<?php
$s = "twoX // threeY";
print_r(preg_split('~\s*(/{2,3})\s*~', $s, -1, PREG_SPLIT_DELIM_CAPTURE
|PREG_SPLIT_NO_EMPTY
));
PD9waHAKCiRzID0gInR3b1ggLy8gdGhyZWVZIjsKcHJpbnRfcihwcmVnX3NwbGl0KCd+XHMqKC97MiwzfSlccyp+JywgJHMsIC0xLCBQUkVHX1NQTElUX0RFTElNX0NBUFRVUkV8UFJFR19TUExJVF9OT19FTVBUWSkpOw==