--- /dev/null
+.TH PCRECOMPAT 3
+.SH NAME
+PCRE - Perl-compatible regular expressions
+.SH "DIFFERENCES BETWEEN PCRE AND 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
+5.10.
+.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
+.\" HTML <a href="pcre.html#utf8support">
+.\" </a>
+section on UTF-8 support
+.\"
+in the main
+.\" HREF
+\fBpcre\fP
+.\"
+page.
+.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
+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
+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 \e0 can be used in the pattern to
+represent a binary zero.
+.P
+5. The following Perl escape sequences are not supported: \el, \eu, \eL,
+\eU, and \eN. 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
+6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
+built with Unicode character property support. The properties that can be
+tested with \ep and \eP 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&. PCRE does support the Cs (surrogate) property, which Perl does not; the
+Perl documentation says "Because Perl hides the need for the user to understand
+the internal representation of Unicode characters, there is no need to
+implement the somewhat messy concept of surrogates."
+.P
+7. PCRE does support the \eQ...\eE 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:
+.sp
+ Pattern PCRE matches Perl matches
+.sp
+.\" JOIN
+ \eQabc$xyz\eE abc$xyz abc followed by the
+ contents of $xyz
+ \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
+ \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
+.sp
+The \eQ...\eE sequence is recognized both inside and outside character classes.
+.P
+8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
+constructions. However, there is support for recursive patterns. This is not
+available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
+feature allows an external function to be called during pattern matching. See
+the
+.\" HREF
+\fBpcrecallout\fP
+.\"
+documentation for details.
+.P
+9. Subpatterns that are called recursively or as "subroutines" are always
+treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
+is a discussion of an example that explains this in more detail in the
+.\" HTML <a href="pcrepattern.html#recursiondifference">
+.\" </a>
+section on recursion differences from Perl
+.\"
+in the
+.\" HREF
+\fBpcrepattern\fP
+.\"
+page.
+.P
+10. There are some differences that are concerned with the settings of captured
+strings when part of a pattern is repeated. For example, matching "aba" against
+the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
+.P
+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).
+.P
+12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
+names is not as general as Perl's. This is a consequence of the fact the PCRE
+works internally just with numbers, using an external table to translate
+between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
+where the two capturing parentheses have the same number but different names,
+is not supported, and causes an error at compile time. If it were allowed, it
+would not be possible to distinguish which parentheses matched, because both
+names map to capturing subpattern number 1. To avoid this confusing situation,
+an error is given at compile time.
+.P
+13. PCRE provides some extensions to the Perl regular expression facilities.
+Perl 5.10 includes new features that are not in earlier versions of Perl, some
+of which (such as named parentheses) have been in PCRE for some time. This list
+is with respect to Perl 5.10:
+.sp
+(a) Although lookbehind assertions in PCRE 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.
+.sp
+(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.
+.sp
+(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.)
+.sp
+(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.
+.sp
+(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.
+.sp
+(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
+PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
+.sp
+(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
+by the PCRE_BSR_ANYCRLF option.
+.sp
+(h) The callout facility is PCRE-specific.
+.sp
+(i) The partial matching facility is PCRE-specific.
+.sp
+(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
+different hosts that have the other endianness.
+.sp
+(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
+different way and is not Perl-compatible.
+.sp
+(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.
+.
+.
+.SH AUTHOR
+.rs
+.sp
+.nf
+Philip Hazel
+University Computing Service
+Cambridge CB2 3QH, England.
+.fi
+.
+.
+.SH REVISION
+.rs
+.sp
+.nf
+Last updated: 04 October 2009
+Copyright (c) 1997-2009 University of Cambridge.
+.fi
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