$re = '/^\d{1,4}[A-Z\d]?$/m';
$str = '1A
11A
111A
1111A
1
11
111
1111
11111
1A11
2A2
222A1
A11
AA
AA1
A1A
';
preg_match_all($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