PCRE2COMPAT(3)PCRE2COMPAT(3)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 asser‐
tions 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 surro‐
gates."
6. PCRE2 does support the \Q...\E escape for quoting substrings. Char‐
acters 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$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
7. Fairly obviously, PCRE2 does not support the (?{code}) and
(??{code}) constructions. However, there is support for recursive pat‐
terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also,
the PCRE2 "callout" feature allows an external function to be called
during pattern matching. See the pcre2callout documentation for
details.
8. Subroutine calls (whether recursive or not) are treated as atomic
groups. Atomic recursion is like Python, but unlike Perl. Captured
values that are set outside a subroutine call can be referenced from
inside in PCRE2, but not in Perl. There is a discussion that explains
these differences in more detail in the section on recursion differ‐
ences from Perl in the pcre2pattern page.
9. If any of the backtracking control verbs are used in a subpattern
that is called as a subroutine (whether or not recursively), their
effect is confined to that subpattern; it does not extend to the sur‐
rounding pattern. This is not always the case in Perl. In particular,
if (*THEN) is present in a group that is called as a subroutine, its
action is limited to that group, even if the group does not contain any
| characters. Note that such subpatterns are processed as anchored at
the point where they are tested.
10. If a pattern contains more than one backtracking control verb, the
first one that is backtracked onto acts. For example, in the pattern
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
it is the same as PCRE2, but there are examples where it differs.
11. Most backtracking verbs in assertions have their normal actions.
They are not confined to the assertion.
12. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE2 it is set to "b".
13. PCRE2's handling of duplicate subpattern numbers and duplicate sub‐
pattern 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 cap‐
turing 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 cur‐
rent 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 liter‐
als. 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.
(c) A backslash followed by a letter with no special meaning is
faulted. (Perl can be made to issue a warning.)
(d) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti‐
fiers is inverted, that is, by default they are not greedy, but if fol‐
lowed 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)
