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<?php $string = '(ab(cd)ef))'; preg_match_all('@\( ( ( (?>[^()]+) | (?R) )* ) \)@x',$string,$matches); var_dump($matches);
Output for 4.3.0 - 7.1.0
array(3) { [0]=> array(1) { [0]=> string(10) "(ab(cd)ef)" } [1]=> array(1) { [0]=> string(8) "ab(cd)ef" } [2]=> array(1) { [0]=> string(2) "ef" } }