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+.TH PCRE 3
+.SH NAME
+PCRE - Perl-compatible regular expressions
+.SH DIFFERENCES FROM PERL
+.rs
+.sp
+This document describes the differences in the ways that PCRE and Perl handle
+regular expressions. The differences described here are with respect to Perl
+5.8.
+
+1. PCRE does not have full UTF-8 support. Details of what it does have are
+given in the
+.\" HTML <a href="pcre.html#utf8support">
+.\" </a>
+section on UTF-8 support
+.\"
+in the main
+.\" HREF
+\fBpcre\fR
+.\"
+page.
+
+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.
+
+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.
+
+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.
+
+5. The following Perl escape sequences are not supported: \\l, \\u, \\L,
+\\U, \\P, \\p, \\N, and \\X. 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.
+
+6. 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:
+
+    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
+
+The \\Q...\\E sequence is recognized both inside and outside character classes.
+
+7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
+constructions. However, there is some experimental support for recursive
+patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the PCRE
+"callout" feature allows an external function to be called during pattern
+matching.
+
+8. 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".
+
+9. PCRE provides some extensions to the Perl regular expression facilities:
+
+(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.
+
+(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.
+
+(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
+meaning is faulted.
+
+(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.
+
+(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first
+matching position in the subject string.
+
+(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
+options for \fBpcre_exec()\fR have no Perl equivalents.
+
+(g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
+matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
+support.)
+
+(h) PCRE supports named capturing substrings, using the Python syntax.
+
+(i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
+package.
+
+(j) The (R) condition, for testing recursion, is a PCRE extension.
+
+(k) The callout facility is PCRE-specific.
+
+.in 0
+Last updated: 09 December 2003
+.br
+Copyright (c) 1997-2003 University of Cambridge.