title: Split string by words in R
body: <p>I would like to split a string by two words:</p>
  
  <pre><code>s &lt;- "PCB153 treated HepG2 cells at T18"
  strsplit(s, split = &lt;treated&gt;&lt;at&gt;)
  </code></pre>
  
  <p>What should I write instead of &lt;>?</p>
  
  <p>I would get:</p>
  
  <pre><code>"PCB153" "HepG2 cells" "T18"
  </code></pre>