QString & QRegExp
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Please use '@' tags to wrap your code here on the forum. It is easier to read this way (I've already done that for you in your post).
Remember that QRegExp is deprecated in Qt 5, you should use QRegularExpression instead.
Regarding your problem: please see the documentation, the usage of QRegExp is explained pretty decently there. "Link":http://qt-project.org/doc/qt-5/QRegExp.html. You can also use "QRegExpValidator":http://qt-project.org/doc/qt-5/qregexpvalidator.html class, if you need your validaion in GUI.