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run code in 200+ php & hhvm versions
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<?php $str = 'a,b\\\,c,d'; var_dump(preg_split('/(?<!(?!\x5c)(?=\x5c)),/', $str));
based on BB9KT
Output for 4.3.0 - 7.2.0
array(4) { [0]=> string(1) "a" [1]=> string(3) "b\\" [2]=> string(1) "c" [3]=> string(1) "d" }