$re = '/(?:(?P<days>[0-9]{0,2})\s*day[^, ]*,? *)?(?:(?P<hrs>[0-9]+)\s*hour[^, ]*,? *)?(?:(?P<min>[0-9]+)\s*minute[^, ]*,? *)?/';
$str = '27 days, 21 hours, 23 minutes ago
21 hours, 23 minutes ago';
preg_match_all($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