title: R list files with multiple conditions
body: <p>I want to list all files in a directory that met certain conditions (date and currency). So with only one condition the argument <code>pattern</code> in <code>list.files</code> works well:</p>
  
  <pre><code>    file.ls &lt;- list.files(path='~/DATA/PiP/Curvas/',pattern='20130801')
  </code></pre>
  
  <p>For multiple conditions I've tried:</p>
  
  <pre><code>    file.ls &lt;- list.files(path='~/DATA/PiP/Curvas/',pattern=c('20130801','USD'))
  </code></pre>
  
  <p>But had the same result as the first one. Is there a way to have multiple criteria in <code>pattern</code> argument of <code>list.files</code>?</p>