Wordpress post validation - I want to make it require AT LEAST one category

A Wordpress post can be assigned to a category. I need a validation hook or a save_post hook and do some logic so a post has to have between 1 and 5 categories selected in order to be saved. There's a save_post hook but I don't know how to us开发者_如何学编程e it to add extra validation.

All help greatly appreciated!