I think I made some progress:

     $search = $top->Entry ('-width'            => 20,
                            '-validate'         => 'key',
                            '-validatecommand'  => sub {$_[1] =~/\w+/ 
+;} ,
                            '-invalidcommand'   => sub {$top -> bell()
+ ;},
               ) ->pack('-side' => 'left');
[download]

I found the code on 'Mastering Perl/Tk' -- I modified it to work with my program. But I still need it to accept ':' and '-'. As for invalid input it should do something like this:

'-invalidcommand' => sub {$_[1] =~/^[[:cntrl:]] ;
                          $top -> bell;
                         }
[download]

2021-03-09 Athanasius added code tags around final snippet.

You could use a character class to accept : and - in the validatecommand sub.

'-validatecommand'  => sub {$_[1] =~/[\w:-]+/ ;} ,
[download]

WRT your invalidcommand sub, the regexp does nothing at the moment. Should the first line be something like this if you want to accept control characters?

'-invalidcommand' => sub {
                          return if $_[1] =~/^[[:cntrl:]] ;
                          $top -> bell;
                         }
[download]

Or do you want the bell if it starts with a control character?

'-invalidcommand' => sub {
                          $top -> bell if $_[1] =~/^[[:cntrl:]] ;
                         }
[download]

I don't code with Tk, but hopefully this is useful.

Here is something new: I also wanna catch '[' (for [(1)). But when my expression is $_1 =~ /^[\w\s:-\[]+/ it spits out an error and says "stack moved" (is that PERL humor?). How do I add '[' to the character class? When I press <RET> with no data, the program still locks up.

Gonna try those two ideas right now. My goal is two fold: 1) if this character-set, then its valid 2) if it begins with a control char (in particular <RET> ) then complain