std::sub_match<BidirIt>::sub_match
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | constexpr sub_match(); | (since C++11) | |
Default constructs a std::sub_match. The matched member is set to false and the inherited members first and second are value-initialized.
This is the only publicly accessible and defined constructor.
Parameters
(none)