Create a Custom registration field/Modify Feild?

Hi,

I’ have a quick question, I’m a rookie at HTML/CSS/Javascript so I have no idea how to do this.

What I want to do is:

Create a custom registration feild or modify the email feild in the registration form. I want to modify it so that it only accepts emails from a certain web domain. EG) If a new user signs up and tries to sign up with an @wordpress.com address they will not be accepted; There will be a pop-up that says – we only accept @wordpress.org emails, Then when they typoe in there @wordpress.org email it will accept them.

If there anyway at all to do this?
Help is much appreciated.
Thanks,

Jack