运用正则表达式不使用内置方法实现计算器

#__author__:"Jay guo"
#__date__:2016/9/12
import re
def check(s):
    if re.findall("[a-zA-Z]",s):
        return "quit"
    else:
        return s

def format(s):
    s = s.replace(" ","")
    s = s.replace("++","+")
    s = s.replace("+-","-")
    s = s.replace("-+","-")
    s = s.replace("--","+")
    return s

def mul_exc(s):
    while True:
        ret = re.search("\d+\.?\d*[*/][+-]?\d+\.?\d*", s)
        if ret:
            x,y = re.split("[*/]",ret.group())
            x = float(x)
            y = float(y)
            if "*" in ret.group():
                end = x*y
            else:
                end = x/y
            s = s.replace(ret.group(),str(end))
        else:
            break
    return s

def add_sub(s):
    while True:
        ret = re.search("\d+\.?\d*[+-]\d+\.?\d*", s)
        if ret:
            x,y = re.split("[+-]", ret.group())
            x = float(x)
            y = float(y)
            if "+" in ret.group():
                end = x + y
            else:
                end = x - y
            s = s.replace(ret.group(), str(end))
        else:
            break
    s = s.replace("(","")
    s = s.replace(")","")
    return s

def main():
    while True:
        user = input("PLZ input>>>>:  ")
        user = check(user)
        if user == "quit":
            print ("False")
            continue
        user = format(user)
        user = "(" + user + ")"
        while True:
            ret = re.search("\([^()]*\)",user)
            if ret:
                ret = ret.group()
                ret1 = mul_exc(ret)
                ret1 = add_sub(ret1)
                user = user.replace(ret,ret1)
                user = format(user)
            else:
                user = add_sub(user)
                break
        print("结果",user)

main()