code: def handleMatch(self, m):
          username = self.unescape(m.group(2))
  
          
          if MARTOR_ENABLE_CONFIGS['mention'] == 'true':
              if username in [u.username for u in User.objects.exclude(is_active=False)]:
                  url = '{0}{1}/'.format(MARTOR_MARKDOWN_BASE_MENTION_URL, username)
                  el = markdown.util.etree.Element('a')
                  el.set('href', url)
                  el.set('class', 'direct-mention-link')
                  el.text = markdown.util.AtomicString('@' + username)
                  return el
docs: Makesure `username` is registered and actived.