How to validate string in line edit

[Indrajeet]

I want to check if the input string in line edit starts with only alphabet or number.

[sierdzio]

Many ways to do this. You can react to QLineEdit::textChanged() or use "QLineEdit::setValidator()":http://qt-project.org/doc/qt-4.8/qlineedit.html#setValidator for example.

[b1gsnak3]

If you do not want to block the input so that he cannot write anything other than alphabet or number, you could check this with QLineEdit::editingFinished() and if the string is not valid display a message

[Indrajeet]

Hi

I have used setValidator in textChanged() of LineEdit but it is not working consistently.
First time it works then if i switch to other window and come back then that line edit will allow all characters.

ui.edit->setValidator(new QRegExpValidator(QRegExp("[a-z0-9]{1}[a-z0-9_-]{0,255}"),NULL));

What is going wrong?