The Qt Reg-Exp engine sometimes gives false positive empty match

STEPS LEADING TO PROBLEM:

Compile and execute the following program:

#include <QDebug>
#include <QRegExp>
int main(int argc, char** argv)
{
  QRegExp r("(a)|(b)") ;
  qDebug() << r.indexIn("xxb", 1) ;
  qDebug() << r.pos(0) << r.pos(1) << r.pos(2) ;
  qDebug() << r.cap(0) << r.cap(1) << r.cap(2) ;
  return 0 ;
}

EXPECTED OUTCOME:

The output of the program is
2
2 -1 2
"b" "" "b"

EXPLANATION:

According to documentation at http://doc.trolltech.com/4.6/qregexp.html#pos
[QUOTATION]
For zero-length matches, pos() always returns -1. (For example, if cap(4)
would return an empty string, pos(4) returns -1.) This is a feature of the
implementation.
[/QUOTATION]

ACTUAL OUTCOME:

The output of the program is
2
2 0 2
"b" "" "b"

WHY IS IT IMPORTANT:
For a generic (...somthing-a...)|(something-b...) kind of regular expression I'm looking for a way to find out, which one of two (or even more) alternatives — "something-a" or "something-b" was found. My first try was to compare pos(1) with -1, but it doesn't work (zero is returned instead, while according to documentation -1 should be returned). A workaround in this case is to compare cap(1).length() to zero.

WHY IT'S NOT ENOUGH ONLY TO FIX THIS BUG:
Furthermore, it would be very interesting to know, how to find out, if the second alternative out of two was found in following case:
QRegExp r("(a*)|(b*)") ;
r.indexIn("axx") ;
r.indexIn("axx", 1) ;
In the first case the answer is "no", in the second case it's "yes", but the function cap(2) is returning "" (empty string) in both cases.
Thus I would like to have a method bool QRegExp::is_matched(int i) returning true if and only if the i-th sub-pattern was matched.