1*22dc650dSSadaf Ebrahimi<html> 2*22dc650dSSadaf Ebrahimi<head> 3*22dc650dSSadaf Ebrahimi<title>pcre2compat specification</title> 4*22dc650dSSadaf Ebrahimi</head> 5*22dc650dSSadaf Ebrahimi<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> 6*22dc650dSSadaf Ebrahimi<h1>pcre2compat man page</h1> 7*22dc650dSSadaf Ebrahimi<p> 8*22dc650dSSadaf EbrahimiReturn to the <a href="index.html">PCRE2 index page</a>. 9*22dc650dSSadaf Ebrahimi</p> 10*22dc650dSSadaf Ebrahimi<p> 11*22dc650dSSadaf EbrahimiThis page is part of the PCRE2 HTML documentation. It was generated 12*22dc650dSSadaf Ebrahimiautomatically from the original man page. If there is any nonsense in it, 13*22dc650dSSadaf Ebrahimiplease consult the man page, in case the conversion went wrong. 14*22dc650dSSadaf Ebrahimi<br> 15*22dc650dSSadaf Ebrahimi<br><b> 16*22dc650dSSadaf EbrahimiDIFFERENCES BETWEEN PCRE2 AND PERL 17*22dc650dSSadaf Ebrahimi</b><br> 18*22dc650dSSadaf Ebrahimi<P> 19*22dc650dSSadaf EbrahimiThis document describes some of the known differences in the ways that PCRE2 20*22dc650dSSadaf Ebrahimiand Perl handle regular expressions. The differences described here are with 21*22dc650dSSadaf Ebrahimirespect to Perl version 5.38.0, but as both Perl and PCRE2 are continually 22*22dc650dSSadaf Ebrahimichanging, the information may at times be out of date. 23*22dc650dSSadaf Ebrahimi</P> 24*22dc650dSSadaf Ebrahimi<P> 25*22dc650dSSadaf Ebrahimi1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not set, the 26*22dc650dSSadaf Ebrahimibehaviour of the '.' metacharacter differs from Perl. In PCRE2, '.' matches the 27*22dc650dSSadaf Ebrahiminext character unless it is the start of a newline sequence. This means that, 28*22dc650dSSadaf Ebrahimiif the newline setting is CR, CRLF, or NUL, '.' will match the code point LF 29*22dc650dSSadaf Ebrahimi(0x0A) in ASCII/Unicode environments, and NL (either 0x15 or 0x25) when using 30*22dc650dSSadaf EbrahimiEBCDIC. In Perl, '.' appears never to match LF, even when 0x0A is not a newline 31*22dc650dSSadaf Ebrahimiindicator. 32*22dc650dSSadaf Ebrahimi</P> 33*22dc650dSSadaf Ebrahimi<P> 34*22dc650dSSadaf Ebrahimi2. PCRE2 has only a subset of Perl's Unicode support. Details of what it does 35*22dc650dSSadaf Ebrahimihave are given in the 36*22dc650dSSadaf Ebrahimi<a href="pcre2unicode.html"><b>pcre2unicode</b></a> 37*22dc650dSSadaf Ebrahimipage. 38*22dc650dSSadaf Ebrahimi</P> 39*22dc650dSSadaf Ebrahimi<P> 40*22dc650dSSadaf Ebrahimi3. Like Perl, PCRE2 allows repeat quantifiers on parenthesized assertions, but 41*22dc650dSSadaf Ebrahimithey do not mean what you might think. For example, (?!a){3} does not assert 42*22dc650dSSadaf Ebrahimithat the next three characters are not "a". It just asserts that the next 43*22dc650dSSadaf Ebrahimicharacter is not "a" three times (in principle; PCRE2 optimizes this to run the 44*22dc650dSSadaf Ebrahimiassertion just once). Perl allows some repeat quantifiers on other assertions, 45*22dc650dSSadaf Ebrahimifor example, \b* , but these do not seem to have any use. PCRE2 does not allow 46*22dc650dSSadaf Ebrahimiany kind of quantifier on non-lookaround assertions. 