I don't think it's worth adding an additional validator just for this, especially as I can't think of any other examples where we'd actually want to use this. Instead, I would actually use the `url` validator here, which will check whether or not the entered domain is a valid one. That should provide a more meaningful validation, takes @sathomas's comment into account and doesn't require a new validator or new error message strings to be set up.

This won't provide upper case validation, but that seems like an edge case that the back-end is now able to deal with anyway and there's nothing stopping people from going through the API directly without validation.