What is the position of an unmatched group in C++?

Question

Let m be of type std::smatch . Suppose there is an unmatched group i. What is m.position(i) ? For that matter, what is m[i]?

For example, consider

std::regex re {"^(a+)|(b+)"};
string target="aa";
std::smatch m;
std::regex_search(target,m,re);
cout<<"m[2] is: "<<m[2]<<" at position: "<<m.position(2);

I cannot figure out from the reference https://en.cppreference.com/w/cpp/regex/match_results/position what is guaranteed to happen here and why.

Answer 1

According to the C++17 Standard:

28.10 Class template match_results [ re.results ]

4 The sub_match object stored at index 0 represents sub-expression 0, i.e., the whole match. In this case the sub_match member matched is always true. The sub_match object stored at index n denotes what matched the marked sub-expression n within the matched expression. If the sub-expression n participated in a regular expression match then the sub_match member matched evaluates to true, and members first and second denote the range of characters [first,second) which formed that match. Otherwise matched is false, and members first and second point to the end of the sequence that was searched.

[ Note: The sub_match objects representing different sub-expressions that did not participate in a regular expression match need not be distinct. — end note ]

Now m.position(n) returns (*this)[n].first.

Given that "[If] matched is false, [then] members first and second point to the end of the sequence that was searched" ...

This means m.position(n) should point "to the end of the sequence that was searched".