Maybe a picture will help. Try running this on a screen that is at least 108 chars wide.

#! perl -slw
use strict;

for my $re ( 
    qw[ .* 
        \d      \D     [^\d]  [^\D] 
        \w      \W     [^\w]  [^\W] 
        [\d\w] [^\d\w] [\D\W] [^\D\W] 
    ] 
) {
    printf "%8s:'%s'\n\n", $re, join '', 
        map{ /$re/ ? $_ : ' ' } map{ chr } 32 .. 126;
};;
[download]

Normally I'd post the output, but the OSU PM wrap feature doesn't like it.