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<html>
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<title>pcrecompat specification</title>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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This HTML document has been generated automatically from the original man page.
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If there is any nonsense in it, please consult the man page, in case the
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conversion went wrong.<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">DIFFERENCES FROM PERL</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">DIFFERENCES FROM PERL</a><br>
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<P>
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This document describes the differences in the ways that PCRE and Perl handle
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regular expressions. The differences described here are with respect to Perl
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5.8.
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</P>
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<P>
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1. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
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them, but they do not mean what you might think. For example, (?!a){3} does
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not assert that the next three characters are not "a". It just asserts that the
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next character is not "a" three times.
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</P>
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<P>
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2. Capturing subpatterns that occur inside negative lookahead assertions are
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counted, but their entries in the offsets vector are never set. Perl sets its
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numerical variables from any such patterns that are matched before the
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assertion fails to match something (thereby succeeding), but only if the
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negative lookahead assertion contains just one branch.
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</P>
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<P>
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3. Though binary zero characters are supported in the subject string, they are
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not allowed in a pattern string because it is passed as a normal C string,
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terminated by zero. The escape sequence "\0" can be used in the pattern to
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represent a binary zero.
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</P>
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<P>
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4. The following Perl escape sequences are not supported: \l, \u, \L,
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\U, \P, \p, \N, and \X. In fact these are implemented by Perl's general
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string-handling and are not part of its pattern matching engine. If any of
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these are encountered by PCRE, an error is generated.
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</P>
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<P>
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5. PCRE does support the \Q...\E escape for quoting substrings. Characters in
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between are treated as literals. This is slightly different from Perl in that $
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and @ are also handled as literals inside the quotes. In Perl, they cause
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variable interpolation (but of course PCRE does not have variables). Note the
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following examples:
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</P>
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<P>
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<pre>
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Pattern PCRE matches Perl matches
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</PRE>
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</P>
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<P>
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<pre>
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\Qabc$xyz\E abc$xyz abc followed by the
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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$xyz
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</PRE>
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</P>
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<P>
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In PCRE, the \Q...\E mechanism is not recognized inside a character class.
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</P>
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<P>
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8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
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constructions. However, there is some experimental support for recursive
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patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the PCRE
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"callout" feature allows an external function to be called during pattern
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matching.
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</P>
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<P>
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9. There are some differences that are concerned with the settings of captured
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strings when part of a pattern is repeated. For example, matching "aba" against
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the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
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</P>
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<P>
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10. PCRE provides some extensions to the Perl regular expression facilities:
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</P>
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<P>
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(a) Although lookbehind assertions must match fixed length strings, each
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alternative branch of a lookbehind assertion can match a different length of
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string. Perl requires them all to have the same length.
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</P>
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<P>
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(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
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meta-character matches only at the very end of the string.
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</P>
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<P>
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© If PCRE_EXTRA is set, a backslash followed by a letter with no special
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meaning is faulted.
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</P>
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<P>
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(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
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inverted, that is, by default they are not greedy, but if followed by a
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question mark they are.
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</P>
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<P>
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(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first
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matching position in the subject string.
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</P>
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<P>
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(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
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options for <b>pcre_exec()</b> have no Perl equivalents.
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</P>
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<P>
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(g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
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matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
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support.)
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</P>
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<P>
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(h) PCRE supports named capturing substrings, using the Python syntax.
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</P>
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<P>
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(i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
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package.
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</P>
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<P>
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(j) The (R) condition, for testing recursion, is a PCRE extension.
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</P>
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<P>
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(k) The callout facility is PCRE-specific.
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</P>
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<P>
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Last updated: 03 February 2003
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<br>
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Copyright © 1997-2003 University of Cambridge.
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