/**
  * Get list of users mentioned in given text.
  */
 protected function findMentionedUsers(string $text) : Collection
 {
     preg_match_all('/@([a-z0-9_-]+)/i', $text, $matches, PREG_SET_ORDER);
     $nicknames = array_pluck($matches, 1);
     return User::whereIn('name', $nicknames)->get();
 }