#!/usr/bin/perl -w
#
# Evaluate simplified arithmetic expressions
#

use re 'eval'; #use recursive regex for parsing
use strict;
    
our $num = qr{\d+};
our $op  = qr{[+*/\-]};
# an expression is a number or a pair of expressions separated by
# an operator, enclosed in parens.
our $exp; 
    $exp = qr{$num|\(\s*(??{$exp})\s*$op\s*(??{$exp})\s*\)}s;
    
# Something to test the evaluator with
my @tests = ("(1+2)","((3+4)+(5+6))","((1+2)+((3+4)+(5*6)))");
for my $t (@tests)
{   
    print "$t\n";
    print "Final Answer: ", simplify($t);
    print "\n\n";
}      

sub simplify                        
{      
    my $e = shift;
       
    if($e =~ /^($num)$/s)
    {
        return $1;
    }
    elsif ($e =~ /^\(($exp)\+($exp)\)$/s)
    {
       my ($l, $r) = ($1,$2);
       my $left  = simplify($l);
       my $right = simplify($r);
       my $sum   = $left + $right;  # You'll need to do more here if
                                    # you're doing fractions.
       print "$left + $right = $sum\n";

       return "$sum";
    }
    elsif ($e =~ /^\(($exp)-($exp)\)$/s)
    {
       #subtraction 
    }
    elsif ($e =~ /^\(($exp)\*($exp)\)$/s)
    {
       my ($l, $r) = ($1,$2);
       my $left  = simplify($l);
       my $right = simplify($r);
       my $prod  = $left * $right;
       print "$left * $right = $prod\n";
       return "$prod";

    }
    elsif ($e =~ /^\(($exp)\/($exp)\)$/s)
    {
        #division
    }
    else
    {
        die "Syntax error\n";
    }
}
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