use strict;
use warnings;

use List::Util qw( sum );

our %ATOM_WEIGHTS = ( C => 12, O => 16, Pb => 207 );


{
   our $rv_compound;
   our $rv_compound_;
   our $rv_group;
   our $rv_factor;
   our $rv_element;


   # Definitions must not be combined with assignment.
   
   my $compound;
   my $compound_;
   my $group;
   my $factor;
   my $element;

   $compound  = qr/
                   (??{ $group     })
                   (??{ $compound_ })

                   # Return value:
                   (?{ local $rv_compound = $rv_group + $rv_compound_; })
                /x;

   $compound_ = qr/
                   (?:
                      (??{ $group     })
                      (??{ $compound_ })

                      # Return value (first prod):
                      (?{ local $rv_compound_ = $rv_group + $rv_compound_; })
                   | 
                      # Nothing

                      # Return value (second prod):
                      (?{ local $rv_compound_ = 0; })
                   )
                /x;

   $group     = qr/
                   (??{ $element })
                   (??{ $factor  })

                   # Return value:
                   (?{ local $rv_group = $rv_element * $rv_factor; })
                /x;

   $factor    = qr/
                   (?:
                      (\d+)

                      # Return value (first prod):
                      (?{ local $rv_factor = $1 })
                   | 
                      # Nothing

                      # Return value (second prod):
                      (?{ local $rv_factor = 1 })
                   )
                /x;

   $element   = qr/
                   (?:
                      ([A-Z][a-z]*(?![a-z]))
 
                      # Return value (first prod):
                      (?{ local $rv_element = $ATOM_WEIGHTS{$1} })
                   | 
                      \(
                      (??{ $compound })
                      \)

                      # Return value (second prod):
                      (?{ local $rv_element = $rv_compound; })
                   )
                /x;


   sub weight {
      local $rv_compound;
      local $rv_compound_;
      local $rv_group;
      local $rv_factor;
      local $rv_element;

      my $rv;

      shift =~ /
                  # To bypass RE bug.
                  ()

                  (??{ $compound })

                  # Return value:
                  (?{ $rv = $rv_compound; })
               /x;

      return $rv;
   }
}

print("The weight of $_ is ", weight($_), ".\n")
   for 'Pb(CO3)2';