Wildcard DNS validation regex.

validate_wildcard_dns.php

<?php
/**
 * Uses a simple regex to validate a wildcard DNS format.
 * 
 * It validates the following entry formats:
 *   *.example.com.
 *   *.exa.m.ple.com.
 *   *.exa-mp.le.com.
 *   *.ex--ample.com. 
 */
function validate_wildcard_dns( $entry ) {
	if ( preg_match( '/^(\\*\\.)+([a-z0-9]([-a-z0-9]*[a-z0-9])?\\.)+([a-z]{2,6})+(\\.)$/i', $entry ) ) {
		return true;
	}
}