Subdomain Validation

Does anyone know where to find a definitive list of subdomain rules? Better still... does anyone have a script for checking subdomains to make sure that they are valid?  ;)

Thanks

It is very early hour right now, so I guess I have to drink my coffee before try to remember about this :)

Anyway the rules I remember is that a hyphen can not be in beginning or end, and that there should be less than 62 characters (I think including the domain)