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<?php preg_match('/b(?<a>a)?/', 'xxxxbxxxx', $matches); var_dump($matches);
Output for 4.4.6 - 4.4.9, 5.2.2 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(1) { [0]=> string(1) "b" }
Output for 5.2.0 - 5.2.1
Warning: preg_match(): Compilation failed: unrecognized character after (?< at offset 4 in /in/1j1d4 on line 3 NULL
Output for 4.3.11 - 4.4.5, 5.0.0 - 5.1.6
Warning: preg_match(): Compilation failed: unrecognized character after (?< at offset 4 in /in/1j1d4 on line 3 array(0) { }
Output for 4.3.0 - 4.3.10
Warning: Compilation failed: unrecognized character after (?< at offset 4 in /in/1j1d4 on line 3 array(0) { }