What is a unmatched match in regex?

Question

From https://en.cppreference.com/w/cpp/regex/match_results/str:

if n >= size(), a string representing the unmatched match is returned.

What is this unmatched match?

Answer 1

The call is equivalent to string_type((*this)[n]);

and operator[] gives more info:

if n >= size(), returns a reference to a std::sub_match representing an unmatched sub-expression (an empty subrange of the target sequence).