#include <algorithm>
#include <iterator>
#include <string>
#include <iostream>

int main()
{
  std::string s = "Hallo, Welt";
  std::string t;
  std::cout << s << '\n';

  std::replace_copy_if(begin(s), end(s), std::back_inserter(t), 
                       [](char c) { return c < 'l'; }, 'x');

  std::cout << t << '\n';
}