title: Regular expression works on regex101.com, but not on prod
body: <p><a href="https://regex101.com/r/sB9wW6/1" rel="noreferrer">https://regex101.com/r/sB9wW6/1</a></p>
  
  <p><code>(?:(?&lt;=\s)|^)@(\S+)</code> &lt;-- the problem in positive lookbehind</p>
  
  <p>Working like this on <code>prod</code>: <code>(?:\s|^)@(\S+)</code>, but I need a correct start index (without space).</p>
  
  <p>Here is in JS: </p>
  
  <pre><code>var regex = new RegExp(/(?:(?&lt;=\s)|^)@(\S+)/g);
  </code></pre>
  
  <blockquote>
    <p>Error parsing regular expression: Invalid regular expression:
    /(?:(?&lt;=\s)|^)@(\S+)/</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>
  
  <p><strong>UPDATE</strong></p>
  
  <p>Ok, no lookbehind in JS :(</p>
  
  <p>But anyways, I need a regex to get the proper start and end index of my match. Without leading space.</p>