title: Regex expression to validate an unquoted alphanumeric csv list in a attribute route regex constraint
body: <p>I'm looking for a RegEx pattern to validate an <strong><em>attribute route</em></strong> with a CSV parameter in the URL.</p>
  
  <p>A valid URL would look like this (list of 0 to n elements):</p>
  
  <pre><code>http://site/prefix/list/a,b1,c123[,...]
  </code></pre>
  
  <p>The attribute route:</p>
  
  <pre><code>Route("List/{Items:alpha:regex(&lt;pattern&gt;)")
  </code></pre>
  
  <p>I don't have much experience with RegEx and am not having much luck with the pattern.</p>