my @strings = qw(
    aaaaa
    eeeee
    iiiii
    abcde
    aeiou
    bdfhj
);

my $range = 'AaEeIiOoUu';
my $alt   = join '|', split //, $range;

my %regexes = (
    alternation => qr/$alt/,
    class       => qr/[$range]/,
    lookahead   => qr/([$range])((?!\1)[$range])((?!\2)[$range])((?!\3)[$range])((?!\4)[$range])/,
);

foreach my $string (@strings)
{
    print "$string:\n";
    while (my ($k, $r) = each %regexes)
    {
        print "\t$k - " . ($string =~ /$r/ ? 'YES' : 'NO') . $/;
    }
}

##</code><code>##

aaaaa:
        alternation - YES
        class - YES
        lookahead - NO
eeeee:
        alternation - YES
        class - YES
        lookahead - NO
iiiii:
        alternation - YES
        class - YES
        lookahead - NO
abcde:
        alternation - YES
        class - YES
        lookahead - NO
aeiou:
        alternation - YES
        class - YES
        lookahead - YES
bdfhj:
        alternation - NO
        class - NO
        lookahead - NO