47*22dc650dSSadaf Ebrahimi</P> 48*22dc650dSSadaf Ebrahimi<P> 49*22dc650dSSadaf Ebrahimi4. If a braced quantifier such as {1,2} appears where there is nothing to 50*22dc650dSSadaf Ebrahimirepeat (for example, at the start of a branch), PCRE2 raises an error whereas 51*22dc650dSSadaf EbrahimiPerl treats the quantifier characters as literal. 52*22dc650dSSadaf Ebrahimi</P> 53*22dc650dSSadaf Ebrahimi<P> 54*22dc650dSSadaf Ebrahimi5. Capture groups that occur inside negative lookaround assertions are counted, 55*22dc650dSSadaf Ebrahimibut their entries in the offsets vector are set only when a negative assertion 56*22dc650dSSadaf Ebrahimiis a condition that has a matching branch (that is, the condition is false). 57*22dc650dSSadaf EbrahimiPerl may set such capture groups in other circumstances. 58*22dc650dSSadaf Ebrahimi</P> 59*22dc650dSSadaf Ebrahimi<P> 60*22dc650dSSadaf Ebrahimi6. The following Perl escape sequences are not supported: \F, \l, \L, \u, 61*22dc650dSSadaf Ebrahimi\U, and \N when followed by a character name. \N on its own, matching a 62*22dc650dSSadaf Ebrahiminon-newline character, and \N{U+dd..}, matching a Unicode code point, are 63*22dc650dSSadaf Ebrahimisupported. The escapes that modify the case of following letters are 64*22dc650dSSadaf Ebrahimiimplemented by Perl's general string-handling and are not part of its pattern 65*22dc650dSSadaf Ebrahimimatching engine. If any of these are encountered by PCRE2, an error is 66*22dc650dSSadaf Ebrahimigenerated by default. However, if either of the PCRE2_ALT_BSUX or 67*22dc650dSSadaf EbrahimiPCRE2_EXTRA_ALT_BSUX options is set, \U and \u are interpreted as ECMAScript 68*22dc650dSSadaf Ebrahimiinterprets them. 69*22dc650dSSadaf Ebrahimi</P> 70*22dc650dSSadaf Ebrahimi<P> 71*22dc650dSSadaf Ebrahimi7. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is 72*22dc650dSSadaf Ebrahimibuilt with Unicode support (the default). The properties that can be tested 73*22dc650dSSadaf Ebrahimiwith \p and \P are limited to the general category properties such as Lu and 74*22dc650dSSadaf EbrahimiNd, the derived properties Any and LC (synonym L&), script names such as Greek 75*22dc650dSSadaf Ebrahimior Han, Bidi_Class, Bidi_Control, and a few binary properties. Both PCRE2 and 76*22dc650dSSadaf EbrahimiPerl support the Cs (surrogate) property, but in PCRE2 its use is limited. See 77*22dc650dSSadaf Ebrahimithe 78*22dc650dSSadaf Ebrahimi<a href="pcre2pattern.html"><b>pcre2pattern</b></a> 79*22dc650dSSadaf Ebrahimidocumentation for details. The long synonyms for property names that Perl 80*22dc650dSSadaf Ebrahimisupports (such as \p{Letter}) are not supported by PCRE2, nor is it permitted 81*22dc650dSSadaf Ebrahimito prefix any of these properties with "Is". 82*22dc650dSSadaf Ebrahimi</P> 83*22dc650dSSadaf Ebrahimi<P> 84*22dc650dSSadaf Ebrahimi8. PCRE2 supports the \Q...\E escape for quoting substrings. Characters 85*22dc650dSSadaf Ebrahimiin between are treated as literals. However, this is slightly different from 86*22dc650dSSadaf EbrahimiPerl in that $ and @ are also handled as literals inside the quotes. In Perl, 87*22dc650dSSadaf Ebrahimithey cause variable interpolation (PCRE2 does not have variables). Also, Perl 88*22dc650dSSadaf Ebrahimidoes "double-quotish backslash interpolation" on any backslashes between \Q 89*22dc650dSSadaf Ebrahimiand \E which, its documentation says, "may lead to confusing results". PCRE2 90*22dc650dSSadaf Ebrahimitreats a backslash between \Q and \E just like any other character. Note the 91*22dc650dSSadaf Ebrahimifollowing examples: 92*22dc650dSSadaf Ebrahimi<pre> 93*22dc650dSSadaf Ebrahimi Pattern PCRE2 matches Perl matches 94*22dc650dSSadaf Ebrahimi 95*22dc650dSSadaf Ebrahimi \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz 96*22dc650dSSadaf Ebrahimi \Qabc\$xyz\E abc\$xyz abc\$xyz 97*22dc650dSSadaf Ebrahimi \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 98*22dc650dSSadaf Ebrahimi \QA\B\E A\B A\B 99*22dc650dSSadaf Ebrahimi \Q\\E \ \\E 100*22dc650dSSadaf Ebrahimi</pre> 101*22dc650dSSadaf EbrahimiThe \Q...\E sequence is recognized both inside and outside character classes 102*22dc650dSSadaf Ebrahimiby both PCRE2 and Perl. 103*22dc650dSSadaf Ebrahimi</P> 104*22dc650dSSadaf Ebrahimi<P> 105*22dc650dSSadaf Ebrahimi9. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code}) 106*22dc650dSSadaf Ebrahimiconstructions. However, PCRE2 does have a "callout" feature, which allows an 107*22dc650dSSadaf Ebrahimiexternal function to be called during pattern matching. See the 108*22dc650dSSadaf Ebrahimi<a href="pcre2callout.html"><b>pcre2callout</b></a> 109*22dc650dSSadaf Ebrahimidocumentation for details. 110*22dc650dSSadaf Ebrahimi</P> 111*22dc650dSSadaf Ebrahimi<P> 112*22dc650dSSadaf Ebrahimi10. Subroutine calls (whether recursive or not) were treated as atomic groups 113*22dc650dSSadaf Ebrahimiup to PCRE2 release 10.23, but from release 10.30 this changed, and 114*22dc650dSSadaf Ebrahimibacktracking into subroutine calls is now supported, as in Perl. 115*22dc650dSSadaf Ebrahimi</P> 116*22dc650dSSadaf Ebrahimi<P> 117*22dc650dSSadaf Ebrahimi11. In PCRE2, if any of the backtracking control verbs are used in a group that 118*22dc650dSSadaf Ebrahimiis called as a subroutine (whether or not recursively), their effect is 119*22dc650dSSadaf Ebrahimiconfined to that group; it does not extend to the surrounding pattern. This is 120*22dc650dSSadaf Ebrahiminot always the case in Perl. In particular, if (*THEN) is present in a group 121*22dc650dSSadaf Ebrahimithat is called as a subroutine, its action is limited to that group, even if 122*22dc650dSSadaf Ebrahimithe group does not contain any | characters. Note that such groups are 123*22dc650dSSadaf Ebrahimiprocessed as anchored at the point where they are tested. 124*22dc650dSSadaf Ebrahimi</P> 125*22dc650dSSadaf Ebrahimi<P> 126*22dc650dSSadaf Ebrahimi12. If a pattern contains more than one backtracking control verb, the first 127*22dc650dSSadaf Ebrahimione that is backtracked onto acts. For example, in the pattern 128*22dc650dSSadaf EbrahimiA(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C 129*22dc650dSSadaf Ebrahimitriggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the 130*22dc650dSSadaf Ebrahimisame as PCRE2, but there are cases where it differs. 