Help: Validate Domain Name by Regular Express

PHP 5.3 PCRE Regular Express to match domain names format according to RFC 1034 - DOMAIN NAMES - CONCEPTS AND FACILITIES /^ (

  [a-z]                 |

[a-z] (?:[a-z]|[0-9]) | [a-z] (?:[a-z]|[0-9]|\-){1,61} (?:[a-z]|[0-9]) ) # One label

(?:\.(?1))*+        # More labels
\.?                 # Root domain name

$/iDx This rule matches only <label> and <label>. but not <label>.<label>... I don't know what wrong with it. Thank you.