Intercepting keyboard input

[Robert Roebling <roebling ruf uni-freiburg de>]

  Good day,

is it possible to intercept keyboard input to
a text widget, e.g. in order to only allow
input of alphanumeric characters? I am not
so much interested in that specific problem,
more in the general way. As far as I understood
can the signal-connect system only be used,
to get notified, when something happens. There is
no way (apparent to me) to actually change
what is going on (as described above e.g.).
Am I right?

   Thanks and regards,

         Robert Roebling

Robert Roebling <roebling@ruf.uni-freiburg.de>