PCRE2COMPAT(3)

📦
version 10.49 · 7 versions
, in alpine 3.23, alpine 3.24, arch rolling

🎀 NAME 🎀

PCRE2COMPAT(3) — straight outta the Library Functions Manual, and it's about PCRE2: Perl-compatible regular expressions (revised API). 💅

✨ DIFFERENCES BETWEEN PCRE2 AND PERL ✨

okay so bestie, this page is the tea ☕ on all the ways PCRE2 and Perl handle regular expressions differently. Everything here is measured against Perl version 5.38.0, but lowkey both Perl and PCRE2 are constantly changing, so sometimes this info can be a lil out of date. no cap. 🦋

  1. When PCRE2_DOTALL is not set (that's the equivalent of Perl's /s qualifier), the way the . metacharacter behaves is different from Perl. In PCRE2, . matches the next character unless it's the start of a newline sequence. Which means: if the newline setting is CR, CRLF, or NUL, then . will match the code point LF (0x0A) in ASCII/Unicode environments, and NL (either 0x15 or 0x25) when you're using EBCDIC. In Perl, . appears to never match LF, even when 0x0A is not a newline indicator. 💀
  2. PCRE2 only has a subset of Perl's Unicode support. The deets on what it actually does have live in the pcre2unicode page. 🧚
  3. Like Perl, PCRE2 lets you put repeat quantifiers on parenthesized assertions — but they do not mean what you think, bestie, I'm so serious. 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 some repeat quantifiers on other assertions too, like \b*, but those don't seem to have any use. PCRE2 does not allow any kind of quantifier on non-lookaround assertions. 🚫
  4. If a braced quantifier like {1,2} shows up somewhere there's nothing to repeat (for example, at the start of a branch), PCRE2 raises an error, whereas Perl just treats the quantifier characters as literal. And when a braced quantifier (...){min,max} has min > max, Perl treats it as an item that fails to match any portion of the subject (since no number of repetitions can meet the condition), and additionally issues a warning when it's in warning mode. PCRE2 has no warning features, so it just gives an error in this case. 🙈
  5. Capture groups that occur inside negative lookaround assertions are counted, but their entries in the offsets vector are set only when a negative assertion is a condition that has a matching branch (that is, the condition is false). Perl may set such capture groups in other circumstances. 🤔
  6. These Perl escape sequences are not supported: \F, \l, \L, \u, \U, and \N when followed by a character name. \N on its own (matching a non-newline character) and \N{U+dd..} (matching a Unicode code point) are supported. The escapes that modify the case of following letters are implemented by Perl's general string-handling and are not part of its pattern matching engine. If PCRE2 encounters any of these, an error is generated by default. However, if either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are interpreted the way ECMAScript interprets them. ✨
  7. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is built with Unicode support (which is the default). The properties you can test with \p and \P are limited to:
    • the general category properties such as Lu and Nd
    • the derived properties Any and Lc (synonym L&)
    • script names such as Greek or Han
    • Bidi_Class, Bidi_Control, and a few binary properties

Both PCRE2 and Perl support the Cs (surrogate) property, but in PCRE2 its use is limited. See the pcre2pattern documentation for details. The long synonyms for property names that Perl supports (such as \p{Letter}) are not supported by PCRE2, and you're also not permitted to prefix any of these properties with "Is". 🎀

  1. PCRE2 supports the \Q...\E escape for quoting substrings — characters in between get treated as literals. But this is slightly different from Perl, because $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (PCRE2 does not have variables, so like... moot point 💅). Also, Perl does "double-quotish backslash interpolation" on any backslashes between \Q and \E, which its documentation says "may lead to confusing results". PCRE2 treats a backslash between \Q and \E just like any other character. Peep these 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
\QA\B\E            A\B               A\B
\Q\\E              \                 \\E

The \Q...\E sequence is recognized both inside and outside character classes by both PCRE2 and Perl. Another difference from Perl: any appearance of \Q or \E inside what might otherwise be a quantifier causes PCRE2 not to recognize the sequence as a quantifier. Perl recognizes a quantifier if (redundantly) either of the numbers is inside \Q...\E, but not if the separating comma is. When it's not recognized as a quantifier, a sequence such as {\Q1\E,2} is treated as the literal string "{1,2}". 🔥

