Win8 Strawberry 5.30.3.1 (64)  Tue 07/26/2022  6:46:09
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

my $input = q/saxyzzy Monday Saturday Thursday Saturday Sat Sun Mon Tue Wen Th/;
my $re =
    join "|",
    map ".*\\b($_\\S*)",
    map m/(..)/,
    qw/monday tuesday wednesday thursday friday saturday sunday/
    ;
print "\$re '$re' \n";
print join '.', $input =~ m/^(?|$re)\b(?{ print "$1\n" })(*FAIL)/i;
^Z
$re '.*\b(mo\S*)|.*\b(tu\S*)|.*\b(we\S*)|.*\b(th\S*)|.*\b(fr\S*)|.*\b(sa\S*)|.*\b(su\S*)'
Mon
Monday
Tue
Wen
Th
Thursday
Sat
Saturday
Saturday
saxyzzy
Sun