(?(condition)yes-pattern)
|
|
(?(condition)yes-pattern|no-pattern)
|
|
</pre>
|
|
If the condition is satisfied, the yes-pattern is used; otherwise the
|
|
no-pattern (if present) is used. If there are more than two alternatives in the
|
|
subpattern, a compile-time error occurs.
|
|
</P>
|
|
<P>
|
|
There are four kinds of condition: references to subpatterns, references to
|
|
recursion, a pseudo-condition called DEFINE, and assertions.
|
|
</P>
|
|
<br><b>
|
|
Checking for a used subpattern by number
|
|
</b><br>
|
|
<P>
|
|
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>
|
|
<P>
|
|
Consider the following pattern, which contains non-significant white space to
|
|
make it more readable (assume the PCRE_EXTENDED option) and to divide it into
|
|
three parts for ease of discussion:
|
|
<pre>
|
|
( \( )? [^()]+ (?(1) \) )
|
|
</pre>
|
|
The first part matches an optional opening parenthesis, and if that
|
|
character is present, sets it as the first captured substring. The second part
|
|
matches one or more characters that are not parentheses. The third part is a
|
|
conditional subpattern that tests whether the first set of parentheses matched
|
|
or not. If they did, that is, if subject started with an opening parenthesis,
|
|
the condition is true, and so the yes-pattern is executed and a closing
|
|
parenthesis is required. Otherwise, since no-pattern is not present, the
|
|
subpattern matches nothing. In other words, this pattern matches a sequence of
|
|
non-parentheses, optionally enclosed in parentheses.
|
|
</P>
|
|
<P>
|
|
If you were embedding this pattern in a larger one, you could use a relative
|
|
reference:
|
|
<pre>
|
|
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
|
|
</pre>
|
|
This makes the fragment independent of the parentheses in the larger pattern.
|
|
</P>
|
|
<br><b>
|
|
Checking for a used subpattern by name
|
|
</b><br>
|
|
<P>
|
|
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
|
|
subpattern by name. For compatibility with earlier versions of PCRE, which had
|
|
this facility before Perl, the syntax (?(name)...) is also recognized. However,
|
|
there is a possible ambiguity with this syntax, because subpattern names may
|
|
consist entirely of digits. PCRE looks first for a named subpattern; if it
|
|
cannot find one and the name consists entirely of digits, PCRE looks for a
|
|
subpattern of that number, which must be greater than zero. Using subpattern
|
|
names that consist entirely of digits is not recommended.
|
|
</P>
|
|
<P>
|
|
Rewriting the above example to use a named subpattern gives this:
|
|
<pre>
|
|
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
|
|
|
|
</PRE>
|
|
</P>
|
|
<br><b>
|
|
Checking for pattern recursion
|
|
</b><br>
|
|
<P>
|
|
If the condition is the string (R), and there is no subpattern with the name R,
|
|
the condition is true if a recursive call to the whole pattern or any
|
|
subpattern has been made. If digits or a name preceded by ampersand follow the
|
|
letter R, for example:
|
|
<pre>
|
|
(?(R3)...) or (?(R&name)...)
|
|
</pre>
|
|
the condition is true if the most recent recursion is into the subpattern whose
|
|
number or name is given. This condition does not check the entire recursion
|
|
stack.
|
|
</P>
|
|
<P>
|
|
At "top level", all these recursion test conditions are false. Recursive
|
|
patterns are described below.
|
|
</P>
|
|
<br><b>
|
|
Defining subpatterns for use by reference only
|
|
</b><br>
|
|
<P>
|
|
If the condition is the string (DEFINE), and there is no subpattern with the
|
|
name DEFINE, the condition is always false. In this case, there may be only one
|
|
alternative in the subpattern. It is always skipped if control reaches this
|
|
point in the pattern; the idea of DEFINE is that it can be used to define
|
|
"subroutines" that can be referenced from elsewhere. (The use of "subroutines"
|
|
is described below.) For example, a pattern to match an IPv4 address could be
|
|
written like this (ignore whitespace and line breaks):
|
|
<pre>
|
|
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
|
|
\b (?&byte) (\.(?&byte)){3} \b
|
|
</pre>
|
|
The first part of the pattern is a DEFINE group inside which a another group
|
|
named "byte" is defined. This matches an individual component of an IPv4
|
|
address (a number less than 256). When matching takes place, this part of the
|
|
pattern is skipped because DEFINE acts like a false condition.