  1. Pretty obviously, PCRE2 does not support the (?{code}) and (??{code}) constructions. However! PCRE2 does have a "callout" feature, which lets an external function get called during pattern matching. See the pcre2callout documentation for details. 📞
  2. Subroutine calls (whether recursive or not) were treated as atomic groups up to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking into subroutine calls is now supported, just like in Perl. 🌈
  3. In PCRE2, if any of the backtracking control verbs are used in a group that is called as a subroutine (whether or not recursively), their effect is confined to that group; 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's called as a subroutine, its action is limited to that group, even if the group doesn't contain any | characters. Note that such groups are processed as anchored at the point where they're tested. PCRE2 also confines all control verbs within atomic assertions, again including (*THEN) in assertions with only one branch. 👑
  4. 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's the same as PCRE2, but there are cases where it differs. 😵‍💫
  5. 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". 🍓
  6. PCRE2's handling of duplicate capture group numbers and names is not as general as Perl's. This is a consequence of the fact that 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 capture groups have the same number but different names, is not supported, and causes an error at compile time. If it were allowed, it wouldn't be possible to distinguish which group matched, because both names map to capture group number 1. To avoid this confusing situation, an error is given at compile time. 💀
  7. Perl used to recognize comments in some places that PCRE2 does not — for example, between the ( and ? at the start of a group. If the /x modifier is set, Perl allowed white space between ( and ?, though the latest Perls give an error (for a while it was just deprecated). There may still be some cases where Perl behaves differently. 🌸
  8. 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're almost certainly user mistakes. 😭
  9. In PCRE2, until release 10.45, the upper/lower case character properties Lu and Ll were not affected when case-independent matching was specified. Perl has changed in this respect, and PCRE2 has now changed to match. When caseless matching is in force, Lu, Ll, and Lt (title case) are all treated as Lc (cased letter). ✨
  10. From release 5.32.0, Perl locks out the use of \K in lookaround assertions. From release 10.38, PCRE2 does the same by default. However, there is an option for re-enabling the previous behaviour. When this option is set, \K is acted on when it occurs in positive assertions, but is ignored in negative assertions. 🚨
  11. PCRE2 provides some extensions to the Perl regular expression facilities. Perl 5.10 included new features that weren't in earlier versions of Perl, some of which (such as named parentheses) were in PCRE2 for some time before. This list is with respect to Perl 5.38:
    • (a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ meta-character matches only at the very end of the string. 💸
    • (b) A backslash followed by a letter with no special meaning is faulted. (Perl can be made to issue a warning.) 🚫
    • (c) 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. 🔄
    • (d) 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. ⚓
    • (e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents. 🙅
    • (f) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF option. 🛤️
    • (g) The callout facility is PCRE2-specific. Perl supports codeblocks and variable interpolation, but not general hooks on every match. 🪝
    • (h) The partial matching facility is PCRE2-specific. 🧩
    • (i) The alternative matching function (pcre2_dfa_match()) matches in a different way and is not Perl-compatible. 🔀
    • (j) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at the start of a pattern. These set overall options that cannot be changed within the pattern. 🔒
    • (k) PCRE2 supports non-atomic positive lookaround assertions. This is an extension to the lookaround facilities. The default, Perl-compatible lookarounds are atomic. 💫
    • (l) There are three syntactical items in patterns that can refer to a capturing group by number: back references such as \g{2}, subroutine calls such as (?3), and condition references such as (?(4)...). PCRE2 supports relative group numbers such as +2 and -4 in all three cases. Perl supports both plus and minus for subroutine calls, but only minus for back references, and no relative numbering at all for conditions. 🔢
  • (m) oh and the scan substring assertion — the syntax is (*scs:(n)...) — that's a PCRE2 original, bestie. Perl doesn't have it, periodt. ✨
  1. Perl and PCRE2 have different limits, fr fr. Go read the pcre2limits docs for the whole tea. Back in 5.10, Perl switched from recursion to iteration and started keeping the intermediate matches on the heap — that's like ~10% slower but it never falls into any stack-overflow limit. PCRE2 did basically the same glow-up at release 10.30, plus it has so many build-time and run-time customizable limits. 💅
  2. Unlike Perl, PCRE2 does not have character set modifiers, and especially no way to set characters by context like Perl's "/d" does. A regex using PCRE2_UTF and PCRE2_UCP will use rules similar to Perl's "/u"; if you want something closer to "/a", you can get there by stacking other PCRE2_EXTRA_ASCII* options on top. 🎀
  3. Some recursive patterns that Perl clocks as infinite recursions? PCRE2 can actually handle them, either through the interpreter or the JIT. Iconic example: /(?:|(?0)abcd)(?(R)|\z)/ — that one matches a sequence of any number of repeated "abcd" substrings at the end of the subject. Ate. 🔥
  4. Both PCRE2 and Perl throw an error when \x{ escapes are invalid, but Perl tries to recover and prints a warning if the problem was that an invalid hexadecimal digit showed up. PCRE2 doesn't have warnings, so it just returns an error instead, no cap. Also: Perl accepts \x{} and generates NUL, which PCRE2 does not. 💀
  5. Starting from release 10.45, PCRE2 errors out if \x isn't followed by a hexadecimal digit or a curly bracket. It used to interpret that as the NUL character. Perl still generates NUL, but it warns you when it's in warning mode in most cases. 🙈

👑 AUTHOR

Philip Hazel Retired from University Computing Service Cambridge, England.

🦋 REVISION

Last updated: 02 June 2025 Copyright (c) 1997-2024 University of Cambridge.

okay bestie that's the whole vibe check, you're officially PCRE2-compat-pilled now 💖 this was PCRE2 10.49, dated 02 June 2025, and you just read PCRE2COMPAT(3) — go forth and slay your regexes, mwah 🫶✨

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