Regular expression that matches valid IPv6 addresses

Answer 1

For PHP 5.2+ users filter_var works great.

I know this doesn't answer the original question (specifically a regex solution), but I post this in the hope it may help someone else in the future.

$is_ip4address = (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4) !== FALSE);
$is_ip6address = (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6) !== FALSE);