|
|
</P>
|
|
<P>
|
|
The rest of the pattern uses references to the named group to match the four
|
|
dot-separated components of an IPv4 address, insisting on a word boundary at
|
|
each end.
|
|
</P>
|
|
<br><b>
|
|
Assertion conditions
|
|
</b><br>
|
|
<P>
|
|
If the condition is not in any of the above formats, it must be an assertion.
|
|
This may be a positive or negative lookahead or lookbehind assertion. Consider
|
|
this pattern, again containing non-significant white space, and with the two
|
|
alternatives on the second line:
|
|
<pre>
|
|
(?(?=[^a-z]*[a-z])
|
|
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
|
|
</pre>
|
|
The condition is a positive lookahead assertion that matches an optional
|
|
sequence of non-letters followed by a letter. In other words, it tests for the
|
|
presence of at least one letter in the subject. If a letter is found, the
|
|
subject is matched against the first alternative; otherwise it is matched
|
|
against the second. This pattern matches strings in one of the two forms
|
|
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
|
|
<a name="comments"></a></P>
|
|
<br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
|
|
<P>
|
|
The sequence (?# marks the start of a comment that continues up to the next
|
|
closing parenthesis. Nested parentheses are not permitted. The characters
|
|
that make up a comment play no part in the pattern matching at all.
|
|
</P>
|
|
<P>
|
|
If the PCRE_EXTENDED option is set, an unescaped # character outside a
|
|
character class introduces a comment that continues to immediately after the
|
|
next newline in the pattern.
|
|
<a name="recursion"></a></P>
|
|
<br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
|
|
<P>
|
|
Consider the problem of matching a string in parentheses, allowing for
|
|
unlimited nested parentheses. Without the use of recursion, the best that can
|
|
be done is to use a pattern that matches up to some fixed depth of nesting. It
|
|
is not possible to handle an arbitrary nesting depth.
|
|
</P>
|
|
<P>
|
|
For some time, Perl has provided a facility that allows regular expressions to
|
|
recurse (amongst other things). It does this by interpolating Perl code in the
|
|
expression at run time, and the code can refer to the expression itself. A Perl
|
|
pattern using code interpolation to solve the parentheses problem can be
|
|
created like this:
|
|
<pre>
|
|
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
|
|
</pre>
|
|
The (?p{...}) item interpolates Perl code at run time, and in this case refers
|
|
recursively to the pattern in which it appears.
|
|
</P>
|
|
<P>
|
|
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
|
|
supports special syntax for recursion of the entire pattern, and also for
|
|
individual subpattern recursion. After its introduction in PCRE and Python,
|
|
this kind of recursion was introduced into Perl at release 5.10.
|
|
</P>
|
|
<P>
|
|
A special item that consists of (? followed by a number greater than zero and a
|
|
closing parenthesis is a recursive call of the subpattern of the given number,
|
|
provided that it occurs inside that subpattern. (If not, it is a "subroutine"
|
|
call, which is described in the next section.) The special item (?R) or (?0) is
|
|
a recursive call of the entire regular expression.
|
|
</P>
|
|
<P>
|
|
In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
|
|
treated as an atomic group. That is, once it has matched some of the subject
|
|
string, it is never re-entered, even if it contains untried alternatives and
|
|
there is a subsequent matching failure.
|
|
</P>
|
|
<P>
|
|
This PCRE pattern solves the nested parentheses problem (assume the
|
|
PCRE_EXTENDED option is set so that white space is ignored):
|
|
<pre>
|
|
\( ( (?>[^()]+) | (?R) )* \)
|
|
</pre>
|
|
First it matches an opening parenthesis. Then it matches any number of
|
|
substrings which can either be a sequence of non-parentheses, or a recursive
|
|
match of the pattern itself (that is, a correctly parenthesized substring).
|
|
Finally there is a closing parenthesis.
|
|
</P>
|
|
<P>
|
|
If this were part of a larger pattern, you would not want to recurse the entire
|
|
pattern, so instead you could use this:
|
|
<pre>
|
|
( \( ( (?>[^()]+) | (?1) )* \) )
|
|
</pre>
|
|
We have put the pattern into parentheses, and caused the recursion to refer to
|
|
them instead of the whole pattern.
|
|
</P>
|
|
<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.)
|
|
Instead of (?1) in the pattern above you can write (?-2) to refer to the second
|
|
most recently opened parentheses preceding the recursion. In other words, a
|
|
negative number counts capturing parentheses leftwards from the point at which
|
|
it is encountered.
