QString::contains ( QRegExp & rx ) doesn't capture regexp

http://doc.qt.nokia.com/4.7/qstring.html#contains-6
bool QString::contains ( QRegExp & rx ) const

If there is a match, the rx regular expression will contain the matched captures (see QRegExp::matchedLength, QRegExp::cap).

int QRegExp::matchedLength () const

Returns the length of the last matched string, or -1 if there was no match.

QString string("abcd");
QRegExp rx("[bc]");
qDebug() << string.contains(rx) << rx.cap(0) << rx.matchedLength();
 
/**
true "" 
Программа неожиданно завершилась. // (CRASH)
*/

"contains()" return true, so I thought "cap()" should return captured text, but...
and I got crash when I try to invoke "matchedLength()"

Tnx.