title: Python regex numbers and underscores
body: <p>I'm trying to get a list of files from a directory whose file names follow this pattern:</p>
  
  <pre><code>PREFIX_YYYY_MM_DD.dat
  </code></pre>
  
  <p>For example</p>
  
  <pre><code>FOO_2016_03_23.dat
  </code></pre>
  
  <p>Can't seem to get the right regex.  I've tried the following:</p>
  
  <pre><code>pattern = re.compile(r'(\d{4})_(\d{2})_(\d{2}).dat')
  &gt;&gt;&gt; []
  
  pattern = re.compile(r'*(\d{4})_(\d{2})_(\d{2}).dat')
  &gt;&gt;&gt; sre_constants.error: nothing to repeat
  </code></pre>
  
  <p>Regex is certainly a weakpoint for me.  Can anyone explain where I'm going wrong?</p>
  
  <p>To get the files, I'm doing:</p>
  
  <pre><code>files = [f for f in os.listdir(directory) if pattern.match(f)] 
  </code></pre>
  
  <p>PS, how would I allow for .dat and .DAT (case insensitive file extension)?</p>
  
  <p>Thanks</p>