title: Validate that first char has to be integer or letter
body: <p>I have this method where I "spot" user input I want to prevent.</p>
  
  <pre><code>def has_forbidden_prefix?(string)
    string =~ %r{^(http://|www.)}
  end
  </code></pre>
  
  <p>If has_forbidden_prefix? is true than I don't want to accept the input.</p>
  
  <p>For example:</p>
  
  <p>Allowed: google.com</p>
  
  <p>Not allowed: www.google.com, <a href="http://google.com" rel="nofollow">http://google.com</a>, <a href="http://www.google.com" rel="nofollow">http://www.google.com</a></p>
  
  <p>Now I want to detect also any beginning special characters in my method.</p>
  
  <p>Not allowed: .google.com, /google.com, ...</p>
  
  <p>What do I have to include in my regex?</p>