C++创建一个解析器

#include<iostream>
#include<string>
#include<regex>

bool is_number(const std::string& s) {
    return !s.empty() && std::find_if(s.begin(), s.end(), [](char c) { return !std::isdigit(c); }) == s.end();
}

int parse_expression(const std::string& expression) {
    std::regex regex("(\\d+|\\+|\\-|\\*|\\/)");
    std::sregex_token_iterator it{expression.begin(), expression.end(), regex, -1};
    std::sregex_token_iterator end;

    int result = 0;
    char op = '+';

    while (it != end) {
        std::string token = *it++;

        if (is_number(token)) {
            int num = std::stoi(token);

            if (op == '+') {
                result += num;
            } else if (op == '-') {
                result -= num;
            } else if (op == '*') {
                result *= num;
            } else if (op == '/') {
                result /= num;
            }
        } else {
            op = token[0];
        }
    }

    return result;
}

int main() {
    std::string expression = "3+5*2-4/2";
    int result = parse_expression(expression);
    std::cout << "The result of the expression "<< expression << " is: "<< result<< std::endl;
    return 0;
}