Ubuntu Manpage: PCRE2 - Perl-compatible regular expressions (revised API)

NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

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.

1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does have are given in the
pcre2unicode page.

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 \b, but these do not seem to have
any use.

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.

4. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N when followed by a
character name or Unicode value. (\N 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, \U and \u are interpreted as ECMAScript interprets them.

5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is built with Unicode 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&. 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."

6. PCRE2 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 PCRE2 does not have variables). Note
the following examples:

Pattern PCRE2 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, 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
pcre2callout documentation for details.

8. Subroutine calls (whether recursive or not) are treated as atomic groups. Atomic recursion is like
Python, but unlike Perl. Captured values that are set outside a subroutine call can be referenced from
inside in PCRE2, but not in Perl. There is a discussion that explains these differences in more detail in
the section on recursion differences from Perl in the pcre2pattern page.

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.

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.

11. Most backtracking verbs in assertions have their normal actions. They are not confined to the
assertion.

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".

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.

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.

15. Perl, when in warning mode, gives warnings for character classes such as [A-\d] 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.

16. In PCRE2, the upper/lower case character properties Lu and Ll are not affected when case-independent
matching is specified. For example, \p{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), \p{Lu} and \p{Ll} match all letters,
regardless of case, when case independence is specified.

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:

(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.

(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.

(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.

(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.

(f) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, and PCRE2_NO_AUTO_CAPTURE
options have no Perl equivalents.

(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF
option.

(h) The callout facility is PCRE2-specific.

(i) The partial matching facility is PCRE2-specific.

(j) The alternative matching function (pcre2_dfa_match() matches in a different way and is not Perl-
compatible.

(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.

AUTHOR


       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION


       Last updated: 15 March 2015
       Copyright (c) 1997-2015 University of Cambridge.

PCRE2 10.20                                       15 March 2015                                   PCRE2COMPAT(3)