title: YYYY/MM/DD date format regular expression
body: <p>I want to use regular expression for matching  these date formats as below in C#.</p>
  
  <ol>
  <li>YYYY/MM/DD  2013/11/12</li>
  <li>YYYY/M/DD   2013/5/11</li>
  <li>YYYY/MM/D   2013/10/5 </li>
  <li>YYYY/M/D    2013/5/6</li>
  </ol>
  
  <p>I have tried some regular expressions but they can't match the 4 date formats.
  such as</p>
  
  <pre><code>^(19|20)\d\d[- /.](0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])
  </code></pre>