title: filter df based on index condition
body: <p>I have a df with lots of rows:</p>
  
  <pre><code>13790226       0.320  0.001976    
  9895d5dis    182.600  0.040450     
  105066007     18.890  0.006432     
  109067019     52.500  0.034011     
  111845014     16.400  0.023974     
  11668574e      7.180  0.070714     
  113307021      4.110  0.017514      
  113679I37      8.180  0.010837     
  </code></pre>
  
  <p>I would like to filter this df in order to obtain the rows where the index last char is not a digit</p>
  
  <p>Desired df: </p>
  
  <pre><code>9895d5dis    182.600  0.040450
  11668574e      7.180  0.070714
  </code></pre>
  
  <p>How can I do it?</p>