Description of problem:

When extracting regex submatches with regex_match function I would expect the first iterator of every sub_match to point to the start of the matched sequence and the second one to point one element past the end.  This does not seem to be the case.  The iterators form open instead of half open interval.


--- regex.cc ---
#include <cstddef>
#include <regex>
#include <string>
#include <iostream>

using namespace std;

int main ()
{ 
  const regex r ("xxx(y*)xxx(y*)xxx(y*)xxx");
  string s;
  smatch m;

  getline (cin, s);

  if (regex_match (s, m, r))
    { 
      for (unsigned int i = 1; i < m.size (); ++i)
        { 
          const size_t b = m[i].first - s.begin ();
          const size_t e = m[i].second - s.begin ();
          string markers (s.size (), ' ');
          markers[b] = '^'; markers[e] = '^';

          cout << "submatch: " << m[i] << '\n';
          cout << "length: " << m[i].length () << '\n';
          cout << "interval: [ " << b << ", " << e << " )\n";
          cout << "first iterator points to: " << *m[i].first << '\n';
          cout << s << '\n';
          cout << markers << endl;
        }
    }
}
--- ---  --- ---


Version-Release number of selected component (if applicable):
libstdc++-4.8.3-9.el7


Steps to Reproduce:
1. g++ -O2 -std=c++11 -g    regex.cc   -o regex
2. ./regex <<<xxxyyyxxxyxxxxxx


Actual results:
submatch: xyyy
length: 4
interval: [ 2, 6 )
first iterator points to: x
xxxyyyxxxyxxxxxx
  ^   ^
submatch: xy
length: 2
interval: [ 8, 10 )
first iterator points to: x
xxxyyyxxxyxxxxxx
        ^ ^
submatch: x
length: 1
interval: [ 12, 13 )
first iterator points to: x
xxxyyyxxxyxxxxxx
            ^^


Expected results:
submatch: yyy
length: 3
interval: [ 3, 6 )
first iterator points to: y
xxxyyyxxxyxxxxxx
   ^  ^         
submatch: y
length: 1
interval: [ 9, 10 )
first iterator points to: y
xxxyyyxxxyxxxxxx
         ^^     
submatch: 
length: 0
interval: [ 13, 13 )
first iterator points to: x
xxxyyyxxxyxxxxxx
             ^  


Additional info: