cpp/regex/match results/operator at

If and, returns a reference to the std representing the part of the target sequence that was matched by the nth captured  marked subexpression).

If, returns a reference to the std representing the part of the target sequence matched by the entire matched regular expression.

if, returns a reference to a std representing an unmatched sub-expression (an empty subrange of the target sequence).

The behavior is undefined unless

Return value
Reference to the std representing the specified matched subrange within the target sequence.