#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use Parse::Earley;

my $parser = Parse::Earley->new();
my $grammar = <<'__GRAMMAR__';
input:   expr

expr:    expr '+' mul_term 
       | expr '-' mul_term
       | mul_term

mul_term:   mul_term '*' exp 
          | mul_term '/' exp
          | exp

exp:  term '^' exp
      | term

term:  '(' expr ')'
       | /\d+/ { $_ < 256 }
__GRAMMAR__
my $str = '1 + 2 - 3';
$parser->grammar($grammar);
$parser->start('input');
$parser->advance($str) for 1..6;
my ($tree) = $parser->matches_all($str, 'input');
print Dumper($tree);