|
|
</P>
|
|
<P>
|
|
It is also possible to refer to subsequently opened parentheses, by writing
|
|
references such as (?+2). However, these cannot be recursive because the
|
|
reference is not inside the parentheses that are referenced. They are always
|
|
"subroutine" calls, as described in the next section.
|
|
</P>
|
|
<P>
|
|
An alternative approach is to use named parentheses instead. The Perl syntax
|
|
for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
|
|
could rewrite the above example as follows:
|
|
<pre>
|
|
(?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
|
|
</pre>
|
|
If there is more than one subpattern with the same name, the earliest one is
|
|
used.
|
|
</P>
|
|
<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
|
|
non-parentheses is important when applying the pattern to strings that do not
|
|
match. For example, when this pattern is applied to
|
|
<pre>
|
|
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
|
|
</pre>
|
|
it yields "no match" quickly. However, if atomic grouping is not used,
|
|
the match runs for a very long time indeed because there are so many different
|
|
ways the + and * repeats can carve up the subject, and all have to be tested
|
|
before failure can be reported.
|
|
</P>
|
|
<P>
|
|
At the end of a match, the values set for any capturing subpatterns are those
|
|
from the outermost level of the recursion at which the subpattern value is set.
|
|
If you want to obtain intermediate values, a callout function can be used (see
|
|
below and the
|
|
<a href="pcrecallout.html"><b>pcrecallout</b></a>
|
|
documentation). If the pattern above is matched against
|
|
<pre>
|
|
(ab(cd)ef)
|
|
</pre>
|
|
the value for the capturing parentheses is "ef", which is the last value taken
|
|
on at the top level. If additional parentheses are added, giving
|
|
<pre>
|
|
\( ( ( (?>[^()]+) | (?R) )* ) \)
|
|
^ ^
|
|
^ ^
|
|
</pre>
|
|
the string they capture is "ab(cd)ef", the contents of the top level
|
|
parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
|
|
has to obtain extra memory to store data during a recursion, which it does by
|
|
using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
|
|
memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
|
|
</P>
|
|
<P>
|
|
Do not confuse the (?R) item with the condition (R), which tests for recursion.
|
|
Consider this pattern, which matches text in angle brackets, allowing for
|
|
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
|
|
recursing), whereas any characters are permitted at the outer level.
|
|
<pre>
|
|
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
|
|
</pre>
|
|
In this pattern, (?(R) is the start of a conditional subpattern, with two
|
|
different alternatives for the recursive and non-recursive cases. The (?R) item
|
|
is the actual recursive call.
|
|
<a name="subpatternsassubroutines"></a></P>
|
|
<br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
|
|
<P>
|
|
If the syntax for a recursive subpattern reference (either by number or by
|
|
name) is used outside the parentheses to which it refers, it operates like a
|
|
subroutine in a programming language. The "called" subpattern may be defined
|
|
before or after the reference. A numbered reference can be absolute or
|
|
relative, as in these examples:
|
|
<pre>
|
|
(...(absolute)...)...(?2)...
|
|
(...(relative)...)...(?-1)...
|
|
(...(?+1)...(relative)...
|
|
</pre>
|
|
An earlier example pointed out that the pattern
|
|
<pre>
|
|
(sens|respons)e and \1ibility
|
|
</pre>
|
|
matches "sense and sensibility" and "response and responsibility", but not
|
|
"sense and responsibility". If instead the pattern
|
|
<pre>
|
|
(sens|respons)e and (?1)ibility
|
|
</pre>
|
|
is used, it does match "sense and responsibility" as well as the other two
|
|
strings. Another example is given in the discussion of DEFINE above.
|
|
</P>
|
|
<P>
|
|
Like recursive subpatterns, a "subroutine" call is always treated as an atomic
|
|
group. That is, once it has matched some of the subject string, it is never
|
|
re-entered, even if it contains untried alternatives and there is a subsequent
|
|
matching failure.
|
|
</P>
|
|
<P>
|
|
When a subpattern is used as a subroutine, processing options such as
|
|
case-independence are fixed when the subpattern is defined. They cannot be
|
|
changed for different calls. For example, consider this pattern:
|
|
<pre>
|
|
(abc)(?i:(?-1))
|
|
</pre>
|
|
It matches "abcabc". It does not match "abcABC" because the change of
|
|
processing option does not affect the called subpattern.
