Python regex tokenizer

import re

# Supported tokens and their operations
ORDER_OF_OPERATIONS = ["MULTIPLY", "DIVIDE", "ADD", "SUBTRACT"]
scanner = re.Scanner([
    (r"([0-9]+)", lambda x, y: int(y)),
    (r"\+", lambda x, y: "ADD"),
    (r"-", lambda x, y: "SUBTRACT"),
    (r"\*", lambda x, y: "MULTIPLY"),
    (r"/", lambda x, y: "DIVIDE"),
    (r"\s+", lambda x, y: None),
], flags=re.DOTALL)

def operation_multiply(a, b): 
    return a * b 

def operation_divide(a, b): 
    return a / b 

def operation_add(a, b): 
    return a + b 

def operation_subtract(a, b): 
    return a - b 

def interpret(results):
    assert isinstance(results[0], int) and isinstance(results[-1], int), "Trailing operators were encoutered!"
    if len(results) == 1:
        return results[0]
    for operation in reversed(ORDER_OF_OPERATIONS):
        try:
            boundary = results.index(operation)
            return globals()["operation_%s" % operation.lower()](
                interpret(results[:boundary]),
                interpret(results[boundary+1:]),
            )
        except ValueError:
            pass
    assert False, "Missing an operator"

def evaluate(expression):
    results, remainder = scanner.scan(expression)
    assert not remainder, "Some of the input couldn't be interpreted"
    return interpret(results)


assert evaluate("14 + 2 * 3 - 6 / 2") == 17
assert evaluate("(1 + 2) * 3") == 9
assert evaluate("__import__('os').remove('foobar.file')")