Sometimes, a parser is a better tool than a regex. I like
Marpa::R2:
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use Marpa::R2;
my $dsl = << '__DSL__';
lexeme default = latm => 1
:default ::= action => ::first
List ::= ('<') Items ('>')
Items ::= Item action => list
| Item (',') Items action => merge
Item ::= List
| digits
digits ~ [\d]+
__DSL__
sub list { [ $_[1] ] }
sub merge { [ $_[1], @{ $_[2] } ] }
my $grammar = 'Marpa::R2::Scanless::G'->new({ source => \$dsl });
my @cases=('<1>',
'<<1>>',
'<1,2>',
'<1,2,<3,4>,5,6>',
'<<<1,2>,3>,<4,5,<6>>,<<7>,8,9>>');
for my $case (@cases) {
my $value_ref = $grammar->parse(\$case, 'main');
use Data::Dumper; print Dumper $$value_ref;
}
map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]