const_reference prefix() const;
Obtains a reference to the std::sub_match object representing the target sequence between the start of the beginning of the target sequence and the start of the entire match of the regular expression.
The behavior is undefined unless ready() == true
 Return value
Reference to the unmatched prefix.
 See also
| returns sub-sequence between the end of the full match and the end of the target sequence |
(public member function)