Tristan Matthews | 0461646 | 2013-11-14 16:09:34 -0500 | [diff] [blame] | 1 | .TH PCRECOMPAT 3 |
| 2 | .SH NAME |
| 3 | PCRE - Perl-compatible regular expressions |
| 4 | .SH "DIFFERENCES BETWEEN PCRE AND PERL" |
| 5 | .rs |
| 6 | .sp |
| 7 | This document describes the differences in the ways that PCRE and Perl handle |
| 8 | regular expressions. The differences described here are with respect to Perl |
| 9 | versions 5.10 and above. |
| 10 | .P |
| 11 | 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what |
| 12 | it does have are given in the |
| 13 | .\" HREF |
| 14 | \fBpcreunicode\fP |
| 15 | .\" |
| 16 | page. |
| 17 | .P |
| 18 | 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do |
| 19 | not mean what you might think. For example, (?!a){3} does not assert that the |
| 20 | next three characters are not "a". It just asserts that the next character is |
| 21 | not "a" three times (in principle: PCRE optimizes this to run the assertion |
| 22 | just once). Perl allows repeat quantifiers on other assertions such as \eb, but |
| 23 | these do not seem to have any use. |
| 24 | .P |
| 25 | 3. Capturing subpatterns that occur inside negative lookahead assertions are |
| 26 | counted, but their entries in the offsets vector are never set. Perl sets its |
| 27 | numerical variables from any such patterns that are matched before the |
| 28 | assertion fails to match something (thereby succeeding), but only if the |
| 29 | negative lookahead assertion contains just one branch. |
| 30 | .P |
| 31 | 4. Though binary zero characters are supported in the subject string, they are |
| 32 | not allowed in a pattern string because it is passed as a normal C string, |
| 33 | terminated by zero. The escape sequence \e0 can be used in the pattern to |
| 34 | represent a binary zero. |
| 35 | .P |
| 36 | 5. The following Perl escape sequences are not supported: \el, \eu, \eL, |
| 37 | \eU, and \eN when followed by a character name or Unicode value. (\eN on its |
| 38 | own, matching a non-newline character, is supported.) In fact these are |
| 39 | implemented by Perl's general string-handling and are not part of its pattern |
| 40 | matching engine. If any of these are encountered by PCRE, an error is |
| 41 | generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, |
| 42 | \eU and \eu are interpreted as JavaScript interprets them. |
| 43 | .P |
| 44 | 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is |
| 45 | built with Unicode character property support. The properties that can be |
| 46 | tested with \ep and \eP are limited to the general category properties such as |
| 47 | Lu and Nd, script names such as Greek or Han, and the derived properties Any |
| 48 | and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the |
| 49 | Perl documentation says "Because Perl hides the need for the user to understand |
| 50 | the internal representation of Unicode characters, there is no need to |
| 51 | implement the somewhat messy concept of surrogates." |
| 52 | .P |
| 53 | 7. PCRE implements a simpler version of \eX than Perl, which changed to make |
| 54 | \eX match what Unicode calls an "extended grapheme cluster". This is more |
| 55 | complicated than an extended Unicode sequence, which is what PCRE matches. |
| 56 | .P |
| 57 | 8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in |
| 58 | between are treated as literals. This is slightly different from Perl in that $ |
| 59 | and @ are also handled as literals inside the quotes. In Perl, they cause |
| 60 | variable interpolation (but of course PCRE does not have variables). Note the |
| 61 | following examples: |
| 62 | .sp |
| 63 | Pattern PCRE matches Perl matches |
| 64 | .sp |
| 65 | .\" JOIN |
| 66 | \eQabc$xyz\eE abc$xyz abc followed by the |
| 67 | contents of $xyz |
| 68 | \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz |
| 69 | \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz |
| 70 | .sp |
| 71 | The \eQ...\eE sequence is recognized both inside and outside character classes. |
| 72 | .P |
| 73 | 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) |
| 74 | constructions. However, there is support for recursive patterns. This is not |
| 75 | available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" |
| 76 | feature allows an external function to be called during pattern matching. See |
| 77 | the |
| 78 | .\" HREF |
| 79 | \fBpcrecallout\fP |
| 80 | .\" |
| 81 | documentation for details. |
| 82 | .P |
| 83 | 10. Subpatterns that are called as subroutines (whether or not recursively) are |
| 84 | always treated as atomic groups in PCRE. This is like Python, but unlike Perl. |
