Your question has been answered well, but since my brain is in parsing mode, I thought I'd bring this up.. The reason it's hard to do this with a regex is that (barring (??{code}) directives in the regex) you can't match arbitrarily deep nested parentheses using regular expressions. Since your data format supports nesting things in parentheses, parsing is a better solution.

Writing a parser for such simple notation is not that hard. What you can do to make it even easier is to combine the weight calculations with actual parsing. This is called syntax-directed evaluation. You don't see syntax-directed evaluation much in the parsing of programming languages, but for simpler expression languages where each part of the expression has a value, and you are parsing the expression for the sole purpose of computing its final value (think of a simple math expression calculator). 

use Parse::RecDescent;
use List::Util 'sum';
use vars '%weights';

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

my $g = Parse::RecDescent->new(<<'END_GRAMMAR');
  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

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This is probably what those other CPAN modules are doing. Actually, since they do more than just compute the weight, they probably parse the chemical formula into a tree structure first, and do the weight calculation on that tree. If you only do the weights, you can save yourself having to use an awkward intermediate tree representation.

blokhead

In reply to Re: Regular Expressions and atomic weights by blokhead
in thread Regular Expressions and atomic weights by hokie

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