title: regular expression - match word only once in line
body: <p>Case:</p>
  
  <ol>
  <li>ehello goodbye hellot hello goodbye</li>
  <li>ehello goodbye hello hello goodbye</li>
  </ol>
  
  <p>I want to match line 1 (only has 'hello' once!)
  DO NOT want to match line 2 (contains 'hello' more than once)</p>
  
  <p>Tried using negative look ahead look behind and what not... without any real success..</p>