std::sub_match::operator string_type, std::sub_match::str
operator string_type() const;
string_type str() const;
Converts to an object of the underlying std::basic_string type.
The first version is an implicit conversion, the second one is explicit.
 Return value
Returns the matched character sequence as an object of the underlying std::basic_string type. If the
matched member is false then the empty string is returned.
Linear in the length of the underlying character sequence.