note grondilu <p>Here is how I'd do it (I skip the part when you parse the weight table and prompt the user):</p> <code> #!/usr/bin/perl use strict; use warnings; # whatever you need to fill %weights ... sub mol_form { my $formula = shift; my $mass; for ($formula =~ /[A-Za-z]\d*/g) { /(?<element>[A-Za-z])(?<amount>\d+)?/; $mass += ($+{amount} // 1) * $weights{$+{element}}; } return $mass; } say mol_form "H2O"; </code> <p>The most tricky part is to separate each group of element+optionalquantity, and then to process them. That's why I ended up with two regexes, not just one.</p> 1004991 1004991