I have a Macintosh / Carbon programming question.  I'm writing an
application in C++ and I have a dialog, created with Interface
Builder.  In this dialog I have a number of text edit controls, and
the text in these controls is a specific format:  A floating point
number with two digits after the decimal point.

I want to restrict input to these controls so that if the user types
invalid characters (anything but a number of a decimal point) the
system will beep, but otherwise allow normal editing (so backspace,
arrow keys etc still work).

The rest of the application uses Carbon Events so I'd like to use that
here too.. I understand you can process text input using Apple events,
but that's not what I want to do here if I can avoid it.
How do I do this?