#!/usr/local/bin/perl
use strict;
use warnings;

use Data::Dumper;

my $number_re = qr{
    (?(DEFINE)
      (?<int>          \d+ )
    )
    (?&int)
}x;

my $num = 15;
if ( $num =~ /^$number_re$/ ) {
    warn "'$num' is valid\n";
    warn Dumper( \%- );
}

# produces:
#'15' is valid
#$VAR1 = {
#          'int' => [
#                     undef
#                   ]
#        };