title: Error I have gotten a few times and I don't know how to fix it
body: <p>I keep seeing undefined <code>(?...)</code> sequence: something. For this particular one I had..</p>
  
  <pre><code>/(?&lt;!\d)[0-3]?\d(?!\d)/
  </code></pre>
  
  <p>but this has happened a few times to me and I'm not sure how to fix it. </p>
  
  <p>These always work in rubular, but then i get that error when I run it? </p>
  
  <p>Help please!</p>