$re = '/^(?<date>\d{4}_?\d{2}_?\d{2})_(?<meeting_type>\w{2,3})_((?<agenda>[a-z0-9]{1,3})_)?(?<description>.+)?\.pdf$/mi';
$str = '20200225_RM_agenda.pdf
20200225_RM_2_memo.pdf
20200225_ss1_3c_presenTATION.pdf
20200225_CA_4d_SiGnEd.pdf
20200225_RM_5_Order1234.pdf
2020_02_25_EV_Notice.pdf';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php