public function isValidJsonpCallback($subject)
    {
        $identifier_syntax
          = '/^[$_\p{L}][$_\p{L}\p{Mn}\p{Mc}\p{Nd}\p{Pc}\x{200C}\x{200D}]*+$/u';

        $reserved_words = array('break', 'do', 'instanceof', 'typeof', 'case',
          'else', 'new', 'var', 'catch', 'finally', 'return', 'void', 'continue',
          'for', 'switch', 'while', 'debugger', 'function', 'this', 'with',
          'default', 'if', 'throw', 'delete', 'in', 'try', 'class', 'enum',
          'extends', 'super', 'const', 'export', 'import', 'implements', 'let',
          'private', 'public', 'yield', 'interface', 'package', 'protected',
          'static', 'null', 'true', 'false');

        return preg_match($identifier_syntax, $subject)
            && ! in_array(mb_strtolower($subject, 'UTF-8'), $reserved_words);
    }