217 lines
8.8 KiB
HTML
217 lines
8.8 KiB
HTML
<html>
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<head>
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<title>pcrecompat specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrecompat man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<br><b>
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DIFFERENCES BETWEEN PCRE AND PERL
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</b><br>
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<P>
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This document describes the differences in the ways that PCRE and Perl handle
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regular expressions. The differences described here are with respect to Perl
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versions 5.10 and above.
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</P>
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<P>
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1. PCRE 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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<a href="pcreunicode.html"><b>pcreunicode</b></a>
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page.
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</P>
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<P>
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2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
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not mean what you might think. For example, (?!a){3} does not assert that the
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next three characters are not "a". It just asserts that the next character is
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not "a" three times (in principle: PCRE optimizes this to run the assertion
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just once). Perl allows repeat quantifiers on other assertions such as \b, but
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these do not seem to have any use.
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</P>
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<P>
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3. Capturing subpatterns that occur inside negative lookahead assertions are
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counted, but their entries in the offsets vector are never set. Perl sets its
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numerical variables from any such patterns that are matched before the
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assertion fails to match something (thereby succeeding), but only if the
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negative lookahead assertion contains just one branch.
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</P>
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<P>
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4. Though binary zero characters are supported in the subject string, they are
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not allowed in a pattern string because it is passed as a normal C string,
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terminated by zero. The escape sequence \0 can be used in the pattern to
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represent a binary zero.
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</P>
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<P>
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5. The following Perl escape sequences are not supported: \l, \u, \L,
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\U, and \N when followed by a character name or Unicode value. (\N 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 PCRE, an error is
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generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
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\U and \u are interpreted as JavaScript interprets them.
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</P>
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<P>
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6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
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built with Unicode character property support. The properties that can be
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tested with \p and \P are limited to the general category properties such as
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Lu and Nd, script names such as Greek or Han, and the derived properties Any
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and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
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Perl documentation says "Because Perl hides the need for the user to understand
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the internal representation of Unicode characters, there is no need to
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implement the somewhat messy concept of surrogates."
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</P>
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<P>
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7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
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between are treated as literals. This is slightly different from Perl in that $
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and @ are also handled as literals inside the quotes. In Perl, they cause
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variable interpolation (but of course PCRE does not have variables). Note the
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following examples:
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<pre>
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Pattern PCRE matches Perl matches
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\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
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\Qabc\$xyz\E abc\$xyz abc\$xyz
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\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
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</pre>
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The \Q...\E sequence is recognized both inside and outside character classes.
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</P>
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<P>
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8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
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constructions. However, there is support for recursive patterns. This is not
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available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
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feature allows an external function to be called during pattern matching. See
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the
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<a href="pcrecallout.html"><b>pcrecallout</b></a>
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documentation for details.
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</P>
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<P>
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9. Subpatterns that are called as subroutines (whether or not recursively) are
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always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
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Captured values that are set outside a subroutine call can be reference from
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inside in PCRE, but not in Perl. There is a discussion that explains these
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differences in more detail in the
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<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
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in the
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<a href="pcrepattern.html"><b>pcrepattern</b></a>
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page.
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</P>
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<P>
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10. If any of the backtracking control verbs are used in an assertion or in a
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subpattern that is called as a subroutine (whether or not recursively), their
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effect is confined to that subpattern; it does not extend to the surrounding
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pattern. This is not always the case in Perl. In particular, if (*THEN) is
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present in a group that is called as a subroutine, its action is limited to
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that group, even if the group does not contain any | characters. There is one
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exception to this: the name from a *(MARK), (*PRUNE), or (*THEN) that is
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encountered in a successful positive assertion <i>is</i> passed back when a
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match succeeds (compare capturing parentheses in assertions). Note that such
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subpatterns are processed as anchored at the point where they are tested.
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</P>
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<P>
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11. 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 PCRE it is set to "b".
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</P>
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<P>
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12. PCRE'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 PCRE
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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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<P>
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13. Perl recognizes comments in some places that PCRE does not, for example,
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between the ( and ? at the start of a subpattern. If the /x modifier is set,
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Perl allows white space between ( and ? but PCRE never does, even if the
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PCRE_EXTENDED option is set.
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</P>
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<P>
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14. PCRE 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) have been in PCRE for some time. This list
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is with respect to Perl 5.10:
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<br>
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<br>
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(a) Although lookbehind assertions in PCRE 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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<br>
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<br>
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(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
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meta-character matches only at the very end of the string.
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<br>
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<br>
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(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
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meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
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(Perl can be made to issue a warning.)
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<br>
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<br>
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(d) If PCRE_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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<br>
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<br>
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(e) PCRE_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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<br>
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<br>
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(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
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PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
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<br>
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<br>
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(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
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by the PCRE_BSR_ANYCRLF option.
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<br>
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<br>
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(h) The callout facility is PCRE-specific.
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<br>
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<br>
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(i) The partial matching facility is PCRE-specific.
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<br>
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<br>
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(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
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different hosts that have the other endianness. However, this does not apply to
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optimized data created by the just-in-time compiler.
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<br>
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<br>
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(k) The alternative matching functions (<b>pcre_dfa_exec()</b>,
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<b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b>,) match in a different way
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and are not Perl-compatible.
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<br>
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<br>
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(l) PCRE recognizes some special sequences such as (*CR) at the start of
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a pattern that set overall options that cannot be changed within the pattern.
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</P>
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<br><b>
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AUTHOR
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</b><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><b>
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REVISION
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</b><br>
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<P>
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Last updated: 25 August 2012
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<br>
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Copyright © 1997-2012 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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