c语言判断字符串是否为合法的十六进制串

键盘输入一串字符，以回车结束，如果输入的字符是合法的16进制串，则输出“yes”，否则输出“no”

一个个字符拿出来判断，判断开头0X，剩下字符是数字或者是A~F

#include <iostream>
#include <string>
using namespace std;
int main()
{
    string str; cin >> str;
    int len = str.length();
    if (str[0] != '0' || str[1] != 'x')
    {
        cout << "no" << endl;
        return 0;
    }
    for (int i = 2; i < len; i++)
    {
        if (str[i] >= '0' && str[i] <= '9' || str[i] >= 'a' && str[i] <= 'f' || str[i] >= 'A' && str[i] <= 'F')
        {
            continue;
        }
        else
        {
            cout << "no" << endl;
            return 0;
        }
    }
    cout << "yes" << endl;
    return 0;
}