Extract strings from nested array in Perl

Question

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]/;
 }
 };
 $loop->($input);
 return \@output;
}

Any idea for improvement without not core dependencies?

• Håkon Hægland: 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.
• Miroslav Popov: It is exactly extract_strings in my edit :)
• Miroslav Popov: 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.
• t3chb0t: It is not allowed to modify the code when answers have been posted. It can invalidate them. I have rolled back your last edit.

Answer 1

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] } ]
}