When used in list context, a regex will return a list of matching expressions - see perlre. Since you have two terms to match here (element and abundance), you could store the results straight to a hash. Consider the following:

$_ = 'CH4N2O';
print $_,"\n";
%hash = /([A-Z][a-z]?)(\d*)/g;
while ( ($key, $value) = each %hash) {
    $value ||= 1;
    print "\t$key\t$value\n";
}

[download]

Note I also changed your grouping a little.

Update:If your chemical formulas encode structural information (e.g. HOH for water), then keys in a hash will get clobbered. You can, of course, substitute an array for the hash, and compensate appropriately. Thanks jwkrahn for reminding me to include a warning.

In reply to Re: Recursive capture of a variable number of elements using regexp by kennethk
in thread Recursive capture of a variable number of elements using regexp by seaver

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