>perl -wMstrict -le
"my $range = quotemeta 'ARKYGZX';
 ;;
 my $rng     = qr{ [$range]  }xms;
 my $not_rng = qr{ [^$range] }xms;
 ;;
 for my $s (qw(
   AAA RR XXK AxA xAxxYxxRx
   A xAx ARKYGZX XxZxGxYxKxRxA AxAxAxA xYYYYx xRRRRRx RRRR
   )) {
   my $match = $s =~ m{
     \A $not_rng* $rng (?: $not_rng* $rng){1,2} $not_rng* \z
     }xms;
   printf qq{%s match : '%s' \n}, $match ? '  ' : 'NO', $s;
   }
"
   match : 'AAA'
   match : 'RR'
   match : 'XXK'
   match : 'AxA'
   match : 'xAxxYxxRx'
NO match : 'A'
NO match : 'xAx'
NO match : 'ARKYGZX'
NO match : 'XxZxGxYxKxRxA'
NO match : 'AxAxAxA'
NO match : 'xYYYYx'
NO match : 'xRRRRRx'
NO match : 'RRRR'