Count the capture groups in a qr regex?

Question

I am working on a project which at one point gets a list of files from an ftp server. At that point it either returns an arrayref of files OR if an optional regex reference

Answer 1

See nparen in Regexp::Parser.

use strictures;
use Carp qw(carp);
use Regexp::Parser qw();
my $parser = Regexp::Parser->new;

sub filter_files {
    my ($files, $pattern) = @_;
    my @files = @$files;
    return \@files unless $pattern;

    carp sprintf('Could not inspect regex "%s": %s (%d)',
        $pattern, $parser->errmsg, $parser->errnum)
        unless $parser->regex($pattern);

    my %versions;
    @files = map {
        if (my ($capture) = $_ =~ $pattern) {
            $parser->nparen
                ? push @{ $versions{$capture} }, $_
                : $_
        } else {
            ()
        }
    } @files;
    carp 'Could not find any matching files' unless @files;

    return (scalar keys %versions)
        ? \%versions
        : \@files;
}

Another possibility to avoid inspecting the pattern is to simply rely on the value of $capture. It will be 1 (Perl true value) in the case of a successful match without capture. You can distinguish it from the occasional capture returning 1 because that one lack the IV flag.