$re = '/[a-zA-Z]{2,3}\d{2,3}\.[a-zA-Z]{3}\d{3}/';
$str = 'rr01.tdw301
www101.red102
xxx000.xxx000
xx00.xxx000';
preg_match_all($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