QJSEngine, Regex regressions (=> 5.12)

Given a simple js snippet like:

var prevString = " ok";
var r = /^(\s*)(([\)\]}]?\s*)*([\)\]]\s*))?;/.exec(prevString);
if (r != null) {
    doSomeThing();
}

In old Qt versions pre 5.12, doSomeThing(); wasn't called, as the regex doesn't match (as there is no ';' in that prevString).

Now it matches.
Still, the complete "r[0].length" is 0, but e.g. r[1].length has the matched leading whitespaces.