my $c = Module->new( { filterX => XXXX } );

    where XXXX could be:
      - a hash ref                --> checks for exists $hashref->{$value}
      - a regex                   --> checks if the regex matches
      - an array ref of regexes   --> checks if any regex matches
      - a coderef                 --> the code is called


##</code><code>##


    my %callbacks = (
        CODE    => \&_init_code_callback,
        HASH    => \&_init_hash_callback,
        ARRAY   => \&_init_array_callback,
    );
    
    #===================================
    sub _init_callback {
    #===================================
        my ( $self, $callback, $filter ) = @_;

        # If nothing set, use default or subclassed version
        unless ($filter) {
            $self->{$callback} = $self->can($callback);
            return;
        }

        # If not in callbacks, then assume it is a single regex
        $filter = [$filter]
            unless exists $callbacks{ref $filter};

        return $callbacks{ref $filter}->($filter);
    }


    #===================================
    sub _init_code_callback { 
    #===================================
        return $_[0] 
    };

    #===================================
    sub _init_hash_callback { 
    #===================================
        my $filter = shift;
        return sub {
            my $self  = shift;
            my $param = shift;
            return exists $filter->{$param};
        };
    }
    
    #===================================
    sub _init_array_callback { 
    #===================================
        my $filter = shift;

        foreach my $value (@$filter) {
            $value ||= '';
            die "'$value' is not a regular expression"
                unless ref $value eq 'Regexp';
        }
        return sub {
            my $self  = shift;
            my $value = shift;
            foreach my $regex (@$filter) {
                return 1 if $value =~ m/$regex/;
            }
            return 0;
        };
    }