$re = '/^[0-9]{1,2}\/[a-zA-Z]-[0-9]\/[A-Z]+\/([0-9.]+\/)?[0-9]+$/m';
$str = '11/A-9/XI/2015
12/A-9/XII/2.99/2015
12/A-9/XII/2.99/a2015
a12/A-9/XII/2.99/2015
12/A-9/XII/2.99/2015a';
preg_match_all($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