Extract the strings from nested arrays in Perl.

It prints: a, b, c, d, E

use strict;
use warnings;
use feature qw(say signatures current_sub); 
no warnings qw(experimental::signatures);

my $nested = [1, 'a', [2, 3, 'b'], [4, [5, 6, 7, ['c']], [7, [8, 9, [10, 'd']]], 11, 'E']];

print join ', ', @{ extract_strings($nested) };

sub extract_strings($input) {
    my @output = ();

    my $loop = sub ($val) {
        if (ref $val eq 'ARRAY') {
            __SUB__->($_) for @{ $val };
        } else {
            push(@output, $val) if $val =~ /^[a-zA-Z]/;


    return \@output;

Any idea for improvement without not core dependencies?

  • 2
    \$\begingroup\$ Looks fine to me :) The name of the sub, flatten might be improved to extract_strings? If wanted, speed could be improved by using Inline::C or XS. \$\endgroup\$ – Håkon Hægland Sep 9 at 12:22
  • \$\begingroup\$ It is exactly extract_strings in my edit :) \$\endgroup\$ – Miroslav Popov Sep 9 at 20:14
  • 1
    \$\begingroup\$ Actually, there is something that might be a bug: push(@output, $val) if /^[a-zA-Z]/; vs push(@output, $val) if $val =~ /^[a-zA-Z]/; I don't understand why this actually work. What is $_ in the else? I'll edit the function. \$\endgroup\$ – Miroslav Popov Sep 9 at 20:24
  • 3
    \$\begingroup\$ It is not allowed to modify the code when answers have been posted. It can invalidate them. I have rolled back your last edit. \$\endgroup\$ – t3chb0t Sep 10 at 7:11

Your concept of "string" seems incomplete. I would look to firm up that definition and precisely match the need. Is "" a string? Do you want "valid identifiers" (/^[^\d\W]\w+$/) or "plausible ASCII words" (/^[A-Za-z]+$/) or just "not numbers" (!/ ^ ( [+-]? \d* \.? \d+ (?:[Ee][+-]?\d+)? ) $/x)?

I like use warnings FATAL => 'all'; so that I don't miss a warning in the midst of other output.

A fat arrow between arguments of different purpose can enhance readability.

Coding in functional style (with grep and map instead of @output) is a natural fit to this kind of problem.

Plain old recursion is suited to the task and would obviate the need for experimental features.

Having extract_strings take a ref (as it does now) and return a list simplifies the logic even further.

print join ', ' => extract_strings($nested);

sub extract_strings {
    grep /^[a-zA-Z]/ => map { ref eq 'ARRAY' ? extract_strings($_) : $_ } @{ $_[0] } 

If returning a ref is necessary, wrap and unwrap accordingly:

sub extract_strings {
    [ grep /^[a-zA-Z]/ => map { ref eq 'ARRAY' ? @{ extract_strings($_) } : $_ } @{ $_[0] } ]

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