title: sed replace brackets with underscore
body: <p>I am a sed beginner and trying to do a replace on the following string:</p>
  
  <p><code>a_b0_24[0][1]</code> should be changed to     <code>a_b0_24_0__1_</code></p>
  
  <p>There are only numbers within brackets. The text before brackets are only numbers, alphabets or underscore.</p>
  
  <p>Thanks</p>