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

int main()
{
    std::string s = "A%sBC%sD";


    std::regex regexp("%s");
    std::string new_s = std::regex_replace(s, regexp, "%|s");
    std::cout << s << '\n';
    std::cout << new_s << '\n';
}