Password hashing is better than message digests or encryption, and you should be using it instead of message digests or encryption. Find out why and how in this article, which supplements the original article on PHP Client Registration, Login, Logo…
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.
Consider email address RFCs:
Look at HTML5 form input element (with type=email) regex pattern: