Bug #80645 [Opn]: PHPs behavior of empty string matching is incorrect

Edit report at https://bugs.php.net/bug.php?id=80645&edit=1 ID: 80645 Updated by: cmb@php.net Reported by: firas dot dib at gmail dot com Summary: PHPs behavior of empty string matching is incorrect Status: Open Type: Bug Package: PCRE related Operating System: n/a PHP Version: 8.0.1 Block user comment: N Private report: N New Comment: Respective commit of PCRE2: <https://vcs.pcre.org/pcre2?view=revision&revision=955>. Previous Comments: ------------------------------------------------------------------------ [2021-01-19 13:03:55] firas dot dib at gmail dot com Description: ------------ PHP is currently not matching empty strings in a perl compatible way. If we use the following regex: /(?<=(\G.{2}))(?!$)/g and the following string: 123456789 We would get the following results in PCRE1 and PHP (both PHP <7.3 and >=7.3): "12", "45", "78" However, in PCRE2 (and Perl), we would get: "12", "34", "56", "78" This can be verified by using pcre2test and perltest that is bundled with PCRE2. The changelog for PCRE2 10.32 states the following: 21. In both pcre2test and pcre2_substitute(), with global matching, a pattern that matched an empty string, but never at the starting match offset, was not handled in a Perl-compatible way. The pattern /(<?=\G.)/ is an example of such a pattern. Because \G is in a lookbehind assertion, there has to be a "bumpalong" before there can be a match. The automatic "advance by one character after an empty string match" rule is therefore inappropriate. A more complicated algorithm has now been implemented. I am not sure if this behavior is intentional or not, but figured I'd highlight it to you. Best, Firas Test script: --------------- <?php preg_match_all("/(?<=(\G.{2}))(?!$)/", "123456789", $matches); var_dump($matches); ?> Expected result: ---------------- "12", "34", "56", "78" Actual result: -------------- "12", "45", "78" ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=80645&edit=1