$re = '/^([A-Z]\d{1,2}|[A-Z]{2}\d(\d|[A-Z])?)( \d[A-Z]{2})?$/mi';
$str = '1 Letter 1 Number
E2
1 Letter 2 Numbers
E22
2 Letters 1 Number
EE2
2 Letters 1 Number 1 Letter
EE2E
2 Letters 2 Numbers
EE22
1 Letter 1 Number (Space) 1 Number 2 Letters
E2 2EE
1 Letter 2 Numbers (Space) 1 Number 2 Letters
E22 2EE
2 Letters 1 Number (Space) 1 Number 2 Letters
EE2 2EE
2 Letters 1 Number 1 Letter (Space) 1 Number 2 Letters
EE2E 2EE
2 Letters 2 Numbers (Space) 1 Number 2 Letters
EE22 2EE
6FW OX28
82XO 6FW
OX28 6FW
E2E
E22E
E33 33E';
preg_match_all($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