$re = '/^PT(\d+H)?(\d+M)?(\d+S)?$/m';
$str = 'PT1H37S
PT1H30M15S
PT0S
PT1H0M0S
PT22H41M59S';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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