title: Create regular expression in ruby on rails for validate username
body: <p>I need create a regular expression in ruby on rails for validate the user name following this rules:</p>
  
  <pre><code> 1)- The user name accept only lowercase letters, numbers,dashes and underscores
   2)- The first character must be a lowercase letter
   3)- The username have as minimum 6 characters and 30 characters as maximun
  </code></pre>
  
  <p>I thought some like this, but I'm not very sure</p>
  
  <pre><code>  /^ ([a-z][a-z\_-0-9]+){6,30}  $/
  </code></pre>