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<?php preg_replace_callback('/a(\w)|b(\w)/','var_dump','axby');
Output for 4.3.0 - 7.2.0
array(2) { [0]=> string(2) "ax" [1]=> string(1) "x" } array(3) { [0]=> string(2) "by" [1]=> string(0) "" [2]=> string(1) "y" }