I have a question. My site generates email addresses for registered users. The email addresses look like this "John265Green@top.MYSITE.com" Can you please show me how to make email verification javascript for the following format

1) All entered email addresses must contain "@top.MYSITE.com"

2) The first part of the email address must be "(LETTERS)(3 Numbers)(LETTERS)@top.MYSITE.com"

Thanks in advance for the help guys. I truly appreciate it!