title: React Router - how to constrain params in route matching?
body: <p>I don't really get how to constrain params with, for example a regex.<br>
  How to differentiate these two routes?</p>
  
  <pre><code>  &lt;Router&gt;
      &lt;Route path="/:alpha_index" component={Child1} /&gt;
      &lt;Route path="/:numeric_index" component={Child2} /&gt;
    &lt;/Router&gt;
  </code></pre>
  
  <p>And prevent "/123" from firing the first route?</p>