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) or re.findall("[*/][^\d(]",s):
        return
    else:
        return s

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


def add_sub(s):
    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))
        s = s.replace("(","")
        s = s.replace(")","")
    return s
def mul_exc(s):
    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))
        s = s.replace("(","")
        s = s.replace(")","")
    return s

def main():
    while True:
        ret = input("PLZ input>>>>:  ")
        ret = check(ret)
        ret = format(ret)
        while True:
            ret = mul_exc(ret)
            ret = add_sub(ret)
            ret = format(ret)
            a = len(re.findall("\d+", ret))
            print ("a",a,ret)
            if a == 2:
                break
        print (re.sub("^\+","",ret))

main()