Regexp negation doesn't work for backreferences

#include <QRegExp>
#include <QDebug>

int main()
{
  QRegExp exp("([^,]+),(\\1)");
  QRegExp exp_neg("([^,]+),(?!\\1)");
  QString line = "foo,foo";
  QString line2 = "foo,bar";
  qDebug() << exp.pattern() << " in " << line << exp.indexIn(line); // 0
  qDebug() << exp.pattern() << " in " << line2 << exp.indexIn(line2); // -1
  qDebug() << exp_neg.pattern() << " in " << line << exp_neg.indexIn(line); // -1
  qDebug() << exp_neg.pattern() << " in " << line2 << exp_neg.indexIn(line2); // -1 (should be 0)
  return 0;
}