How do I make a form to require certain strings of text?

[JasMate]

Here is the basic input field that I want to change:

PHP Code:

<input type=text maxlength='40' name=ProductName size=31> 


I want to make it so people can only enter domain names into that field. So I need either one of the following; .com, .net, .org, etc, etc. to be required in that field. So if someone types in "holiday units" it will come up with an error message stating that the domain name is not formated properly (or something like that).
How do I do this?
Many thanks for helping out.