I guess I skimmed your question a bit too fast, as I noticed you were asking for radiobutton-like behaviour. That is somewhat more difficult to implement, but my function's button-disabling behaviour might suffice.

Indeed, as the monk below me said, for immediate feedback, you need JS. It, however, would not hurt to just say somewhere on the UI that the "E" button, if checked, overrides the others; you will need to just implement it in code then.

# If 'E' box is checked, then ignore the others
my @checked = $qry->param("GRU$Za");
if (grep { $_ eq 'E' } @checked) {
  @checked = ('E');
}

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In reply to Re^2: How to uncheck one checkbox if some other is checked by Anonymous Monk
in thread How to uncheck one checkbox if some other is checked by SerZKO

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