title: Regex for optional first special character
body: <p>Is it possible to create a regex for an optional first character?</p>
  
  <p>My requirement is to create a regex which</p>
  
  <ul>
  <li>accepts alphabets and numbers</li>
  <li>Is between 6 and 100 characters</li>
  <li><p>only contain special character colon as the first character</p></li>
  <li><p>The colon : is an optional character</p></li>
  </ul>
  
  <p>I have made <code>^([:][a-zA-Z0-9]{6,100}$)</code></p>
  
  <p>How can I make the first character : as optional? At the moment it is mandatory.</p>
  
  <p>Thanks in advance!</p>