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<?php $str = 'a,b\\\,а\\c,d'; var_dump(preg_split('/(?<![\x00-\x5b\x5d-\xFF]\x5c),/', $str));
based on pI7Ud
Output for 4.3.0 - 7.1.0
array(4) { [0]=> string(1) "a" [1]=> string(3) "b\\" [2]=> string(4) "а\c" [3]=> string(1) "d" }