title: Regex to find string with only numbers, but match only when preceeded with # or \s and followed by space
body: <p>I am attempting to find a regex that will find a string of numbers and only match if they are preceded with white space of a pound sign and followed by either white space or a line break. For example, the following would match: </p>
  
  <ol>
  <li><code>#1234</code></li>
  <li><code>#001234</code></li>
  <li><code>000123</code></li>
  <li><code>1234</code></li>
  </ol>
  
  <p>But the following would not:</p>
  
  <ol>
  <li><code>123-456</code></li>
  <li><code>@1234</code></li>
  <li><code>123kok</code></li>
  </ol>
  
  <p>Using one of those online regex sandboxes, I tried to use a negative look behind: 
  <code>\d*(?&lt;=#|\s)\d{1,10}</code>  but I can't get the <em>following</em> to work. So out of these:</p>
  
  <ol>
  <li><code>123-456</code></li>
  <li><code>@1234</code></li>
  <li><code>123kok</code></li>
  </ol>
  
  <p><code>456</code> would match</p>