Allow binding to specific interface addresses

https://trello.com/c/aWazKGQc/404-allow-binding-to-specified-ip-address
Match the whole string using ^$ anchors instead of \b:
>  ^(?:(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.){3}(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])$