https://twitter.com/ushitora_anqou/status/1112004970356727810

newline_c.cpp

#include <iostream>
#include <optional>

int main()
{
    std::string input;
    while (std::getline(std::cin, input)) {
        std::optional<int> begin, end;
        for (int i = 0; i < input.size(); i++) {
            char ch = input[i];
            if (ch != '"') continue;

            if (!begin) {
                begin = i;
                continue;
            }

            end = i;
            break;
        }

        if (!begin || !end) {
            std::cout << input << std::endl;
            continue;
        }

        std::cout << input.substr(0, *begin) << "\"\\" << std::endl;
        for (int i = *begin + 1; i < *end; i++)
            std::cout << input[i] << "\\" << std::endl;
        std::cout << "\"" << input.substr(*end + 1) << std::endl;
    }
}