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+<html>
+<head>
+<title>pcrecompat specification</title>
+</head>
+<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
+<h1>pcrecompat man page</h1>
+<p>
+Return to the <a href="index.html">PCRE index page</a>.
+</p>
+<p>
+This page is part of the PCRE HTML documentation. It was generated automatically
+from the original man page. If there is any nonsense in it, please consult the
+man page, in case the conversion went wrong.
+<br>
+<br><b>
+DIFFERENCES BETWEEN PCRE AND PERL
+</b><br>
+<P>
+This document describes the differences in the ways that PCRE and Perl handle
+regular expressions. The differences described here are mainly with respect to
+Perl 5.8, though PCRE versions 7.0 and later contain some features that are
+expected to be in the forthcoming Perl 5.10.
+</P>
+<P>
+1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
+it does have are given in the
+<a href="pcre.html#utf8support">section on UTF-8 support</a>
+in the main
+<a href="pcre.html"><b>pcre</b></a>
+page.
+</P>
+<P>
+2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
+them, but they do not mean what you might think. For example, (?!a){3} does
+not assert that the next three characters are not "a". It just asserts that the
+next character is not "a" three times.
+</P>
+<P>
+3. Capturing subpatterns that occur inside negative lookahead assertions are
+counted, but their entries in the offsets vector are never set. Perl sets its
+numerical variables from any such patterns that are matched before the
+assertion fails to match something (thereby succeeding), but only if the
+negative lookahead assertion contains just one branch.
+</P>
+<P>
+4. Though binary zero characters are supported in the subject string, they are
+not allowed in a pattern string because it is passed as a normal C string,
+terminated by zero. The escape sequence \0 can be used in the pattern to
+represent a binary zero.
+</P>
+<P>
+5. The following Perl escape sequences are not supported: \l, \u, \L,
+\U, and \N. In fact these are implemented by Perl's general string-handling
+and are not part of its pattern matching engine. If any of these are
+encountered by PCRE, an error is generated.
+</P>
+<P>
+6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
+built with Unicode character property support. The properties that can be
+tested with \p and \P are limited to the general category properties such as
+Lu and Nd, script names such as Greek or Han, and the derived properties Any
+and L&.
+</P>
+<P>
+7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
+between are treated as literals. This is slightly different from Perl in that $
+and @ are also handled as literals inside the quotes. In Perl, they cause
+variable interpolation (but of course PCRE does not have variables). Note the
+following examples:
+<pre>
+ Pattern PCRE matches Perl matches
+
+ \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
+ \Qabc\$xyz\E abc\$xyz abc\$xyz
+ \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
+</pre>
+The \Q...\E sequence is recognized both inside and outside character classes.
+</P>
+<P>
+8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
+constructions. However, there is support for recursive patterns. This is not
+available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
+feature allows an external function to be called during pattern matching. See
+the
+<a href="pcrecallout.html"><b>pcrecallout</b></a>
+documentation for details.
+</P>
+<P>
+9. Subpatterns that are called recursively or as "subroutines" are always
+treated as atomic groups in PCRE. This is like Python, but unlike Perl.
+</P>
+<P>
+10. There are some differences that are concerned with the settings of captured
+strings when part of a pattern is repeated. For example, matching "aba" against
+the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
+</P>
+<P>
+11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
+(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
+argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing
+parentheses, PCRE does not set that capture group; this is different to Perl.
+</P>
+<P>
+12. PCRE provides some extensions to the Perl regular expression facilities.
+Perl 5.10 will include new features that are not in earlier versions, some of
+which (such as named parentheses) have been in PCRE for some time. This list is
+with respect to Perl 5.10:
+<br>
+<br>
+(a) Although lookbehind assertions must match fixed length strings, each
+alternative branch of a lookbehind assertion can match a different length of
+string. Perl requires them all to have the same length.
+<br>
+<br>
+(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
+meta-character matches only at the very end of the string.
+<br>
+<br>
+(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
+meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
+(Perl can be made to issue a warning.)
+<br>
+<br>
+(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
+inverted, that is, by default they are not greedy, but if followed by a
+question mark they are.
+<br>
+<br>
+(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
+only at the first matching position in the subject string.
+<br>
+<br>
+(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
+options for <b>pcre_exec()</b> have no Perl equivalents.
+<br>
+<br>
+(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
+by the PCRE_BSR_ANYCRLF option.
+<br>
+<br>
+(h) The callout facility is PCRE-specific.
+<br>
+<br>
+(i) The partial matching facility is PCRE-specific.
+<br>
+<br>
+(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
+different hosts that have the other endianness.
+<br>
+<br>
+(k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
+different way and is not Perl-compatible.
+<br>
+<br>
+(l) PCRE recognizes some special sequences such as (*CR) at the start of
+a pattern that set overall options that cannot be changed within the pattern.
+</P>
+<br><b>
+AUTHOR
+</b><br>
+<P>
+Philip Hazel
+<br>
+University Computing Service
+<br>
+Cambridge CB2 3QH, England.
+<br>
+</P>
+<br><b>
+REVISION
+</b><br>
+<P>
+Last updated: 11 September 2007
+<br>
+Copyright © 1997-2007 University of Cambridge.
+<br>
+<p>
+Return to the <a href="index.html">PCRE index page</a>.
+</p>