title: Using regex to match date format in yyyymmdd
body: <p>The regex should match valid dates in a string in the format <code>YYYYMMDD</code>. For example, <code>aaa_20150327_bbb</code> should be matched but <code>aaa_20150229_bbb</code> not because 2015 is not a leap year.</p>
  
  <p>Only year from <em>2000</em> to <em>2099</em> need to be considered.</p>