This document describes the differences in the ways that PCRE and
Perl handle regular expressions. The differences described here
are with respect to Perl versions 5.10 and above.
1. PCRE has only a subset of Perl's Unicode support. Details of
what it does have are given in the pcreunicode page.
2. PCRE 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: PCRE 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. 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, 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 PCRE, an error is generated by
default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, \U
and \u are interpreted as JavaScript interprets them.
6. The Perl escape sequences \p, \P, and \X are supported only if
PCRE is built with Unicode character property 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&. 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."
7. 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$xyz
The \Q...\E sequence is recognized both inside and outside
character classes.
8. Fairly obviously, PCRE does not support the (?{code}) and
(??{code}) constructions. However, there is support for recursive
patterns. This is not available in Perl 5.8, but it is in Perl
5.10. Also, the PCRE "callout" feature allows an external
function to be called during pattern matching. See the
pcrecallout documentation for details.
9. Subpatterns that are called as subroutines (whether or not
recursively) are always treated as atomic groups in PCRE. This is
like Python, but unlike Perl. Captured values that are set
outside a subroutine call can be reference from inside in PCRE,
but not in Perl. There is a discussion that explains these
differences in more detail in the section on recursion
differences from Perl in the pcrepattern page.
10. If any of the backtracking control verbs are used in a
subpattern that is called as a subroutine (whether or not
recursively), their effect is confined to that subpattern; it
does not extend to the surrounding pattern. This is not always
the case in Perl. In particular, if (*THEN) is present in a group
that is called as a subroutine, its action is limited to that
group, even if the group does not contain any | characters. Note
that such subpatterns are processed as anchored at the point
where they are tested.
11. If a pattern contains more than one backtracking control
verb, the first one that is backtracked onto acts. For example,
in the pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers
(*COMMIT), but a failure in C triggers (*PRUNE). Perl's behaviour
is more complex; in many cases it is the same as PCRE, but there
are examples where it differs.
12. Most backtracking verbs in assertions have their normal
actions. They are not confined to the assertion.
13. There are some differences that are concerned with the
settings of captured strings when part of a pattern is repeated.
For example, matching "aba" against the pattern /^(a(b)?)+$/ in
Perl leaves $2 unset, but in PCRE it is set to "b".
14. 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.
15. Perl recognizes comments in some places that PCRE 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 PCRE
never does, even if the PCRE_EXTENDED option is set.
16. Perl, when in warning mode, gives warnings for character
classes such as [A-\d] or [a-[:digit:]]. It then treats the
hyphens as literals. PCRE has no warning features, so it gives an
error in these cases because they are almost certainly user
mistakes.
17. In PCRE, 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.
18. 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:
(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.
(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. Otherwise, like Perl, the
backslash is quietly ignored. (Perl can be made to issue a
warning.)
(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 at matching time 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,
PCRE_NOTEMPTY_ATSTART, and PCRE_NO_AUTO_CAPTURE options for
pcre_exec() have no Perl equivalents.
(g) The \R escape sequence can be restricted to match only CR,
LF, or CRLF by the PCRE_BSR_ANYCRLF option.
(h) The callout facility is PCRE-specific.
(i) The partial matching facility is PCRE-specific.
(j) Patterns compiled by PCRE can be saved and re-used at a later
time, even on different hosts that have the other endianness.
However, this does not apply to optimized data created by the
just-in-time compiler.
(k) The alternative matching functions (pcre_dfa_exec(),
pcre16_dfa_exec() and pcre32_dfa_exec(),) match in a different
way and are not Perl-compatible.
(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.
