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

my $grammar = q{
    evaluate        : ADD_SUB

    ADD_SUB         : MULT_DIV_MOD ADD_SUB_OP ADD_SUB
                      { [@item[1,2,3]] }
                      | MULT_DIV_MOD

    ADD_SUB_OP      : '+'  |  '-'

    MULT_DIV_MOD    : GROUP MULT_DIV_MOD_OP MULT_DIV_MOD { [@item[1,2,3]] }
                      | GROUP

    MULT_DIV_MOD_OP : '*'  |  '/'  |  '%'

    GROUP           : '(' ADD_SUB ')'
                      { $item[2] }
                      | NUMBER

    NUMBER          : INTEGER  |  FLOAT  |  NAN
    INTEGER         : /[+-]?\d+/
    FLOAT           : /([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?/
    NAN             : /(Inf(inity)?|NaN)/i
};
my $parser = new Parse::RecDescent $grammar;
print Dumper $parser->evaluate('42 - 5 + 1');
# Sees the result as 42 - (5 + 1)