Undo the last entered symbol in QTextEditor

Question

While developing a program on Qt I noticed that there is no setValidator() method in QTextEdit , so I had to implement one myself. Here is what I wrote:

QRegExp textRegExp = QRegExp("[0-9a-fA-F]*");
QTextEdit *text = qobject_cast<QTextEdit*>(sender());
if (!textRegExp.exactMatch(text->toPlainText()))
    //undo the change

This is a slot connected to the textChanged() of the required object of class QTextEdit . The question is what should be instead of the comment? I've tried text->undo() , but it erases all the text, while I just need the last symbol to be undone. Is this even possible in Qt?