title: Email Regex with firstName.LastName limiting domain name
body: <p>I have this regex</p>
  
  <pre><code>"^\w+@(email1|email2|email3|email4|email5)+?\.[a-zA-Z]{2,3}$"
  </code></pre>
  
  <p>which works to limit what domain name people can use to register. However some people have addresses like this:</p>
  
  <pre><code>FirstName.LastName@email1.com
  </code></pre>
  
  <p>So even though their domain name matches, they're not able to register.</p>
  
  <p>Any suggestions as to how I can include an optional <code>"first.last"</code> in this Regex?</p>