NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

       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 5.26, but as
       both  Perl  and  PCRE2  are  continually changing, the information may sometimes be out of
       date.

       1. PCRE2 has only a subset of Perl's Unicode support. Details of what  it  does  have  are
       given in the pcre2unicode page.

       2. Like Perl, PCRE2 allows repeat quantifiers 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  some  repeat
       quantifiers  on  other assertions, for example, \b* (but not \b{3}), but these do not seem
       to have any use.

       3. Capture groups that occur inside negative lookaround assertions are counted, but  their
       entries  in  the offsets vector are set only when a negative assertion is a condition that
       has a matching branch (that is, the condition is false).

       4. The following Perl escape sequences are not supported: \F, \l, \L, \u, \U, and \N  when
       followed  by  a  character  name.  \N  on  its  own, matching a non-newline character, and
       \N{U+dd..}, matching a Unicode code point, are supported. The escapes that modify the case
       of following letters 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 either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX
       options is set, \U and \u are interpreted as ECMAScript interprets them.

       5. The Perl escape sequences \p, \P, and \X are supported only  if  PCRE2  is  built  with
       Unicode  support  (the  default).  The  properties  that  can be tested with \p and \P 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."

       6. PCRE2 supports the \Q...\E escape for quoting substrings.  Characters  in  between  are
       treated  as  literals.  However,  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). Also, Perl does "double-quotish backslash
       interpolation" on any backslashes between \Q and \E which, its  documentation  says,  "may
       lead to confusing results". PCRE2 treats a backslash between \Q and \E just like any other
       character. Note the following examples:

           Pattern            PCRE2 matches     Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
           \QA\B\E            A\B               A\B
           \Q\\E              \                 \\E

       The \Q...\E sequence is recognized both inside and outside character classes.

       7. Fairly obviously, PCRE2 does not support the (?{code})  and  (??{code})  constructions.
       However,  PCRE2  does  have  a  "callout" feature, which allows an external function to be
       called during pattern matching. See the pcre2callout documentation for details.

       8. Subroutine calls (whether recursive or not) were treated as atomic groups up  to  PCRE2
       release 10.23, but from release 10.30 this changed, and backtracking into subroutine calls
       is now supported, as in Perl.

       9. If any of the backtracking control verbs are used in  a  group  that  is  called  as  a
       subroutine  (whether  or not recursively), their effect is confined to that group; 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 groups are
       processed as anchored at the point where they are tested.

       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 cases where it differs.

       11. Most backtracking verbs in assertions have their normal actions. They are not confined
       to the assertion.

       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".

       13.  PCRE2's  handling  of  duplicate capture group numbers and 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  capture  groups  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 group matched, because  both  names
       map  to  capture  group  number 1. To avoid this confusing situation, an error is given at
       compile time.

       14. Perl used to recognize comments in some places  that  PCRE2  does  not,  for  example,
       between the ( and ? at the start of a group. If the /x modifier is set, Perl allowed white
       space between ( and ? though the latest Perls give an error  (for  a  while  it  was  just
       deprecated). There may still be some cases where Perl behaves differently.

       15.  Perl,  when  in  warning mode, gives warnings for character classes such as [A-\d] 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.

       16.  In  PCRE2,  the upper/lower case character properties Lu and Ll are not affected when
       case-independent matching is specified. For example, \p{Lu} always matches an  upper  case
       letter.  I  think  Perl has changed in this respect; in the release at the time of writing
       (5.24), \p{Lu} and \p{Ll} match all letters, regardless of case, when case independence is
       specified.

       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)  were in PCRE2 for some time before. This list is with respect to Perl
       5.26:

       (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.

       (b) From  PCRE2  10.23,  backreferences  to  groups  of  fixed  length  are  supported  in
       lookbehinds,  provided  that there is no possibility of referencing a non-unique number or
       name. Perl does not support backreferences in lookbehinds.

       (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set,  the  $  meta-character
       matches only at the very end of the string.

       (d) A backslash followed by a letter with no special meaning is faulted. (Perl can be made
       to issue a warning.)

       (e) 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.

       (f) 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.

       (g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART options have
       no Perl equivalents.

       (h)  The  \R  escape  sequence  can  be  restricted  to  match only CR, LF, or CRLF by the
       PCRE2_BSR_ANYCRLF option.

       (i) The  callout  facility  is  PCRE2-specific.  Perl  supports  codeblocks  and  variable
       interpolation, but not general hooks on every match.

       (j) The partial matching facility is PCRE2-specific.

       (k) The alternative matching function (pcre2_dfa_match() matches in a different way and is
       not Perl-compatible.

       (l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at the start  of  a
       pattern that set overall options that cannot be changed within the pattern.

       (m)  PCRE2 supports non-atomic positive lookaround assertions. This is an extension to the
       lookaround facilities. The default, Perl-compatible lookarounds are atomic.

       18. The Perl /a modifier restricts  /d  numbers  to  pure  ascii,  and  the  /aa  modifier
       restricts  /i  case-insensitive  matching  to  pure  ascii,  ignoring  Unicode rules. This
       separation cannot be represented with PCRE2_UCP.

       19. Perl has different limits than PCRE2. See the pcre2limit  documentation  for  details.
       Perl  went  with  5.10 from recursion to iteration keeping the intermediate matches on the
       heap, which is ~10% slower but does not fall into any stack-overflow limit. PCRE2  made  a
       similar  change  at  release 10.30, and also has many build-time and run-time customizable
       limits.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 13 July 2019
       Copyright (c) 1997-2019 University of Cambridge.