title: Valid characters for URI schemes?
body: <p>I was thinking about <a href="http://msdn.microsoft.com/en-us/library/aa767914.aspx" rel="nofollow noreferrer">Registering an Application to a URL Protocol</a> and I'd like to know, what characters are allowed in a scheme?</p>
  <p>Some examples:</p>
  <ul>
  <li><a href="https://www.rfc-editor.org/rfc/rfc3508" rel="nofollow noreferrer">h323</a> (has numbers)
  <ul>
  <li><code>h323:[&lt;user&gt;@]&lt;host&gt;[:&lt;port&gt;][;&lt;parameters&gt;]</code></li>
  </ul>
  </li>
  <li><a href="https://www.rfc-editor.org/rfc/rfc2056" rel="nofollow noreferrer">z39.50r</a> (has a <code>.</code> as well)
  <ul>
  <li><code>z39.50r://&lt;host&gt;[:&lt;port&gt;]/&lt;database&gt;?&lt;docid&gt;[;esn=&lt;elementset&gt;][;rs=&lt;recordsyntax&gt;]</code></li>
  </ul>
  </li>
  <li><a href="http://derailer.org/paparazzi/faq" rel="nofollow noreferrer">paparazzi:http</a> (has a <code>:</code>)
  <ul>
  <li><code>paparazzi:http:[//&lt;host&gt;[:[&lt;port&gt;][&lt;transport&gt;]]/</code></li>
  </ul>
  </li>
  </ul>
  <p>So, what characters can I fancy using?<br />
  Can we have...</p>
  <ul>
  <li><code>@:TwitterUser</code></li>
  <li><code>#:HashTag</code></li>
  <li><code>$:CapitalStock</code></li>
  <li><code>?:ID-10T</code></li>
  </ul>
  <p>...etc., as desired, or characters in the scheme are restricted by standard?</p>