The problem is that  m// binds by default to  $_ in the absence of an explicit binding, e.g.,  $s =~ m{pattern}; A poor, naked  qr// doesn't know what to bind to and must always be bound explicitly as BrowserUk has done.

In general, the interpolation of a constant into a regex is messy:
    use constant FOO => qr{ foo }xms;
    print 'got foo' if 'foo' =~ m{ @{[ FOO ]} }xms;

The use of a Readonly constant makes things neater:
    use Readonly;
    Readonly my $FOO => qr{ foo }xms;
    print 'got foo' if 'foo' =~ m{ $FOO }xms;
but BrowserUk will tell you that the Readonly module has some dirty little secrets of its own. YMMV.

Ah! This is where I blundered; I didn't understand regex binding.

Thank you all for your patience & taking the time to share your insight. This will help me write more robust code.

Its not pretty, but this works:

use constant DOT => qr[\.];;

print grep $_ =~ DOT, qw[ a b c . d e f];;
.
[download]

Though it is really hard to see the benefit of making constants of those particular regex.

It's not like they are going to change anytime soon, and if they did, then you'd want to change their names also, which means you'd have to edit the place of use as well anyway.

Agreed as far as the directory semantics staying the same.

However, the question is larger than just directory filtering. What constants gain is lessening the chance that a regular expression gets changed in one location when it needs to be changed in all locations, or it gets changed incorrectly in a code branch which isn't executed often or unfortunately tested. I only used directory filtering as an example.

Thanks for your contribution.

use warnings;
use strict;
use Data::Dumper;
use File::Slurp;

my @lst = read_dir('.');
print Data::Dumper->Dump([\@lst], [qw/lst/]);
[download]

Thanks! That is good to know.

However, I'm most interested in figuring out the syntax needed to move regular expressions to constants. My Perl foo is weak.

Thanks, again for responding. Maybe my foo isn't quite as weak now...

1. Define a variable holding the regex you want to use.
   • my $dot = qr/^[^\.{1,2}]/;
   • This checks to see if the string begins with a period or two (.) or (..)
2. Check out http://perldoc.perl.org/functions/our.html
for more info... about 'our' and 'my' for defining variables. In your example - my will work fine.
3. Use your defined variables in the grep, shown below.

my @list = qw(. .. test.xls file100.pl);

my $dot = qr/^[^\.{1,2}]/;

my @files = grep { /$dot/ } @list;

foreach my $file(@files) {
        print "$item\n";
}
[download]