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+.TH PCRE2COMPAT 3 "28 September 2014" "PCRE2 10.0"
+.SH NAME
+PCRE2 - Perl-compatible regular expressions (revised API)
+.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
+.rs
+.sp
+This document describes the differences in the ways that PCRE2 and Perl handle
+regular expressions. The differences described here are with respect to Perl
+versions 5.10 and above.
+.P
+1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
+have are given in the
+.\" HREF
+\fBpcre2unicode\fP
+.\"
+page.
+.P
+2. PCRE2 allows repeat quantifiers only on parenthesized assertions, 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 (in principle: PCRE2 optimizes this to run the assertion
+just once). Perl allows repeat quantifiers on other assertions such as \eb, but
+these do not seem to have any use.
+.P
+3. Capturing subpatterns that occur inside negative lookahead assertions are
+counted, but their entries in the offsets vector are never set. Perl sometimes
+(but not always) sets its numerical variables from inside negative assertions.
+.P
+4. The following Perl escape sequences are not supported: \el, \eu, \eL,
+\eU, and \eN when followed by a character name or Unicode value. (\eN on its
+own, matching a non-newline character, is supported.) 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 PCRE2, an error is
+generated by default. However, if the PCRE2_ALT_BSUX option is set,
+\eU and \eu are interpreted as ECMAScript interprets them.
+.P
+5. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is
+built with Unicode 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&. PCRE2 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
+6. PCRE2 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 PCRE2 does not have variables).
+Note the following examples:
+.sp
+    Pattern            PCRE2 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
+7. Fairly obviously, PCRE2 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 PCRE2 "callout"
+feature allows an external function to be called during pattern matching. See
+the
+.\" HREF
+\fBpcre2callout\fP
+.\"
+documentation for details.
+.P
+8. Subpatterns that are called as subroutines (whether or not recursively) are
+always treated as atomic groups in PCRE2. This is like Python, but unlike Perl.
+Captured values that are set outside a subroutine call can be reference from
+inside in PCRE2, but not in Perl. There is a discussion that explains these
+differences in more detail in the
+.\" HTML <a href="pcre2pattern.html#recursiondifference">
+.\" </a>
+section on recursion differences from Perl
+.\"
+in the
+.\" HREF
+\fBpcre2pattern\fP
+.\"
+page.
+.P
+9. If any of the backtracking control verbs are used in a subpattern that is
+called as a subroutine (whether or not recursively), their effect is confined
+to that subpattern; it does not extend to the surrounding pattern. This is not
+always the case in Perl. In particular, if (*THEN) is present in a group that
+is called as a subroutine, its action is limited to that group, even if the
+group does not contain any | characters. Note that such subpatterns are
+processed as anchored at the point where they are tested.
+.P
+10. If a pattern contains more than one backtracking control verb, the first
+one that is backtracked onto acts. For example, in the pattern
+A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
+triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
+same as PCRE2, but there are examples where it differs.
+.P
+11. Most backtracking verbs in assertions have their normal actions. They are
+not confined to the assertion.
+.P
+12. 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 PCRE2 it is set to
+"b".
+.P
+13. PCRE2'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 PCRE2
+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
+14. Perl recognizes comments in some places that PCRE2 does not, for example,
+between the ( and ? at the start of a subpattern. If the /x modifier is set,
+Perl allows white space between ( and ? (though current Perls warn that this is
+deprecated) but PCRE2 never does, even if the PCRE2_EXTENDED option is set.
+.P
+15. Perl, when in warning mode, gives warnings for character classes such as
+[A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no
+warning features, so it gives an error in these cases because they are almost
+certainly user mistakes.
+.P
+16. In PCRE2, the upper/lower case character properties Lu and Ll are not
+affected when case-independent matching is specified. For example, \ep{Lu}
+always matches an upper case letter. I think Perl has changed in this respect;
+in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all
+letters, regardless of case, when case independence is specified.
+.P
+17. PCRE2 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 PCRE2 for some time. This
+list is with respect to Perl 5.10:
+.sp
+(a) Although lookbehind assertions in PCRE2 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 PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $
+meta-character matches only at the very end of the string.
+.sp
+(c) A backslash followed by a letter with no special meaning is faulted. (Perl
+can be made to issue a warning.)
+.sp
+(d) If PCRE2_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) PCRE2_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 PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, and
+PCRE2_NO_AUTO_CAPTURE options have no Perl equivalents.
+.sp
+(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
+by the PCRE2_BSR_ANYCRLF option.
+.sp
+(h) The callout facility is PCRE2-specific.
+.sp
+(i) The partial matching facility is PCRE2-specific.
+.sp
+(j) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a
+different way and is not Perl-compatible.
+.sp
+(k) PCRE2 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: 28 September 2014
+Copyright (c) 1997-2014 University of Cambridge.
+.fi