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.

Answer 2

You could use something like:

sub capturing_groups{
    my $re = shift;
    "" =~ /|$re/;
    return $#+;
}

say capturing_groups qr/fo(.)b(..)/;

Output:

2

Answer 3

You could use YAPE::Regex to parse the regular expression to see if there is a capture present:

use warnings;
use strict;
use YAPE::Regex;

filter_files(qr/foo.*/);
filter_files(qr/(foo).*/);

sub filter_files {
    my ($pattern) = @_;
    print "$pattern ";
    if (has_capture($pattern)) {
        print "yes capture\n";
    }
    else {
        print "no capture\n";
    }
}

sub has_capture {
    my ($pattern) = @_;
    my $cap = 0;
    my $p = YAPE::Regex->new($pattern);
    while ($p->next()) {
        if (scalar @{ $p->{CAPTURE} }) {
            $cap = 1;
            last;
        }
    }
    return $cap;
}

__END__

(?-xism:foo.*) no capture
(?-xism:(foo).*) yes capture