#!/usr/bin/env perl

use 5.01800;
use warnings;
my @cases=(
        "<1>"
        ,"<1,2>"
        ,"<1,2,<3,4>,5,6>"
        );
my $re=qr{(?x) (?&LIST)
        (?(DEFINE)
            (?<LIST> < (?&ITEM)  (?: , (?&ITEM))*+ > )
            (?<ITEM> \d*+ | (?&LIST) )
        )
        };

for my $case (@cases) {
    say qq{$case\n},
        $case =~ qr{$re} ? "matches: '$&'" : "doesn't match";
    };
__END__

##</code><code>##

perl Conway_01.pl
<1>
matches! <1>
<1,2>
matches! <1,2>
<1,2,<3,4>,5,6>
matches! <3,4>      !!!! shouldn't this be <1,2,<3,4>,5,6>