my $chars = q{abcde};           # Specify the characters.
my $matchset = "[$chars]";      # Set up a character class.
my $reg_match = qr{$matchset};  # Turn it into a regexp.
my @strings = qw/ apple logs frog elmo /;
foreach my $string ( @strings ) {
    print "$_ matched\n" if $string =~ $reg_match;
}
__END__
apple
elmo

##</code><code>##


my $matchset = "[^$chars]"; # <-- Changed to a negated char class.
# ....
foreach my $string ( @strings ) {
    print "$_ is pure" if not $string =~ $reg_match; # Disqualify strings with illegal chars.
}

##</code><code>##

package Multiset;

use strict;
use warnings;

sub new {
    my $class = shift;
    my %self;
    $self{set_chars} = shift;
    my $neg_char_class = "[^$self{set_chars}]";
    $self{set_regexp} = qr/$neg_char_class/;
    $self{entries} = {};
    bless \%self, $class;
}

sub is_valid {
    my $self = shift;
    my $string = shift;
    return 0 if $string =~ $self->{set_regexp}; # Contains bad chars.
    return 1;
}

sub _normalize {
    my $self = shift;
    my $string = shift;
    return( join '', sort split //, $string );
}

sub is_unique {
    my $self = shift;
    my $string = shift;
    my $normalized = _normalize( $self, $string );
    return 0 if exists $self->{entries}{$normalized};
    return 1;
}

sub add_entry {
    my $self = shift;
    my $string = shift;
    die "Invalid entry." unless is_valid( $self, $string );
    my $entry = _normalize( $self, $string );
    return 0 unless is_unique( $self, $entry );
    $self->{entries}{$entry}++;
}

sub remove_entry {
    my $self = shift;
    my $string = shift;
    my $entry = _normalize( $self, $string );
    die "Invalid entry." unless is_valid( $self, $entry );
    die "Entry doesn't exist." if not is_unique( $self, $entry );
    delete $self->{entries}{$entry};
    return 1;
}

sub list_seen {
    my $self = shift;
    return keys $self->{entries};
}

1;

package main;

use strict;
use warnings;

my @strings = qw/ ab ac ca bb dc abc cba /;
my $set = q/abcde/;

my $multi = Multiset->new( $set );

foreach my $string ( @strings ) {
    print "$string\n";
    if( $multi->is_valid( $string ) ) {
        print "\tis valid.\n";
    } else {
        print "\tis not valid.\n";
        next;
    }
    if( $multi->is_unique( $string ) ) {
        print "\tis unique.\n";
        $multi->add_entry( $string );
    } else {
        print "\tis not unique.\n";
    }
}

my @found = $multi->list_seen();
print "Entries:\n\t@found\n";


##</code><code>##

ab
	is valid.
	is unique.
ac
	is valid.
	is unique.
ca
	is valid.
	is not unique.
bb
	is valid.
	is unique.
dc
	is valid.
	is unique.
abc
	is valid.
	is unique.
cba
	is valid.
	is not unique.
Entries:
	ab cd abc bb ac