title: extract date from filename string
body: <p>Im looking to extract the date from this kind of string format. It is fixed in length and pattern</p>
  <pre><code>Files/2020/AUGUST/9009142541082020PT019.txt
  </code></pre>
  <p>Date will always be in the last 15 to 10 characters of the string: 082020.<br />
  Its format will always be %m%Y</p>
  <p>So far i have been using <code>substring(pt19.df$id, 29)</code> but output will still have additional characters at the end <code>082020PT019.txt</code></p>
  <p>can you please advise if there is a more efficient approach?</p>