Effects: Constructs an end-of-sequence iterator.
regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
const regex_type& re,
regex_constants::match_flag_type m = regex_constants::match_default);
Effects: Initializes begin and end to a and b, respectively, sets pregex to &re, sets flags to m, then calls regex_search(begin, end, match, *pregex, flags). If this call returns false the constructor sets *this to the end-of-sequence iterator.