title: possible characters base64 url safe function
body: <p>What is the range of possible characters returned from this string?</p>
  
  <pre><code>function base64url_encode($data) 
  { 
    return rtrim(strtr(base64_encode($data), '+/', '-_'), '='); 
  } 
  </code></pre>
  
  <p>My guess is <code>[a-z0-9-_]</code></p>