disallow internal links / validation

Hey guys,

I am running an external API which needs a special link-format.

The format is an absolute link type like: http://www.domainname.com oder http://domainname.com

How can I validate that my link has always an "http://" infront of the domain name and that there is a top-level ending?

Some users just don't get it to enter the right value. :-)

Thanks alot.