title: Only an exact match number in a string
body: <p>How to match (without capturing): 5678</p>
  
  <p>In this string: <code>performer=5678,25678,56342,56782</code></p>
  
  <p>This is what I have so far: <code>5678(?!\d)</code></p>
  
  <p><a href="http://rubular.com/r/WqdK6sQjOK" rel="nofollow">http://rubular.com/r/WqdK6sQjOK</a></p>