title: sort dataFrame index containing string and number
body: <p>I have a dataframe where the index value is a mixture of string and number separated by an underscore.</p>
  
  <pre><code>    sub_int1_ICA_int2  # 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/C1ZJa.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/C1ZJa.jpg" alt="enter image description here"></a></p>
  
  <p>I would like to sort the column index using int1 first and after that int2
  The expected output would be:</p>
  
  <pre><code>    sub_1_ICA_1
      sub_1_ICA_2
      sub_1_ICA_3
      ...........
      sub_2_ICA_1
      sub_2_ICA_2
      ...........
  </code></pre>
  
  <p>I tried to use convert_numeric as I saw in many posts, but I get an error </p>
  
  <pre><code>     X.convert_objects(convert_numeric=True).sort_values(['id] , ascending=[True], inplace=True)
      &gt;&gt;(KeyError: 'id')
  </code></pre>
  
  <p>Any help would be nice!</p>