| .TH PCRE2COMPAT 3 "15 March 2015" "PCRE2 10.20" |
| .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. 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 |
| .\" 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, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 15 March 2015 |
| Copyright (c) 1997-2015 University of Cambridge. |
| .fi |