Edit digit at cursor position in a QlineEdit

[Egeris]

I desire to make a smart-edit feature to a program, so that it's possible to edit a numeric value on a QlineEdit by pressing up- or down-arrow-key.

See attachment to get the idea:
IBeam.png

What I have so far:
- QLineEdit in a mainwindow
- An eventFilter which detects when I press the up-arrow-key on the QLineEdit

bool MainWindow::eventFilter(QObject *obj, QEvent *event){
    QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
    if ((event->type() == QEvent::KeyPress) && keyEvent->key() == 16777235) {
        qDebug("Pressed the up-arrow-key. Should now increase the digit (of the numeric value) which is in front of the IBeam/cursor (on the numeric value associated with the QLineEdit for this filter event)");
        return true;
    } else {
        // standard event processing
        return QObject::eventFilter(obj, event);
    }
}

bool MainWindow::eventFilter(QObject *obj, QEvent *event){
    QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
    if ((event->type() == QEvent::KeyPress) && keyEvent->key() == 16777235) {
        qDebug("Pressed the up-arrow-key. Should now increase the digit (of the numeric value) which is in front of the IBeam/cursor (on the numeric value associated with the QLineEdit for this filter event)");
        return true;
    } else {
        // standard event processing
        return QObject::eventFilter(obj, event);
    }
}

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The question: Is there some practical method allowing me to increase or decrease the digit in front of the Ibeam on a keystroke event?