Good way to get CCK link to enforce user input of valid link?

It's convenient that the CCK field can automatically parse both internal and external links, but what if I want to enforce the user entering a valid URL? As it is now, if a user enters "cat" in the field, the node the user creates will have a link pointing to mysite.com/cat. I want to ensure only links of type http://www.example.com are entered.

Thanks!