boost::text::v1::prev_sentence_break
// In header: <boost/text/sentence_break.hpp> template<code_point_range R> std::ranges::borrowed_iterator_t< R > prev_sentence_break(R && r, std::ranges::iterator_t< R > it);
Finds the nearest sentence break at or before before it. If it == r.begin(), that is returned. Otherwise, the first code point of the sentence that it is within is returned (even if it is already at the first code point of a sentence).
Returns a grapheme_iterator to the nearest sentence break at or before before it. If it == r.begin(), that is returned. Otherwise, the first grapheme of the sentence that it is within is returned (even if it is already at the first grapheme of a sentence).