Thank you for this great tutorial. This was a very helpful introduction.

I tried to play a bit with your example. In the following code I do not get the Error "Bad Expression" for my second expression although it is not valid.

use strict;
use warnings;


use Parse::RecDescent ();

my $grammar = <<'__END_OF_GRAMMAR__';

   {
      use strict;
      use warnings;
   }


{
    sub eval_sum {
        my $acc = shift(@_);
        while (@_) {
            my $op = shift(@_);
            if    ($op eq '+') { $acc += shift(@_); }
            elsif ($op eq '-') { $acc -= shift(@_); }
        }
        return $acc;
    }
}

sum :  NUM sum_         { eval_sum( ($item[1], @{$item[2]}) ) }
sum_:  /[+-]/ NUM sum_  { $return = [$item[1], $item[2], @{$item[3]}] 
+}
      |                 { $return = [] }
NUM : /\d+/             { $return = $item[1] }

__END_OF_GRAMMAR__


my $parser = Parse::RecDescent->new($grammar)
   or die("Bad grammar\n");

foreach my $expr ('4-5+6-2','4*5')
{
    my $sum = $parser->sum($expr) or die "Bad expression";
    print "$sum" . "\n";
}
[download]

Why do I not get the "Bad expression" error for my second expression?

Thank you

Dirk