如何从字符串中仅提取有效的公式

import re

def extract_valid_formula(string):
    pattern = r"([a-zA-Z]+|[0-9]+|\+|\-|\*|\/|\(|\))"
    tokens = re.findall(pattern, string)
    
    stack = []
    valid_formula = ""
    
    for token in tokens:
        if token == "(":
            stack.append(token)
        elif token == ")":
            if len(stack) > 0 and stack[-1] == "(":
                stack.pop()
            else:
                return "Invalid formula"
        valid_formula += token
    
    if len(stack) > 0:
        return "Invalid formula"
    
    return valid_formula

# 示例用法
string = "3 + (4 * 5) / 2 - 6"
valid_formula = extract_valid_formula(string)
print(valid_formula)