to_slug

template <typename InputIterator, typename OutputIterator>
auto to_slug(InputIterator first, InputIterator last, OutputIterator out, std::locale const& locale)
    -> std::pair<OutputIterator, bool>
{
    using char_t = decltype(*first);
    constexpr auto hyphen = char_t {'-'};

    auto const next = [&](InputIterator it) {
        return std::find_if(it, last, [&](char_t ch) {
            return std::isalnum(ch, locale);
        });
    };

    auto it = next(first);
    if (it == last) {
        return std::make_pair(out, false);
    }

    bool const is_digit = std::isdigit(*it, locale);
    out++ = std::tolower(*it++, locale);

    while (it != last) {
        if (std::isalnum(*it, locale)) {
            out++ = std::tolower(*it++, locale);
        } else if ((it = next(++it)) != last) {
            out++ = hyphen;
        }
    }

    return std::make_pair(out, !is_digit);
}

template <typename InputIterator, typename OutputIterator>
auto to_slug(InputIterator first, InputIterator last, OutputIterator out) {
    return to_slug(first, last, out, std::locale::classic());
}

void main() {
    to_slug(std::istream_iterator<char>(std::cin)
          , std::istream_iterator<char>()
          , std::ostream_iterator<char>(std::cout));
}