200 lines
9.1 KiB
Groff
200 lines
9.1 KiB
Groff
.TH PCRE2COMPAT 3 "18 April 2017" "PCRE2 10.30"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
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.rs
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.sp
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This document describes the differences in the ways that PCRE2 and Perl handle
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regular expressions. The differences described here are with respect to Perl
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versions 5.26, but as both Perl and PCRE2 are continually changing, the
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information may sometimes be out of date.
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.P
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1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
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have are given in the
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.\" HREF
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\fBpcre2unicode\fP
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.\"
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page.
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.P
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2. Like Perl, PCRE2 allows repeat quantifiers on parenthesized assertions, but
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they do not mean what you might think. For example, (?!a){3} does not assert
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that the next three characters are not "a". It just asserts that the next
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character is not "a" three times (in principle: PCRE2 optimizes this to run the
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assertion just once). Perl allows some repeat quantifiers on other assertions,
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for example, \eb* (but not \eb{3}), but these do not seem to have any use.
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.P
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3. Capturing subpatterns that occur inside negative lookaround assertions are
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counted, but their entries in the offsets vector are set only when a negative
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assertion is a condition that has a matching branch (that is, the condition is
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false).
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.P
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4. The following Perl escape sequences are not supported: \el, \eu, \eL,
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\eU, and \eN when followed by a character name or Unicode value. (\eN on its
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own, matching a non-newline character, is supported.) In fact these are
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implemented by Perl's general string-handling and are not part of its pattern
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matching engine. If any of these are encountered by PCRE2, an error is
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generated by default. However, if the PCRE2_ALT_BSUX option is set,
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\eU and \eu are interpreted as ECMAScript interprets them.
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.P
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5. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is
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built with Unicode support (the default). The properties that can be tested
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with \ep and \eP are limited to the general category properties such as Lu and
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Nd, script names such as Greek or Han, and the derived properties Any and L&.
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PCRE2 does support the Cs (surrogate) property, which Perl does not; the Perl
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documentation says "Because Perl hides the need for the user to understand the
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internal representation of Unicode characters, there is no need to implement
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the somewhat messy concept of surrogates."
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.P
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6. PCRE2 does support the \eQ...\eE escape for quoting substrings. Characters
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in between are treated as literals. This is slightly different from Perl in
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that $ and @ are also handled as literals inside the quotes. In Perl, they
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cause variable interpolation (but of course PCRE2 does not have variables).
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Note the following examples:
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.sp
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Pattern PCRE2 matches Perl matches
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.sp
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.\" JOIN
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\eQabc$xyz\eE abc$xyz abc followed by the
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contents of $xyz
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\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
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\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
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.sp
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The \eQ...\eE sequence is recognized both inside and outside character classes.
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.P
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7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})
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constructions. However, there is support PCRE2's "callout" feature, which
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allows an external function to be called during pattern matching. See the
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.\" HREF
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\fBpcre2callout\fP
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.\"
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documentation for details.
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.P
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8. Subroutine calls (whether recursive or not) were treated as atomic groups up
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to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking
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into subroutine calls is now supported, as in Perl.
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.P
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9. If any of the backtracking control verbs are used in a subpattern that is
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called as a subroutine (whether or not recursively), their effect is confined
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to that subpattern; it does not extend to the surrounding pattern. This is not
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always the case in Perl. In particular, if (*THEN) is present in a group that
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is called as a subroutine, its action is limited to that group, even if the
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group does not contain any | characters. Note that such subpatterns are
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processed as anchored at the point where they are tested.
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.P
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10. If a pattern contains more than one backtracking control verb, the first
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one that is backtracked onto acts. For example, in the pattern
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A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
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triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
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same as PCRE2, but there are cases where it differs.
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.P
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11. Most backtracking verbs in assertions have their normal actions. They are
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not confined to the assertion.
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.P
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12. There are some differences that are concerned with the settings of captured
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strings when part of a pattern is repeated. For example, matching "aba" against
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the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to
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"b".
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.P
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13. PCRE2's handling of duplicate subpattern numbers and duplicate subpattern
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names is not as general as Perl's. This is a consequence of the fact the PCRE2
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works internally just with numbers, using an external table to translate
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between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b>B),
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where the two capturing parentheses have the same number but different names,
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is not supported, and causes an error at compile time. If it were allowed, it
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would not be possible to distinguish which parentheses matched, because both
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names map to capturing subpattern number 1. To avoid this confusing situation,
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an error is given at compile time.
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.P
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14. Perl used to recognize comments in some places that PCRE2 does not, for
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example, between the ( and ? at the start of a subpattern. If the /x modifier
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is set, Perl allowed white space between ( and ? though the latest Perls give
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an error (for a while it was just deprecated). There may still be some cases
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where Perl behaves differently.
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.P
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15. Perl, when in warning mode, gives warnings for character classes such as
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[A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no
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warning features, so it gives an error in these cases because they are almost
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certainly user mistakes.
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.P
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16. In PCRE2, the upper/lower case character properties Lu and Ll are not
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affected when case-independent matching is specified. For example, \ep{Lu}
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always matches an upper case letter. I think Perl has changed in this respect;
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in the release at the time of writing (5.24), \ep{Lu} and \ep{Ll} match all
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letters, regardless of case, when case independence is specified.
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.P
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17. PCRE2 provides some extensions to the Perl regular expression facilities.
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Perl 5.10 includes new features that are not in earlier versions of Perl, some
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of which (such as named parentheses) were in PCRE2 for some time before. This
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list is with respect to Perl 5.26:
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.sp
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(a) Although lookbehind assertions in PCRE2 must match fixed length strings,
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each alternative branch of a lookbehind assertion can match a different length
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of string. Perl requires them all to have the same length.
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.sp
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(b) From PCRE2 10.23, back references to groups of fixed length are supported
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in lookbehinds, provided that there is no possibility of referencing a
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non-unique number or name. Perl does not support backreferences in lookbehinds.
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.sp
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(c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $
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meta-character matches only at the very end of the string.
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.sp
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(d) A backslash followed by a letter with no special meaning is faulted. (Perl
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can be made to issue a warning.)
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.sp
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(e) If PCRE2_UNGREEDY is set, the greediness of the repetition quantifiers is
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inverted, that is, by default they are not greedy, but if followed by a
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question mark they are.
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.sp
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(f) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried
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only at the first matching position in the subject string.
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.sp
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(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART
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options have no Perl equivalents.
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.sp
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(h) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
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by the PCRE2_BSR_ANYCRLF option.
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.sp
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(i) The callout facility is PCRE2-specific. Perl supports codeblocks and
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variable interpolation, but not general hooks on every match.
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.sp
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(j) The partial matching facility is PCRE2-specific.
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.sp
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(k) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a
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different way and is not Perl-compatible.
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.sp
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(l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at
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the start of a pattern that set overall options that cannot be changed within
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the pattern.
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.P
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18. The Perl /a modifier restricts /d numbers to pure ascii, and the /aa
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modifier restricts /i case-insensitive matching to pure ascii, ignoring Unicode
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rules. This separation cannot be represented with PCRE2_UCP.
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.P
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19. Perl has different limits than PCRE2. See the
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.\" HREF
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\fBpcre2limit\fP
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.\"
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documentation for details. Perl went with 5.10 from recursion to iteration
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keeping the intermediate matches on the heap, which is ~10% slower but does not
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fall into any stack-overflow limit. PCRE2 made a similar change at release
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10.30, and also has many build-time and run-time customizable limits.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 18 April 2017
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Copyright (c) 1997-2017 University of Cambridge.
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.fi
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