Question

How might I split a string (with postfix) into one with parentheses? What I mean is, if the user inputs 3 (3 6 *) * I want the interpreter to be able to multiply 3 18 * and understand the parentheses in the text. Is this possible? Like a str.split() but with a start and end delimiter?

Am I not being realistic? I found the shlex module similar, but it only does quotes AFAIK.

Answer 1

I don’t think using re is such a bad idea. You could just go through replacing the nested parenthesis one at a time with the result of their operation. Seems pretty straight forward to me:

import re
regex = re.compile(r'\([^\(\)]+\)')

def eval_expr(expr):
    match = regex.search(expr)
    if match:
        sub_expr = match.group()
        return eval_expr(expr.replace(sub_expr, str(eval_expr(sub_expr[1:-1]))))
    args = expr.split()
    op = args.pop()
    return eval(op.join(args))

And use like so:

>>> eval_expr('(1 (2 3 -) 4 +) (5 6 *) 7 +')
<<< 41