title: Prevent sequential repeated chars using Regular Expression
body: <p>I'm trying to learn regular expression. I wrote this regex.</p>
  
  <pre><code>^[a-zA-Z]{1}[a-zA-Z0-9.]*@{1}([a-z]+).[a-z]{2,5}$
  </code></pre>
  
  <p>in fact, I'm not going to use this as an email validator. just testing.<br>
  results of this regex are :</p>
  
  <pre><code>exchange@stack.com          // match
  s.@.com                     // not
  @overflow.c@com             // not
  stack..@o.net               // match
  stackov@@.com               // not
  @overflow.com@s.stack       // not
  1#stack.@s.com              // not
  </code></pre>
  
  <p>but as you see, <code>stack..@o.net</code> is also known as a valid expression.
  How can I detect those sequentially repeated <code>.</code>s in string?</p>