Limited chars input into entry widget.

[Damian Kramer <dkramer cup cam ac uk>]

I'm trying to use the entry widget for text input of a limited nature, i.e.
I only wnat to allow certain chars to be added. At the moment I'm catching
all keypresses, scanning the text for chars that are not allowed, removing
them and then setting the text back.

This has the side effect of leaving the cursor dangling out in the middle of
nowhere because the text length has changed. Trying to set the cursor
position manually doesn't work.

Is there a nicer way to do this, or am I stuck with this kludge?

Thanks in advance.

~Damian