Perfect! Thank you so much!You need to use the Lookbehind in the GREP
So it looks behind - but doesn't include it in the search result.
GREP is case sensitive
so this will find
any 2 after co2 Co2 cO2 CO2
Then you change the formating to subscript.
Alternatively - if you're using styles - you can insert the GREP style into the Paragraph Style. And then it will automatically update any text that is added.