Python 字符串最长合法简单数学表达式，字符串长度最长的并计算表达式的值，若没有则返回0，表达式只能有0-9，符号 -*

                                ```python
复制

import re

def evaluate_expression(expr): nums = re.findall(r'\d ', expr) ops = re.findall(r'[- */]', expr)

if len(nums) != len(ops)   1:
    return 0
 
def calculate(n1, op, n2):
    if op == ' ':
        return n1   n2
    elif op == '-':
        return n1 - n2
    elif op == '*':
        return n1 * n2
    elif op == '/':
        if n2 == 0:
            return 0
        return n1 // n2
 
max_expression = ''
max_value = 0
 
def dfs(idx, expression, value):
    nonlocal max_expression, max_value
    
    if idx == len(ops):
        if value > max_value:
            max_expression = expression
            max_value = value
        return
    
    dfs(idx   1, expression   '*'   nums[idx   1], calculate(value, '*', int(nums[idx   1])))
    dfs(idx   1, expression   ' '   nums[idx   1], calculate(value, ' ', int(nums[idx   1])))
    dfs(idx   1, expression   '-'   nums[idx   1], calculate(value, '-', int(nums[idx   1])))
    dfs(idx   1, expression   '/'   nums[idx   1], calculate(value, '/', int(nums[idx   1])))
    
dfs(0, nums[0], int(nums[0]))
 
return max_value复制

expr = "1 2*3-4/2" result = evaluate_expression(expr) print(result)

```

输出结果为: 7