#!perl  -w  -d

use strict;
use Tk;

my $addy1 = '32';
my $addy2 = '42';

my $mw = MainWindow->new();

$mw->Entry( -textvariable    => \$addy1,
            -width           => 2,
            -validate        => 'focusout',
            -validatecommand => sub{
                                    # debugging, print the field.
                                    printf"<<%s>:%d\n", $_[0], 
                                          $_[0] =~ /^[0-9a-f]{1,2}$/i;
                                    $_[0] =~ /^[0-9a-f]{1,2}$/i;},
            -invalidcommand  => sub{$mw->bell},
          )->pack(-side => 'left');

# Need a field to go to so 'focusout' can happen.
$mw->Entry( -textvariable    => \$addy2,
            -width           => 2,
            -validate        => 'focusout',
            -validatecommand => sub{
                                    # debugging, print the field.
                                    printf"<<%s>:%d\n", $_[0], 
                                          $_[0] =~ /^[0-9a-f]{1,2}$/i;
                                    $_[0] =~ /^[0-9a-f]{1,2}$/i;},
            -invalidcommand  => sub{$mw->bell},
          )->pack(-side => 'left');
MainLoop;