title: extract numbers from file names
body: <p>I got a lot of files whose names are like this:</p>
  
  <pre><code>tmp1.csv
  tmp32.csv
  tmp9.csv
  tmp76.csv
  ...
  </code></pre>
  
  <p>They are in the same dir, and I want to extract the numbers in the file name. How can I do that in <code>bash</code>?</p>
  
  <p><strong>PS</strong></p>
  
  <p>I tried <code>grep</code>, but can't make it. Also I tried <code>${filename##[a-z]}</code>. Are they the right way to go?</p>