title: regex to find between nth to nth occurrence
body: <p>I would like to capture the characters between the 1st and 2nd occurrence of '_' in this string:</p>
  
  <blockquote>
    <p>C2_Sperd20A_XXX_20170301_20170331</p>
  </blockquote>
  
  <p>That is:</p>
  
  <blockquote>
    <p>Sperd20A</p>
  </blockquote>
  
  <p>Thank you</p>