I allow the ability for users to create additional accounts as part of their Company.
IE. Nicky has an account, and her company is Nike. So her email address is nicky@nike.com.
Because her company is 'Nike', I'd only like email addresses she associates with new accounts to be @nike.com. If she tried to create bob@nike.com.au, it wouldn't be allowed.
I'm trying to find the regex that would match Nicky's domain to the one she enters in a new account field. If it's accepted, she can create the new account. Otherwise, it'll throw an error.