use v5.12;
use warnings;
use Data::Dump;

#ddx
my @words = map { "..X${_}X.." } <{a,b,c,d}{a,b,c,d}{a,b,c,d}>;


my $class = "[abc]";
#$class    = '.';                        #uncomment to match d too


my $len = 3;

for (@words) {
    say "$_"
      if $_ =~ / X                      # start anchor
                 (
                     ($class)
                     (?!
                         .{0,$len}
                         \g{-1}         # relative backreference
                         .*?
                         X
                      )
                 ){$len}
                 X                      # end anchor
               /x;
}

##</code><code>##

..XabcX..
..XacbX..
..XbacX..
..XbcaX..
..XcabX..
..XcbaX..

##</code><code>##

.*?
X              # end anchor