or download this
my $input = q/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 join ' ', $input =~ m/^(?|$re)\b(?{ print "$1\n" })(*FAIL)/i;