title: Invalid pattern in look-behind
body: <p>Why does this regex work in Python but not in Ruby:</p>
  
  <pre><code>/(?&lt;!([0-1\b][0-9]|[2][0-3]))/
  </code></pre>
  
  <p>Would be great to hear an explanation and also how to get around it in Ruby</p>
  
  <p>EDIT w/ the whole line of code:</p>
  
  <pre><code>re.sub(r'(?&lt;!([0-1\b][0-9]|[2][0-3])):(?!([0-5][0-9])((?i)(am)|(pm)|(a\.m)|(p\.m)|(a\.m\.)|(p\.m\.))?\b)' , ':\n' , s)
  </code></pre>
  
  <p>Basically, I'm trying to add <code>'\n'</code> when there is a colon and it is not a time.</p>