sub  {
    my $s = shift;
    return undef if !defined $s;
    my $pos = 0;
    my $state = 1;
    my $done;
    while (1) {
        my $char = substr($s, $pos, 1);
        if ($state == 4) {
            $done = $pos;
            last if !defined $char;
            if ($char eq "a") {
                $state = 4;
                next;
            }
            if ($char eq "b") {
                $state = 4;
                next;
            }
            last;
        }
        if ($state == 1) {
            last if !defined $char;
            if ($char eq "a") {
                $state = 2;
                next;
            }
            if ($char eq "b") {
                $state = 3;
                next;
            }
            last;
        }
        if ($state == 3) {
            last if !defined $char;
            if ($char eq "a") {
                $state = 2;
                next;
            }
            if ($char eq "b") {
                $state = 4;
                next;
            }
            last;
        }
        if ($state == 2) {
            last if !defined $char;
            if ($char eq "a") {
                $state = 4;
                next;
            }
            if ($char eq "b") {
                $state = 3;
                next;
            }
            last;
        }
        die "error: Unknown state: $state";
    } continue {
        $pos++;
    }
    if (!defined $done) { return undef; }
    substr($s, 0, $done);
}