#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    chomp;

    print "$_: ", /^(?{ %choices = map {;"\Q$_" => 1} qw !X Y Z! })
                   (?: (?!)?  # Will never match, but keeps perl from 
+whining.
                       ((??{ keys %choices ? join "|" => keys %choices
                                           : "(?!)" }))
                       (?{ delete $choices {$1} }))+
                       (?(?{ keys %choices })(?!)|)$/x ? "ok\n" : "not
+ ok\n";
}

__DATA__
XYZ
ZYX
ZXY
XY
ZYYX
ABC


XYZ: ok
ZYX: ok
ZXY: ok
XY: not ok
ZYYX: not ok
ABC: not ok
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