使用正则写一个计算器

eval 不能直接计算数字表达式
去掉s中的所有空格
写正则表达式 把内部不再有括号的表达式提取出来
写正则表达式 把a*b或者a/b这样的表达式从左到右依次匹配出来
写正则表达式 把加减法匹配出来
能够级匹配整数表达式 也匹配小数表达式

如果 你已经拿到一个 a+b 或者 a*b
写一个函数,计算这样表达式的结果
'1+5'
'1.345+2.567'
s = '1 - 2 * ( (60-30 +(-40/5) * (9-2*5/3 + 7 /3*99/4*2998 +10 * 568/14 )) - (-4*3)/ (16-3*2) )'

import re
s = '1 - 2 * ( (60-30 +(-40/5) * (9-2*5/3 + 7 /3*99/4*2998 +10 * 568/14 )) - (-4*3)/ (16-3*2) )'
s = s.replace(' ','')

def mul_di(num):
    if '/' in num:
        a,b = num.split('/')
        return str(float(a)/float(b))
    elif '*' in num:
        a,b = num.split('*')
        return str(float(a)*float(b))

def mul_div(num):
    while True:
        mu = re.search('\d+(\.\d+)?[*/]-?\d+(\.\d+)?',num)
        if mu:
            mul = mu.group()
            mun = mul_di(mul)
            num = num.replace(mul,mun)
        else:break
    return num

def cha_sym(num):
    num = num.replace('++','+')
    num = num.replace('+-','-')
    num = num.replace('-+','-')
    num = num.replace('--','+')
    return num
def add_sub(num):
    nu = re.findall('[+-]?\d+(?:\.\d+)?', num)
    count = 0
    for i in nu:
        count += float(i)
    return str(count)
def no_sp(num):
    while True:
        nu = re.search('\([^()]+\)',num)
        if nu:
            ret = nu.group()
            Value = mul_div(ret)
            Value = cha_sym(Value)
            new_Value = add_sub(Value)
            num = num.replace(ret,new_Value)
        else:break
    ret = mul_div(num)
    Value = cha_sym(ret)
    return add_sub(Value)
print(no_sp(s))

 

 

递归做法

import re
def calculator(e,n=0,round_num=5):
    if ' ' in e:e = e.replace(' ','')
    regex_lst = ['\(([\d\-\+\.\*\/])+\)', '(\d+\.\d+|\d+)(\*|\/)(\-?\d+\.\d+|\d+)', '(\+|\-){2,}','(\-|\+)?((\d+\.\d+)|\d+)']
    for step in range(n,len(regex_lst)):
        regex = re.compile(regex_lst[step])
        while 1:
            tmp = regex.finditer(e) if step==3 else regex.search(e)
            if not tmp:
                if step==0:return None if ('(' in e or ')' in e) else round(calculator(e,1),round_num)
                break
            if step == 3: return sum([float(i.group()) for i in tmp])
            dic = {0:lambda:calculator(tmp.group()[1:-1],1),
                   1:lambda:float(tmp.group(1))*float(tmp.group(3)) if '*' in tmp.group(2) else float(tmp.group(1))/float(tmp.group(3)),
                   2:lambda:'+' if tmp.group().count('-')%2==0 else '-'}
            e = e.replace(tmp.group(), str(dic[step]()))

s1 = '1+(2-3-4*5/ (-4 - 2)+ 4 5 )-494+5+54+45*452-45245/4524*525-452-(56+45)'
print(calculator(s1))