std::match_results<BidirIt,Alloc>::begin, std::match_results<BidirIt,Alloc>::cbegin
From cppreference.com
                    
                                        
                    < cpp | regex | match results
                    
                                                            
                    |   iterator begin() noexcept;  | 
(since C++11) | |
|   const_iterator begin() const noexcept;  | 
(since C++11) | |
|   const_iterator cbegin() const noexcept;  | 
(since C++11) | |
Returns an iterator to the beginning of the list of sub-matches. If match was successful, the iterator will point to the entire matched expression.
Parameters
(none)
Return value
Iterator to the first sub-match.
Complexity
Constant.
See also
|   returns iterator to the end of the list of sub-matches  (public member function)  |