def get_ids_by_expression(self, expression, threshold=0.001, func=np.sum):
        """ Use a PEG to parse expression and return study IDs."""
        lexer = lp.Lexer()
        lexer.build()
        parser = lp.Parser(
            lexer, self.dataset, threshold=threshold, func=func)
        parser.build()
        return parser.parse(expression).keys().values