QScriptEngine eng;
qDebug() << eng.newRegExp(QRegExp("foo")).isFunction(); // returned false prior to 4.6

Similarly, the expression "typeof /foo/" returned "object" prior to 4.6, but now returns "function".

JavaScriptCore, like SpiderMonkey (FireFox) and V8 (Chrome), allows regular expressions objects to be called as functions, whereas the old QtScript back-end did not.
Technically, regular expression objects are callable, but they are not functions. Still, QScriptValue::isFunction() behaves like isCallable(), and that's really what it should have been called. (See also http://www.davidflanagan.com/2009/08/typeof-isfuncti.html)

If you want to treat regular expressions unlike other callable objects, since 4.6 you have to write e.g. object.isFunction() && !object.isRegExp().