<?php $re = "/(?<=^\S|\s\S|\s)[RD]/"; $str = "-D-K4_NNNN_M116_R1_001.gz _D-K4_NNNN_M56_R1_001.gz R-K4_NNNN_KQ9_R1_001.gz D-K4_NNNN_M987_R1_001.gz _R-K4_NNNN_M987_R1_001.gz"; preg_match_all($re, $str, $matches); echo '<pre>'; print_r($matches); echo '</pre>';
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