title: regex - return all before the second occurrence
body: <p>Given this string:</p>
  
  <pre><code>DNS000001320_309.0/121.0_t0
  </code></pre>
  
  <p>How would I return everything before the second occurrence of "_"?</p>
  
  <pre><code>DNS000001320_309.0/121.0
  </code></pre>
  
  <p>I am using R.</p>
  
  <p>Thanks.</p>