131*22dc650dSSadaf Ebrahimi</P> 132*22dc650dSSadaf Ebrahimi<P> 133*22dc650dSSadaf Ebrahimi13. There are some differences that are concerned with the settings of captured 134*22dc650dSSadaf Ebrahimistrings when part of a pattern is repeated. For example, matching "aba" against 135*22dc650dSSadaf Ebrahimithe pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to 136*22dc650dSSadaf Ebrahimi"b". 137*22dc650dSSadaf Ebrahimi</P> 138*22dc650dSSadaf Ebrahimi<P> 139*22dc650dSSadaf Ebrahimi14. PCRE2's handling of duplicate capture group numbers and names is not as 140*22dc650dSSadaf Ebrahimigeneral as Perl's. This is a consequence of the fact the PCRE2 works internally 141*22dc650dSSadaf Ebrahimijust with numbers, using an external table to translate between numbers and 142*22dc650dSSadaf Ebrahiminames. In particular, a pattern such as (?|(?<a>A)|(?<b>B)), where the two 143*22dc650dSSadaf Ebrahimicapture groups have the same number but different names, is not supported, and 144*22dc650dSSadaf Ebrahimicauses an error at compile time. If it were allowed, it would not be possible 145*22dc650dSSadaf Ebrahimito distinguish which group matched, because both names map to capture group 146*22dc650dSSadaf Ebrahiminumber 1. To avoid this confusing situation, an error is given at compile time. 147*22dc650dSSadaf Ebrahimi</P> 148*22dc650dSSadaf Ebrahimi<P> 149*22dc650dSSadaf Ebrahimi15. Perl used to recognize comments in some places that PCRE2 does not, for 150*22dc650dSSadaf Ebrahimiexample, between the ( and ? at the start of a group. If the /x modifier is 151*22dc650dSSadaf Ebrahimiset, Perl allowed white space between ( and ? though the latest Perls give an 152*22dc650dSSadaf Ebrahimierror (for a while it was just deprecated). There may still be some cases where 153*22dc650dSSadaf EbrahimiPerl behaves differently. 154*22dc650dSSadaf Ebrahimi</P> 155*22dc650dSSadaf Ebrahimi<P> 156*22dc650dSSadaf Ebrahimi16. Perl, when in warning mode, gives warnings for character classes such as 157*22dc650dSSadaf Ebrahimi[A-\d] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no 158*22dc650dSSadaf Ebrahimiwarning features, so it gives an error in these cases because they are almost 159*22dc650dSSadaf Ebrahimicertainly user mistakes. 160*22dc650dSSadaf Ebrahimi</P> 161*22dc650dSSadaf Ebrahimi<P> 162*22dc650dSSadaf Ebrahimi17. In PCRE2, the upper/lower case character properties Lu and Ll are not 163*22dc650dSSadaf Ebrahimiaffected when case-independent matching is specified. For example, \p{Lu} 164*22dc650dSSadaf Ebrahimialways matches an upper case letter. I think Perl has changed in this respect; 165*22dc650dSSadaf Ebrahimiin the release at the time of writing (5.38), \p{Lu} and \p{Ll} match all 166*22dc650dSSadaf Ebrahimiletters, regardless of case, when case independence is specified. 167*22dc650dSSadaf Ebrahimi</P> 168*22dc650dSSadaf Ebrahimi<P> 169*22dc650dSSadaf Ebrahimi18. From release 5.32.0, Perl locks out the use of \K in lookaround 170*22dc650dSSadaf Ebrahimiassertions. From release 10.38 PCRE2 does the same by default. However, there 171*22dc650dSSadaf Ebrahimiis an option for re-enabling the previous behaviour. When this option is set, 172*22dc650dSSadaf Ebrahimi\K is acted on when it occurs in positive assertions, but is ignored in 173*22dc650dSSadaf Ebrahiminegative assertions. 