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run code in 150+ php & hhvm versions
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<?php $str = 'a,b\\\,ё\\,c,d'; var_dump(preg_split('/(?<![\x00-\x5b\x5d-\xFF]\x5c),/', $str));
based on 0UiEG
Output for 4.3.3 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(4) { [0]=> string(1) "a" [1]=> string(3) "b\\" [2]=> string(5) "ё\,c" [3]=> string(1) "d" }
Output for 4.3.0 - 4.3.2
array(5) { [0]=> string(1) "a" [1]=> string(3) "b\\" [2]=> string(3) "ё\" [3]=> string(1) "c" [4]=> string(1) "d" }