Process the first group of a rule.

def _process_first_group(self, group):
        """
        Process the first group of a rule.
        """
        if "-" in group:
            # range
            if len(group.split("-")) == 2:
                arr = group.split("-")
                start = self._parse_codepoint(arr[0])
                end = self._parse_codepoint(arr[1])
        else:
            # single char/U+xxxx
            start = self._parse_codepoint(group)
            end = start
        result = []
        if (start > -1) and (end >= start):
            for index in range(start, end + 1):
                result.append(gf.safe_unichr(index))
        return result