Postfix math evaluator

gistfile1.py

'''
   Postfix math evaluator
   See: http://en.wikipedia.org/wiki/Reverse_Polish_notation
   by Adrian Statescu <mergesortv@gmail.com>
   MIT Style License
'''
import re

operators = {'+': lambda x,y: x+y,
             '*': lambda x,y: x*y,
             '/': lambda x,y: x/y,
             '-': lambda x,y: x-y}

def postfixeval(str):
    stack = []
    n = len(str)
    ch = None
    for i in range(0,n):
        ch = str[i]
        if re.match('\d',ch):
           stack.append(int(ch))
        elif ch in operators:           
           a = stack.pop(-1)
           b = stack.pop(-1)
           val = operators[ch](a,b)
           stack.append(val)
    return stack[0]         

print postfixeval("2 3 + 7 * 3 *")