title: extract number from string
body: <p>I have a string ABCD20110420.txt and I want to extract the date out of it. Expected <strong>2011-04-20</strong>
  I can use replace to remove the text part, but how do I insert the "-" ?</p>
  
  <pre><code># echo "ABCD20110420.txt" | replace 'ABCD' '' | replace '.txt' ''
  20110420
  </code></pre>