title: Convert string values in clomun to other string
body: <p>consider input df as:</p>
  <pre><code>name         value
  abcd_1       dfgh
  abcd_2       erty
  abcd_3       yuio
  abcd_4       uiop
  abcd_5       yuio
  abcd_6       yuio
  abcd 7       tyui
  abcd_8       tyui
  abcd_9       ghjk
  </code></pre>
  <p>to  be converted as</p>
  <pre><code>name         value
  a1       dfgh
  a1       erty
  a1       yuio
  a2       uiop
  a2       yuio
  a2       yuio
  a3       tyui
  a3       tyui
  a3       ghjk
  </code></pre>
  <p>What I want to happen here is in name column from abcd_1 to abcd_3 has to be replaced by a1 based on index(like abcd_1,2,3...) same ways for others.
  Do not replace string manually but have to be done based on range like 1 to 3 of abcd with a1</p>
  <p>Any help will be appreciated</p>
  <p>Thanks</p>