title: list.files pattern argument in R, extended regular expression use
body: <p>I run</p>
  
  <pre><code>  dir.create('./junk_data')
    file.create(paste('./junk_data/QWE',01:12,01:31,2005:2015,'.3',sep=''))
    file.create(paste('./junk_data/RTY',01:12,01:31,2005:2015,'.3',sep=''))
  </code></pre>
  
  <p>and want to list all the files that <em>begin</em> with <code>QWE</code> and <em>end</em> with <code>2011.3</code>. I tried</p>
  
  <pre><code>list.files('./junk_data/',pattern='QWE....2011.3',full.names=T)
  </code></pre>
  
  <p>and </p>
  
  <pre><code>list.files('./junk_data/',pattern='QWE....2011.3',full.names=T,perl=T)
  </code></pre>
  
  <p>but I guess <code>'.'</code> doesn't mean one what I think, as I get none of the files I want.</p>
  
  <p>I tried a few tutorials on regex, but no joy.</p>