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<?php preg_match('~a(?=\Kb)~', 'abc', $matches); print_r($matches); preg_match('~a(?=b\K)~', 'abc', $matches); print_r($matches);
Output for 7.3.8, 8.2.0 - 8.2.29, 8.3.0 - 8.3.26, 8.4.1 - 8.4.13
Array ( [0] => ) Warning: preg_match(): Get subpatterns list failed in /in/W9s8q on line 6 Array ( )

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