cck form data validation

I'm not sure if I'm posting in the right place, but here goes.

I created a very simple CCK based node which is use to add web sites links to a database. there is the "name", the actual "web address", and "body" describing the link and site.

I want to make sure I can validate the "website address" field as a real website (ie www.example.com) instead of someone putting garbage in there.

I have the regular expression script to handle the data, I just don't konw where to stick it. I know drupal has an internal form error handling engine I can hook into. Can someone tell me how this is done?

thanks,
blue