title: RelaxNG choice and regular expression
body: <p>I have a RelaxNG based XML instance which should allow (choice) <code>align="right"</code>, <code>align="left"</code> and <code>align="{ anything here }"</code>. The right/left is easy:</p>
  
  <pre><code> attribute name="align"&gt;
     &lt;a:documentation&gt;doc for attrib&lt;/a:documentation&gt;
     &lt;choice&gt;
        &lt;value&gt;left&lt;/value&gt;
        &lt;a:documentation&gt;doc for left&lt;/a:documentation&gt;
        &lt;value&gt;right&lt;/value&gt;
        &lt;a:documentation&gt;doc for right&lt;/a:documentation&gt;
     &lt;/choice&gt;
  &lt;/attribute&gt;
  </code></pre>
  
  <p>But how can I allow the user to enter <code>{ ... }</code> as an alternative to the two values above?</p>