Basic calculator built in Python3

calculator.py

import re

print("Welcome to The Calculator")
print("Type 'quit' to exit\n")
previous = 0
run = True


def perform_math():
    global run
    global previous
    equation = ""
    if previous == 0:
        equation = input("Enter equation:")
    else:
        equation = input("Enter equation (or type clear for new equation): " + str(previous))

    if equation == "quit":
        print("Goodbye, human")
        run = False
    elif equation == "clear":
        previous = 0
    else:
        equation = re.sub('[a-zA-z,.:()" "]', '', equation)

        if previous == 0:
            previous = eval(equation)
        else:
            previous = eval(str(previous) + equation)


while run:
    perform_math()