Iterating regex submatches represented as std::basic_string_view

Question

Is there a direct efficient way to convert std::sub_match to std::basic_string_view (without constructing an intermediate std::basic_string and without intermediate heap allocation)? Or one abstraction level further, is there an alternative to std::regex_token_iterator for iterating regex submatches represented as std::basic_string_view instead of std::sub_match using the std (C++17)?

The reasons why I rather like to use std::basic_string_view over std::sub_match are:

• std::basic_string_view refers to a constant contiguous sequence of char-like objects with the first element of the sequence at position zero. This enables the usage of charconv's std::from_chars (which surprisingly is not implemented using ForwardIterators). This does not seem to be the case for std::sub_match, since it is represented as a pair of BidirectionalIterators.
• std::basic_string_view has a much richer string-like interface facilitating additional context-sensitive tokenization in some exceptional cases for some file formats.

Answer 1

There's no general way to detect whether an iterator is contiguous. We can still handle known contiguous iterators - such as those of std::string:

std::string_view as_sv(std::ssub_match m) {
    if(!m.matched) return {};
    return { &*m.first, m.second - m.first };
}

Handling the remaining named specializations of sub_match is left as an exercise for the reader.