- preg_match_all: documentation ( source)
- var_export: documentation ( source)
<?php
$tag = "MU";
$string = 'D109 MU, worked from home today
Walked the dog, later took the kids to swimming. D110 MU. Went to the gym in the evening for the 9th time this month.';
var_export(preg_match_all("~D\K\d+(?= $tag)~", $string, $out) ? $out[0] : 'fail');