|
|
<a name="onigurumasubroutines"></a></P>
|
|
<br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
|
|
<P>
|
|
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
|
|
a number enclosed either in angle brackets or single quotes, is an alternative
|
|
syntax for referencing a subpattern as a subroutine, possibly recursively. Here
|
|
are two of the examples used above, rewritten using this syntax:
|
|
<pre>
|
|
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
|
|
(sens|respons)e and \g'1'ibility
|
|
</pre>
|
|
PCRE supports an extension to Oniguruma: if a number is preceded by a
|
|
plus or a minus sign it is taken as a relative reference. For example:
|
|
<pre>
|
|
(abc)(?i:\g<-1>)
|
|
</pre>
|
|
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
|
|
synonymous. The former is a back reference; the latter is a subroutine call.
|
|
</P>
|
|
<br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
|
|
<P>
|
|
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
|
|
code to be obeyed in the middle of matching a regular expression. This makes it
|
|
possible, amongst other things, to extract different substrings that match the
|
|
same pair of parentheses when there is a repetition.
|
|
</P>
|
|
<P>
|
|
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
|
|
code. The feature is called "callout". The caller of PCRE provides an external
|
|
function by putting its entry point in the global variable <i>pcre_callout</i>.
|
|
By default, this variable contains NULL, which disables all calling out.
|
|
</P>
|
|
<P>
|
|
Within a regular expression, (?C) indicates the points at which the external
|
|
function is to be called. If you want to identify different callout points, you
|
|
can put a number less than 256 after the letter C. The default value is zero.
|
|
For example, this pattern has two callout points:
|
|
<pre>
|
|
(?C1)abc(?C2)def
|
|
</pre>
|
|
If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
|
|
automatically installed before each item in the pattern. They are all numbered
|
|
255.
|
|
</P>
|
|
<P>
|
|
During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
|
|
set), the external function is called. It is provided with the number of the
|
|
callout, the position in the pattern, and, optionally, one item of data
|
|
originally supplied by the caller of <b>pcre_exec()</b>. The callout function
|
|
may cause matching to proceed, to backtrack, or to fail altogether. A complete
|
|
description of the interface to the callout function is given in the
|
|
<a href="pcrecallout.html"><b>pcrecallout</b></a>
|
|
documentation.
|
|
</P>
|
|
<br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
|
|
<P>
|
|
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>
|
|
<P>
|
|
Since these verbs are specifically related to backtracking, most of them can be
|
|
used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
|
|
a backtracking algorithm. With the exception of (*FAIL), which behaves like a
|
|
failing negative assertion, they cause an error if encountered by
|
|
<b>pcre_dfa_exec()</b>.
|
|
</P>
|
|
<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:
|
|
</P>
|
|
<br><b>
|
|
Verbs that act immediately
|
|
</b><br>
|
|
<P>
|
|
The following verbs act as soon as they are encountered:
|
|
<pre>
|
|
(*ACCEPT)
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
A(A|B(*ACCEPT)|C)D
|
|
</pre>
|
|
This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
|
|
captured.
|
|
<pre>
|
|
(*FAIL) or (*F)
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
a+(?C)(*FAIL)
|
|
</pre>
|
|
A match with the string "aaaa" always fails, but the callout is taken before
|
|
each backtrack happens (in this example, 10 times).
|
|
</P>
|
|
<br><b>
|
|
Verbs that act after backtracking
|
|
</b><br>
|
|
<P>
|
|
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.
|
|
<pre>
|
|
(*COMMIT)
|
|
</pre>
|
|
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, <b>pcre_exec()</b> is committed to finding a match at the current
|
|
starting point, or not at all. For example:
|
|
<pre>
|
|
a+(*COMMIT)b
|
|
</pre>
|
|
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."
|
|
<pre>
|
|
(*PRUNE)
|
|
</pre>
|
|
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.
|
|
<pre>
|
|
(*SKIP)
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
a+(*SKIP)b
|
|
</pre>
|
|
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".
|
|
<pre>
|
|
(*THEN)
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
|
|
</pre>
|
|
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).
|
|
</P>
|
|
<br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
|
|
<P>
|
|
<b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
|
|
</P>
|
|
<br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge CB2 3QH, England.
|
|
<br>
|
|
</P>
|
|
<br><a name="SEC28" href="#TOC1">REVISION</a><br>
|
|
<P>
|
|
Last updated: 11 April 2009
|
|
<br>
|
|
Copyright © 1997-2009 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE index page</a>.
|
|
</p>
|