std::match_results<BidirIt,Alloc>::empty
From cppreference.com
                    
                                        
                    < cpp | regex | match results
                    
                                                            
                    |   bool empty() const;  | 
(since C++11) | |
Checks whether the match was successful.
Parameters
(none)
Return value
true if *this represents the result of a failed match, false otherwise, i.e. size() == 0.
Exceptions
May throw implementation-defined exceptions.
Complexity
Constant.
See also
|   returns the number of matches in a fully-established result state  (public member function)  |