Effects: Value-initializes the pair base class subobject and the member matched.
difference_type length() const;
Returns: (matched ? distance(first, second) : 0).
Returns: matched ? string_type(first, second) : string_type().
Returns: matched ? string_type(first, second) : string_type().
int compare(const sub_match& s) const;
Returns: str().compare(s.str()).
int compare(const string_type& s) const;
Returns: str().compare(s).
int compare(const value_type* s) const;
Returns: str().compare(s).