Not a clue

Ok, so I started with this:

LEFT = '{[('
RIGHT = '}])'

def checkio(expression):
    l_brackets = []
    r_brackets = []
    for i in expression:
        if i in LEFT:
            l_brackets.append(i)
        elif i in RIGHT:
            r_brackets.append(i)
    for l, r in zip(l_brackets, r_brackets[::-1]):
        lc, rc = ord(l), ord(r)
        if lc != rc-1 and lc != rc-2:
            return False
    return True

This worked for the first few examples, but then it fails with the expression checkio("[1+1]+(2*2)-{3/3}"). Now, I am clueless. That one expression renders my entire code useless. Any clues for me? Because this is doing my head in!