#!/usr/bin/perl -l

use warnings;
use strict;

my $input = join "|", qw/Monday Saturday Thursday/;
"monday tuesday wednesday thursday friday saturday sunday" =~ m/\b($input)\b(?{ print "[$1]" })(*FAIL)/i; 

print join ' ', grep s/,//, split ' ', "monday tuesday wednesday thursday friday saturday sunday" =~ s/\b($input)\b\K/,/igr; 

##</code><code>##

[monday]
[thursday]
[saturday]
monday thursday saturday