利用正则表达式模拟计算器进行字符串的计算实现eval()内置函数功能

代码感觉有点绕，刚开始学习python，相关知识点还没全部学习到，还请各位大神多多指教

import re

# 定义乘法
def mul(string):
    mul1 = re.search('-?\d+(?:\.\d+)?\*-?\d+(?:\.\d+)?', string)
    str1 = mul1.group()
    li = re.split('\*', str1)
    mul_ret = float(li[0]) * float(li[1])
    if mul_ret < 0:
        return string.replace(string[mul1.start(): mul1.end()], str(mul_ret))
    else:
        return string.replace(string[mul1.start(): mul1.end()], '+' + str(mul_ret))

# 定义除法
def div(string):
    div1 = re.search('-?\d+(?:\.\d+)?/-?\d+(?:\.\d+)?', string)
    str1 = div1.group()
    li = re.split('/', str1)
    div_ret = float(li[0]) / float(li[1])
    if div_ret < 0:
        return string.replace(string[div1.start(): div1.end()], str(div_ret))
    else:
        return string.replace(string[div1.start(): div1.end()], '+' + str(div_ret))

# 定义加法
def add(string):
    add1 = re.search('-?\d+(?:\.\d+)?\+-?\d+(?:\.\d+)?', string)
    str1 = add1.group()
    li = re.split('\+', str1)
    add_ret = float(li[0]) + float(li[1])
    return string.replace(string[add1.start(): add1.end()], str(add_ret))

# 定义减法
def sub(string):
    sub1 = re.search('-?\d+(?:\.\d+)?--?\d+(?:\.\d+)?', string)
    str1 = sub1.group()
    if str1[0] == '-':
        str1 = str1[1:]
        li = re.split('-', str1, 1)
        sub_ret = 0 - float(li[0]) - float(li[1])
        return string.replace(string[sub1.start(): sub1.end()], str(sub_ret))
    else:
        li = re.split('-', str1, 1)
        sub_ret = float(li[0]) - float(li[1])
        return string.replace(string[sub1.start(): sub1.end()], str(sub_ret))

# 去除括号
def brackets(string):
    string = re.sub(r' ', '', string)
    n = string.count('(')
    while n:
        n -= 1
        m1 = re.search('(?<=\()[^\(\)]*(?=\))', string)
        str1 = m1.group()
        str1_ret = cal(str1)
        string = string.replace(string[m1.start() - 1: m1.end() + 1], str(str1_ret))
    return string

# 定义不含括号的运算

def cal(string):
    li1 = re.findall('\*|/', string)
    for i in li1:
        if i == '*':
            string = mul(string)
        elif i == '/':
            string = div(string)
    if string[0] == '-' or '+':
        string = '0' + string
    li2 = re.findall('\+|-', string)
    n = len(li2)
    while n:
        string = string.replace('+-', '-')
        string = string.replace('--', '+')
        string = string.replace('-+', '-')
        string = string.replace('++', '+')
        n -= 1
    li3 = re.findall('\+|-', string)
    for i in li3:
        if i == '+':
            string = add(string)
        elif i == '-':
            string = sub(string)
    return string

def jm_cal(string): 
　　cal_ret = cal(brackets(string)) 
　　return cal_ret

输入：

s = '-45 + 15 * (12 - 68 / (11 - 5 * 2 + (-9 )/ 4)* 6) / 4 + 6 - (14 / 9 + 8 * (-7) - 6 * 2 + 13) * 12'

print(jm_cal(s))

输出：

1871.3333333333333

与直接调用eval()内置函数计算结果一致