174*22dc650dSSadaf Ebrahimi</P> 175*22dc650dSSadaf Ebrahimi<P> 176*22dc650dSSadaf Ebrahimi19. PCRE2 provides some extensions to the Perl regular expression facilities. 177*22dc650dSSadaf EbrahimiPerl 5.10 included new features that were not in earlier versions of Perl, some 178*22dc650dSSadaf Ebrahimiof which (such as named parentheses) were in PCRE2 for some time before. This 179*22dc650dSSadaf Ebrahimilist is with respect to Perl 5.38: 180*22dc650dSSadaf Ebrahimi<br> 181*22dc650dSSadaf Ebrahimi<br> 182*22dc650dSSadaf Ebrahimi(a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ 183*22dc650dSSadaf Ebrahimimeta-character matches only at the very end of the string. 184*22dc650dSSadaf Ebrahimi<br> 185*22dc650dSSadaf Ebrahimi<br> 186*22dc650dSSadaf Ebrahimi(b) A backslash followed by a letter with no special meaning is faulted. (Perl 187*22dc650dSSadaf Ebrahimican be made to issue a warning.) 188*22dc650dSSadaf Ebrahimi<br> 189*22dc650dSSadaf Ebrahimi<br> 190*22dc650dSSadaf Ebrahimi(c) If PCRE2_UNGREEDY is set, the greediness of the repetition quantifiers is 191*22dc650dSSadaf Ebrahimiinverted, that is, by default they are not greedy, but if followed by a 192*22dc650dSSadaf Ebrahimiquestion mark they are. 193*22dc650dSSadaf Ebrahimi<br> 194*22dc650dSSadaf Ebrahimi<br> 195*22dc650dSSadaf Ebrahimi(d) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried 196*22dc650dSSadaf Ebrahimionly at the first matching position in the subject string. 197*22dc650dSSadaf Ebrahimi<br> 198*22dc650dSSadaf Ebrahimi<br> 199*22dc650dSSadaf Ebrahimi(e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART 200*22dc650dSSadaf Ebrahimioptions have no Perl equivalents. 201*22dc650dSSadaf Ebrahimi<br> 202*22dc650dSSadaf Ebrahimi<br> 203*22dc650dSSadaf Ebrahimi(f) The \R escape sequence can be restricted to match only CR, LF, or CRLF 204*22dc650dSSadaf Ebrahimiby the PCRE2_BSR_ANYCRLF option. 205*22dc650dSSadaf Ebrahimi<br> 206*22dc650dSSadaf Ebrahimi<br> 207*22dc650dSSadaf Ebrahimi(g) The callout facility is PCRE2-specific. Perl supports codeblocks and 208*22dc650dSSadaf Ebrahimivariable interpolation, but not general hooks on every match. 209*22dc650dSSadaf Ebrahimi<br> 210*22dc650dSSadaf Ebrahimi<br> 211*22dc650dSSadaf Ebrahimi(h) The partial matching facility is PCRE2-specific. 212*22dc650dSSadaf Ebrahimi<br> 213*22dc650dSSadaf Ebrahimi<br> 214*22dc650dSSadaf Ebrahimi(i) The alternative matching function (<b>pcre2_dfa_match()</b> matches in a 215*22dc650dSSadaf Ebrahimidifferent way and is not Perl-compatible. 216*22dc650dSSadaf Ebrahimi<br> 217*22dc650dSSadaf Ebrahimi<br> 218*22dc650dSSadaf Ebrahimi(j) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at 219*22dc650dSSadaf Ebrahimithe start of a pattern. These set overall options that cannot be changed within 220*22dc650dSSadaf Ebrahimithe pattern. 221*22dc650dSSadaf Ebrahimi<br> 222*22dc650dSSadaf Ebrahimi<br> 223*22dc650dSSadaf Ebrahimi(k) PCRE2 supports non-atomic positive lookaround assertions. This is an 224*22dc650dSSadaf Ebrahimiextension to the lookaround facilities. The default, Perl-compatible 225*22dc650dSSadaf Ebrahimilookarounds are atomic. 