Replace "CO2" by a "2" in subscript

[moussab33871616]

Hi everyone !
I have an InDesign document with more than 400 times "CO2". 
I absolutely need to replace all "CO2" by CO2 : the "2" has to be in subscript.
Is there a way to use the tool "Replace" with subscript ? 
Thanks

[Frans v.d. Geest · Accepted solution]

Use GREP!

Create a subscript character style, then create a Paragraph style that had that character style to be used with this GREP:

(?<=CO)2

Do you have NO2 and H2O as well? You can catch them all in one GREP:

([CN]O)\K2|(H)\K2(?=O)