#!/usr/bin/perl

use 5.010;

use strict;
use warnings;

use YAML;
use Regexp::Grammars;

my $str = <<'EOT';
(root
  (class living_thing
   (spec human
        (prop has_legs))
   (spec bird (prop has_wings))))
EOT

my $parser = qr{
    ^ <List> $

    <rule: List> (?: \( <[MATCH=Atom]>* \) )
    <rule: Atom> <MATCH=List> | <MATCH=Word>
    <token: Word> \w+

}x;

print Dump $/{List} if $str =~ /$parser/;

__END__
---
- root
-
  - class
  - living_thing
  -
    - spec
    - human
    -
      - prop
      - has_legs
  -
    - spec
    - bird
    -
      - prop
      - has_wings