No idea what CTCI is finding hard here

calculate.py

import re

OPERATIONS = {
    '*': lambda a, b: a * b,
    '/': lambda a, b: a / b,
    '+': lambda a, b: a + b,
    '-': lambda a, b: a - b,
}


def perform_operation(numbers, op):
    first = numbers.pop()
    second = numbers.pop()
    result = OPERATIONS[op](first, second)
    numbers.append(result)


def calculate(equation):
    # Turn into stack
    numbers = [float(i) for i in reversed(re.findall(r'\d+', equation))]
    operations = re.findall(r'[*/+-]', equation)[::-1]

    while operations:
        op = operations.pop()
        if op in '*/':
            perform_operation(numbers, op)
        else:
            temp = numbers.pop()
            while operations and operations[-1] in '*/':
                perform_operation(numbers, operations.pop())
            numbers.append(temp)
            perform_operation(numbers, op)

    return numbers[-1]
  
def main():
    print(calculate('2*3+5/6*3+15'))