226*22dc650dSSadaf Ebrahimi<br> 227*22dc650dSSadaf Ebrahimi<br> 228*22dc650dSSadaf Ebrahimi(l) There are three syntactical items in patterns that can refer to a capturing 229*22dc650dSSadaf Ebrahimigroup by number: back references such as \g{2}, subroutine calls such as (?3), 230*22dc650dSSadaf Ebrahimiand condition references such as (?(4)...). PCRE2 supports relative group 231*22dc650dSSadaf Ebrahiminumbers such as +2 and -4 in all three cases. Perl supports both plus and minus 232*22dc650dSSadaf Ebrahimifor subroutine calls, but only minus for back references, and no relative 233*22dc650dSSadaf Ebrahiminumbering at all for conditions. 234*22dc650dSSadaf Ebrahimi</P> 235*22dc650dSSadaf Ebrahimi<P> 236*22dc650dSSadaf Ebrahimi20. Perl has different limits than PCRE2. See the 237*22dc650dSSadaf Ebrahimi<a href="pcre2limit.html"><b>pcre2limit</b></a> 238*22dc650dSSadaf Ebrahimidocumentation for details. Perl went with 5.10 from recursion to iteration 239*22dc650dSSadaf Ebrahimikeeping the intermediate matches on the heap, which is ~10% slower but does not 240*22dc650dSSadaf Ebrahimifall into any stack-overflow limit. PCRE2 made a similar change at release 241*22dc650dSSadaf Ebrahimi10.30, and also has many build-time and run-time customizable limits. 242*22dc650dSSadaf Ebrahimi</P> 243*22dc650dSSadaf Ebrahimi<P> 244*22dc650dSSadaf Ebrahimi21. Unlike Perl, PCRE2 doesn't have character set modifiers and specially no way 245*22dc650dSSadaf Ebrahimito set characters by context just like Perl's "/d". A regular expression using 246*22dc650dSSadaf EbrahimiPCRE2_UTF and PCRE2_UCP will use similar rules to Perl's "/u"; something closer 247*22dc650dSSadaf Ebrahimito "/a" could be selected by adding other PCRE2_EXTRA_ASCII* options on top. 248*22dc650dSSadaf Ebrahimi</P> 249*22dc650dSSadaf Ebrahimi<P> 250*22dc650dSSadaf Ebrahimi22. Some recursive patterns that Perl diagnoses as infinite recursions can be 251*22dc650dSSadaf Ebrahimihandled by PCRE2, either by the interpreter or the JIT. An example is 252*22dc650dSSadaf Ebrahimi/(?:|(?0)abcd)(?(R)|\z)/, which matches a sequence of any number of repeated 253*22dc650dSSadaf Ebrahimi"abcd" substrings at the end of the subject. 254*22dc650dSSadaf Ebrahimi</P> 255*22dc650dSSadaf Ebrahimi<br><b> 256*22dc650dSSadaf EbrahimiAUTHOR 257*22dc650dSSadaf Ebrahimi</b><br> 258*22dc650dSSadaf Ebrahimi<P> 259*22dc650dSSadaf EbrahimiPhilip Hazel 260*22dc650dSSadaf Ebrahimi<br> 261*22dc650dSSadaf EbrahimiRetired from University Computing Service 262*22dc650dSSadaf Ebrahimi<br> 263*22dc650dSSadaf EbrahimiCambridge, England. 264*22dc650dSSadaf Ebrahimi<br> 265*22dc650dSSadaf Ebrahimi</P> 266*22dc650dSSadaf Ebrahimi<br><b> 267*22dc650dSSadaf EbrahimiREVISION 268*22dc650dSSadaf Ebrahimi</b><br> 269*22dc650dSSadaf Ebrahimi<P> 270*22dc650dSSadaf EbrahimiLast updated: 30 November 2023 271*22dc650dSSadaf Ebrahimi<br> 272*22dc650dSSadaf EbrahimiCopyright © 1997-2023 University of Cambridge. 273*22dc650dSSadaf Ebrahimi<br> 274*22dc650dSSadaf Ebrahimi<p> 275*22dc650dSSadaf EbrahimiReturn to the <a href="index.html">PCRE2 index page</a>. 276*22dc650dSSadaf Ebrahimi</p> 277