use strict;
use Tk;
use Data::Dumper;
                                                                                
my $addy1 = '32';
my $addy2 = '42';
                                                                                
my $mw = MainWindow->new(-title => 'Mac Address Input Example');
my $fr = $mw->Frame()->pack(-expand => 1, -fill => 'both');
my $lb = $fr->Label(-text => 'MAC Address')->pack(-side => 'left');
                                                                                
my $e1 = $fr->Entry();
$e1->configure(
    -textvariable    => \$addy1,
    -width           => 2,
    -validate        => 'focusout',
    -validatecommand => \&validate,
    -invalidcommand  => [ \&show_invalid, $e1 ],
);
$e1->pack(-side => 'left');
                                                                                
# Need a field to go to so 'focusout' can happen.
my $e2 = $fr->Entry();
$e2->configure(
    -textvariable    => \$addy2,
    -width           => 2,
    -validate        => 'focusout',
    -validatecommand => \&validate,
    -invalidcommand  => [ \&show_invalid, $e2 ],
);
$e2->pack(-side => 'left');
MainLoop;
                                                                                
                                                                                
sub validate {
    my ($val) = @_;
    my $b_valid = ($val =~ /^[0-9a-f]{1,2}$/i)? 1: 0;
                                                                                
    # debugging, print the field.
    printf"<%s>:  %s valid\n", $val, $b_valid? "IS": "is NOT";
    return $b_valid;
}
                                                                                
sub show_invalid {
    my ($widget) = @_;
    $widget->focus();
    my $bg = $widget->cget(-background);
    $widget->configure(-background => 'red');
    $widget->update();
    select(undef, undef, undef, 0.2);
    $widget->configure(-background => $bg);
    $widget->update();
}