def build_groups(self, tokens):
        """Build dict of groups from list of tokens"""
        groups = {}
        for token in tokens:
            match_type = MatchType.start if token.group_end else MatchType.single
            groups[token.group_start] = (token, match_type)
            if token.group_end:
                groups[token.group_end] = (token, MatchType.end)
        return groups