my @days    = qw( Sun Mon Tue Wed Thu Fri Sat );
  my $days_re = do { my $tmp = join '|', @days; qr/$tmp/; }

##</code><code>##

  sub compile_regex ( $ ) { qr/$_[0]/; }

  my $days_re = compile_regex join '|', @days;

##</code><code>##

  my $days_re = qr->( join '|', @days );