namespace dsl = lexy::dsl;

// Parse an IPv4 address into a `std::uint32_t`.
struct ipv4_address
{
    // What is being matched.
    static constexpr auto rule = []{
        // Match a sequence of (decimal) digits and convert it into a std::uint8_t.
        auto octet = dsl::integer<std::uint8_t>;

        // Match four of them separated by periods.
        return dsl::times<4>(octet, dsl::sep(dsl::period)) + dsl::eof;
    }();

    // How the matched output is being stored.
    static constexpr auto value
        = lexy::callback<std::uint32_t>([](std::uint8_t a, std::uint8_t b, std::uint8_t c, std::uint8_t d) {
            return (a << 24) | (b << 16) | (c << 8) | d;
        });
};