Below is a refactoring that avoids using stuff from main. Using stuff from main prevents you from precompiling your grammar into a module. Also, using stuff from main makes the script incompatible with mod_perl. (I also lined up the productions.)

use Parse::RecDescent;

my $g = Parse::RecDescent->new(<<'END_GRAMMAR');
  {
     use List::Util 'sum';
     use vars '%weights';

     %weights = qw( C 12 O 16 Pb 207 );
  }

  weight   : compound         { $item[1] }
  compound : group(s)         { sum( @{$item[1]} ) }
  group    : element /\d+/    { $item[1] * $item[2] }
           | element          { $item[1] }
  element  : /[A-Z][a-z]*/    { $weights{ $item[1] } }
           | "(" compound ")" { $item[2] }
END_GRAMMAR

print $g->weight("Pb(CO3)2"), $/;
# prints 327

[download]

I was planning on doing P::RD solution for the OP, but I abandonned the idea when others pointed to existing specialized modules. Thanks for filling in the gap.

Update: The common start of both group productions is very innefficient. Fix:

  weight   : compound         { $item[1] }
  compound : group(s)         { sum( @{$item[1]} ) }
  group    : element factor   { $item[1] * $item[2] }
  factor   : /\d+/            { $item[1] }
           |                  { 1 }
  element  : /[A-Z][a-z]*/    { $weights{ $item[1] } }
           | "(" compound ")" { $item[2] }

[download]

In reply to Re^2: Regular Expressions and atomic weights by ikegami
in thread Regular Expressions and atomic weights by hokie

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.