Problem with enabling QPushButton!

[Krish]

Hello! Friends,
I am planning to design GUI wherein unless any user enters in QLineEdit a valid input it shouldn't enable one of the pushbuttons say Calculate. In the designer i have disabled the Calculate button. I have also set the inputMask in the designer to accept only 1-9 integers. I have disabled the button as soon as the calculate starts.

After which i used QLineEdit's "textChanged( const QString &)" property to enable the button and it worked properly, but it even enabled the button if "EmptyString" or anyother string which is inserted to LineEdit even if those are not displayed OR when Clear function is used which clears LineEdit's data.

Can anyone please suggest me which of the QLineEdit's property like ::isModified(), ::selectedText() or any other has to be used to solve my problem?

I will be obliged if helped.---->Thanks in Advance.