$re = '/^(E|EC|N|NW|SE|SW|W|WC|BR|CR|DA|EN|HA|IG|SL|TN|KT|RM|SM|TW|UB|WD|CM)/m';
$str = 'E1 7AA
E1 7AD
M1 1AG';
preg_match_all($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