title: Split and sort Pandas DataFrame in Python
body: <p>I have a pandas dataframe</p>
  <pre><code>         values
  0      x15_2_30
  1     x15_3_137
  2      x15_6_26
  3     x15_9_139
  4    x15_10_143
  ..          ...
  266  x208_6_153
  267  x210_2_147
  268  x211_1_155
  269   x212_3_28
  270   x212_5_25
  </code></pre>
  <p>I would like to sort this dataframe according to the ascending of number in the middle than number at last.</p>
  <p>Preferred output is like that</p>
  <pre><code>x15_1_4
  x12_1_8
  x25_1_12
  ....
  </code></pre>