Created attachment 34156 [details]
g++ -v

Please see the following sample.

========================================== sample code ==========================================
#include <iostream>
#include <regex>
#include <string>

void print(const char* t, const std::string& s, const std::ssub_match& sub)
{
    std::cout << "  " << t << ": " << (sub.matched ? "matched  " : "unmatched") << ", "
        "length() = " << sub.length() << ", str() = '" << sub.str() << "\', "
        "pair = (" << sub.first - s.begin() << ", " << sub.second - s.begin() << "), "
        "'" << std::string(sub.first, sub.second) << '\'' << std::endl;
}

int main()
{
    const std::regex e("z*");
    const std::string s("ab");

    int i = 0;
    for (auto&& it = std::sregex_iterator(s.begin(), s.end(), e), end = std::sregex_iterator();
         it != end; ++it) {
        std::cout << i++ << ':' << std::endl;
        print("prefix", s, it->prefix());
        print("match ", s, (*it)[0]);
        std::cout << std::endl;
    }
}
=================================================================================================

============================= output =============================
0:
  prefix: unmatched, length() = 0, str() = '', pair = (0, 0), ''
  match : matched  , length() = 0, str() = '', pair = (0, 0), ''

1:
  prefix: unmatched, length() = 0, str() = '', pair = (0, 1), 'a'
  match : matched  , length() = 0, str() = '', pair = (1, 1), ''

2:
  prefix: unmatched, length() = 0, str() = '', pair = (1, 2), 'b'
  match : matched  , length() = 0, str() = '', pair = (2, 2), ''
==================================================================

cf. http://melpon.org/wandbox/permlink/JSkP6tl2QWFxmOEv


According to C++11 standard 28.11.3[re.alg.search]/p.3 Table 143, prefix().matched should be true
if prefix().first != prefix().second.

(prefix().first is correct, because 28.12.1.4[re.regiter.incr]/p.5 says "match.prefix().first
shall be equal to the previous value of match[0].second".)

So, I think that the output should be 

============================= output =============================
0:
  prefix: unmatched, length() = 0, str() = '', pair = (0, 0), ''
  match : matched  , length() = 0, str() = '', pair = (0, 0), ''

1:
  prefix: matched  , length() = 1, str() = 'a', pair = (0, 1), 'a'
  match : matched  , length() = 0, str() = '', pair = (1, 1), ''

2:
  prefix: matched  , length() = 1, str() = 'b', pair = (1, 2), 'b'
  match : matched  , length() = 0, str() = '', pair = (2, 2), ''
==================================================================