title: Sort DataFrame index that has a string and number
body: <p>My <code>df</code> DataFrame index looks like this:</p>
  
  <pre><code>Com_Lag_01
  Com_Lag_02
  Com_Lag_03
  Com_Lag_04
  Com_Lag_05
  Com_Lag_06
  Com_Lag_07
  Com_Lag_08
  Com_Lag_09
  Com_Lag_10
  Com_Lag_101
  Com_Lag_102
  Com_Lag_103
  ...
  Com_Lag_11
  Com_Lag_111
  Com_Lag_112
  Com_Lag_113
  Com_Lag_114
  ...
  Com_Lag_12
  Com_Lag_120
  ...
  Com_Lag_13
  Com_Lag_14
  Com_Lag_15
  </code></pre>
  
  <p>I want to sort this index so that the numbers go from <code>Com_Lag_1</code> to <code>Com_Lag_120</code>. If I use <code>df.sort_index()</code> I will get the same thing as above. Any suggestion on how to sort this index properly? </p>