# sort_weekdays_2.pl

# method relies on left-anchored nature of "short" day names,
# e.g., th/thu/thur/thurs/thursday.
# automatic pattern generation.  possibility of false-positive
# matches exists, e.g., 'wedne' for wednesday.

use 5.018;  # need reliable regex interaction with lexicals
use 5.010;  # need regex extended patterns (?|...) branch reset

use strict;
use warnings;

use Test::More 'tests' => 2;
use Test::NoWarnings;

use Data::Dump qw(dd);

# works - false positive matches possible.
my ($rx_days) =
    map qr{ (?i) $_ }xms,
    join ' | ',
  # map { dd $_;  $_; }  # for debug
    map ".*? \\b (${ \day_parts($_) })",  # works
  # map { dd $_;  $_; }  # for debug
    map { @$_ == 3 or die "bad day '$_'";  $_; }  # minimal validation
    map [ m{ \A ([motuwehfrsa]{2}) ([nesdritu]+?) (day) \z }xms ],  # 
+works
  # map [ m{ \A ([a-z]{2}) ([a-z]+?) (day) \z }xms ],  # works
    qw/monday tuesday wednesday thursday friday saturday sunday/
    ;
# print "\$rx_days $rx_days \n"; # for debug

my $test = <<'EOS';
Monday Saturday Thursday Saturday Sat Sun Mon Tue Th
nothing on this or the following line should extract
shdhsd mond s mondaytuesday mondayxtuesday xsun sunx xsunx saxyzzy Wen
mo tu we th fr sa su  mon tue wed thu fri sat sun

blank lines don't matter

mOnDaY TuE wEd ThUrS mO sUn

false positive matches are possible with automatic pattern generation
satu wednes

EOS

# day-names extracted in day order, then by order of
# appearance in the source string.
# (?{ ... }) interface to lexical variable only reliable for perl ver 
+5.18+
# this problem can be avoided by using package-global variables.
my @got;  # lexical unreliable in older perls
$test =~ m{ \A (?| $rx_days) \b (?{ push @got, $^N }) (*FAIL) }xms;
# dd \@got;  # for debug

is_deeply \@got, [ qw(
    Monday    Mon    mo    mon    mOnDaY    mO
    Tue    tu    tue    TuE
    we    wed    wEd    wednes
    Thursday    Th    th    thu    ThUrS
    fr    fri
    Saturday    Saturday    Sat    sa    sat    satu
    Sun    su    sun    sUn
    )], 'extraction sorted by day-order';

sub day_parts {

    my ($ar_parts,  # substrings to assemble to nested regex
        ) = @_;

    # starts like  [ 'tu', 'es', 'day' ]
    # intermediate ( 'tu', 'e', 's', 'day' )
    # ends like   tu (?: e (?: s (?: day)?)?)?

    my $start = $ar_parts->[0];
    my @parts = (split('', $ar_parts->[1]), $ar_parts->[2]);

    return "$start @{[ _day_parts(@parts) ]}";

    }

sub _day_parts { return @_ ? "(?: @{[ shift, _day_parts(@_) ]})?" : ()
+; }
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