hm theoretically it should be possible to swap it and generate a (complicated) regex from "monday .. sunday" which sorts the input without explicit sort like the others showed.

But I'm better opting out now! =)

I find a way to do so only with searching from the beginning of the string ("^.*"):

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;
[download]

Mon
Monday
Tue
Wen
Th
Thursday
Sat
Saturday
Saturday
Sun
[download]

Note 'saxyzzy' and'Wen' in the output:

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 Tu
+e 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
[download]

This can be avoided with more complete automatic pattern generation, but some false positives will remain, e.g., 'satu' for Saturday.

File sort_weekdays_2.pl:

Read more... (3 kB)

False-positive day-name matches can be entirely avoided with a custom regex:

    my $rx_days = qr{ (?i)  # custom patterns
        .*? \K \b mo (?: n (?: day)?)?                 |
        .*? \K \b tu (?: e (?: s (?: day)?)?)?         |
        .*? \K \b we (?: d (?: nes day)?)?             |
        .*? \K \b th (?: u (?: r (?: s (?: day)?)?)?)? |
        .*? \K \b fr (?: i (?: day)?)?                 |
        .*? \K \b sa (?: t (?: ur day)?)?              |
        .*? \K \b su (?: n (?: day)?)?
        }xms;
[download]

My somewhat arbitrary use of \K in this regex means the extraction regex must be changed. This works:
    $test =~ m{ \A $rx_days \b (?{ push @got, ${^MATCH} }) (*FAIL) }xmsp;
Use of the ${^MATCH} match variable (and /p modifier) can be avoided by use of the
    substr $test, $-[0], $+[0]-$-[0]
expression.

---

Give a man a fish:  <%-{-{-{-<

what about ... :)

  DB<189> p "@a"                                                      
+                                                       
Monday Tuesday Wednesday Thursday Friday Saturday Sunday

  DB<190> $a = join "", map { "(?=.*($_))*" } @a

  DB<191> x grep {defined} "Sunday Friday Monday" =~ /^$a/i
0  'Monday'                                                           
+                                                       
1  'Friday'                                                           
+                                                       
2  'Sunday'                                                           
+                                                         
  DB<192>
[download]

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}