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<?php $str = 'a\\\\,b,c\\,d'; $str2 = 'a\\\\,b,c\\,d'; var_dump(preg_split('/(?<!(?<!\\\\)\\\\),/', $str)); var_dump(preg_split('/(?<![^\\\\])(?<!\\\\),/', $str2));

preferences:
41.96 ms | 402 KiB | 5 Q