main.cpp

#include <iostream>
using namespace std;

std::string unfuck_windows_path(std::string path);
int main() {
    auto in = "C:\\Users\\Мистер Йож\\Мои Документы\\Мои Игры";
    auto out = unfuck_windows_path(in);
    std::cout << out << std::endl;
    return 0;
}

std::string unfuck_windows_path(std::string path) {
    const int backSlashChar = '\\';
    int firstChar = path.find(backSlashChar);
    std::string result = path.substr(0, firstChar);
    result.reserve(path.size());
    for (size_t i = firstChar, end = path.size(); i < end; ++i) {
        switch (auto symbol = path[i]) {
            case 'a' ... 'z': [[fallthrough]];
            case 'A' ... 'Z': [[fallthrough]];
            case '0' ... '9': [[fallthrough]];
            case '-': [[fallthrough]];
            case '_': [[fallthrough]];
            case '/': [[fallthrough]];
            case '.':
                result.push_back(symbol);
                break;
            case backSlashChar:
                result.push_back('/');
                break;
            default:
                result.push_back('_');
                break;
        }
    }
    return result;
}