title: Regular Expression doesn't work in Firefox
body: <p>A regular expression which works correctly in Chrome fails with this error message in Firefox: </p>
  
  <blockquote>
    <p>SyntaxError: invalid regexp group</p>
  </blockquote>
  
  <p>The regexp looks like this: <code>/(?&lt;=\{\{)[a-zA-Z0-9_]+(?=\}\})/g</code></p>
  
  <p>I want to get all chars between {{ }}.</p>
  
  <p>Why doesn't this work in Firefox?</p>