use strict;
use Parse::RecDescent;

my $grammar = '
        start:          a_oder_b a_oder_b(?) a_oder_b(?) eos
        a_oder_b:       "a" | "b"
        eos:            "?"
';
my $parser = Parse::RecDescent->new ($grammar);
my $re = qr/^[ab]{1,3}$/;

my $input = "";
my $switch = "regexp";
my $res = 0;

print "exit beendet, switch schaltet um\n\n";
while ($input ne "exit") {
        printf "%-7s: gib irgendwas ein: ", $switch;
        chomp ($input = <STDIN>);

        if ($input eq "switch") {
                $switch eq "regexp" and $switch = "grammar" or $switch = "regexp";
                next;
        } elsif ($input eq "exit") {
                next;
        } elsif ($switch eq "regexp") {
                $res = "$input" =~ $re ? 1 : 0;
        } else {
                $res = $parser->start ("$input?") ? 1 : 0;
        }

        print "[$switch] [$input] wird", $res ? "" : " nicht", " erkannt\n";
}