<?php
$str = 'a-1 90 b55 0 -4 4 c9';
$array = preg_split('#(?=[abc])#',$str, -1, PREG_SPLIT_NO_EMPTY
);
PD9waHAKCiRzdHIgPSAnYS0xIDkwIGI1NSAwIC00IDQgYzknOwokYXJyYXkgPSBwcmVnX3NwbGl0KCcjKD89W2FiY10pIycsJHN0ciwgLTEsIFBSRUdfU1BMSVRfTk9fRU1QVFkpOwpwcmludF9yKCRhcnJheSk7