GREP Style Help - Subscripting Chemical Formulas

Hello there! 

I'd like to create a GREP script for a project I'm working on. This particular document is going to be talking about CO2, N2, and CH4. I would like those numbers to be subscripted. Here's the kicker though...the company I work for, CO2Meter, is also mentioned in this document. I DON'T want that 2 to be subscripted. Below is a script I've found that is working for the subscripting. I just need to add in how to NOT have the '2' in CO2Meter get subscripted. Any suggestions would be incredibly helpful!

((?<=Na|Cl)|(?<=H|C|O|S|N))\d{1,3}(?!\d)