| 85 | Captured values that are set outside a subroutine call can be reference from |
| 86 | inside in PCRE, but not in Perl. There is a discussion that explains these |
| 87 | differences in more detail in the |
| 88 | .\" HTML <a href="pcrepattern.html#recursiondifference"> |
| 89 | .\" </a> |
| 90 | section on recursion differences from Perl |
| 91 | .\" |
| 92 | in the |
| 93 | .\" HREF |
| 94 | \fBpcrepattern\fP |
| 95 | .\" |
| 96 | page. |
| 97 | .P |
| 98 | 11. If (*THEN) is present in a group that is called as a subroutine, its action |
| 99 | is limited to that group, even if the group does not contain any | characters. |
| 100 | .P |
| 101 | 12. There are some differences that are concerned with the settings of captured |
| 102 | strings when part of a pattern is repeated. For example, matching "aba" against |
| 103 | the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". |
| 104 | .P |
| 105 | 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern |
| 106 | names is not as general as Perl's. This is a consequence of the fact the PCRE |
| 107 | works internally just with numbers, using an external table to translate |
| 108 | between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B), |
| 109 | where the two capturing parentheses have the same number but different names, |
| 110 | is not supported, and causes an error at compile time. If it were allowed, it |
| 111 | would not be possible to distinguish which parentheses matched, because both |
| 112 | names map to capturing subpattern number 1. To avoid this confusing situation, |
| 113 | an error is given at compile time. |
| 114 | .P |
| 115 | 14. Perl recognizes comments in some places that PCRE does not, for example, |
| 116 | between the ( and ? at the start of a subpattern. If the /x modifier is set, |
| 117 | Perl allows whitespace between ( and ? but PCRE never does, even if the |
| 118 | PCRE_EXTENDED option is set. |
| 119 | .P |
| 120 | 15. PCRE provides some extensions to the Perl regular expression facilities. |
| 121 | Perl 5.10 includes new features that are not in earlier versions of Perl, some |
| 122 | of which (such as named parentheses) have been in PCRE for some time. This list |
| 123 | is with respect to Perl 5.10: |
| 124 | .sp |
| 125 | (a) Although lookbehind assertions in PCRE must match fixed length strings, |
| 126 | each alternative branch of a lookbehind assertion can match a different length |
| 127 | of string. Perl requires them all to have the same length. |
| 128 | .sp |
| 129 | (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ |
| 130 | meta-character matches only at the very end of the string. |
| 131 | .sp |
| 132 | (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special |
| 133 | meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. |
| 134 | (Perl can be made to issue a warning.) |
| 135 | .sp |
| 136 | (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is |
| 137 | inverted, that is, by default they are not greedy, but if followed by a |
| 138 | question mark they are. |
| 139 | .sp |
| 140 | (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried |
| 141 | only at the first matching position in the subject string. |
| 142 | .sp |
| 143 | (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and |
| 144 | PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents. |
| 145 | .sp |
| 146 | (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF |
| 147 | by the PCRE_BSR_ANYCRLF option. |
| 148 | .sp |
| 149 | (h) The callout facility is PCRE-specific. |
| 150 | .sp |
| 151 | (i) The partial matching facility is PCRE-specific. |
| 152 | .sp |
| 153 | (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on |
| 154 | different hosts that have the other endianness. However, this does not apply to |
| 155 | optimized data created by the just-in-time compiler. |
| 156 | .sp |
| 157 | (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a |
| 158 | different way and is not Perl-compatible. |
| 159 | .sp |
| 160 | (l) PCRE recognizes some special sequences such as (*CR) at the start of |
| 161 | a pattern that set overall options that cannot be changed within the pattern. |
| 162 | . |
| 163 | . |
| 164 | .SH AUTHOR |
| 165 | .rs |
| 166 | .sp |
| 167 | .nf |
| 168 | Philip Hazel |
| 169 | University Computing Service |
| 170 | Cambridge CB2 3QH, England. |
| 171 | .fi |
| 172 | . |
| 173 | . |
| 174 | .SH REVISION |
| 175 | .rs |
| 176 | .sp |
| 177 | .nf |
| 178 | Last updated: 14 November 2011 |
| 179 | Copyright (c) 1997-2011 University of Cambridge. |
| 180 | .fi |