grep - Automatically convert H2 to H₂ when typing in Indesign?

I have to regularly type the word H2 into documents for my job. I always forget to make the 2 subscript

I've been watching David Blatner's Indesign secrets and I figured I could use either, commonly misspelled word or to create a GREP Style.

I tried \d., this works but, when I now type any number on its own it will be formatted to subscript.

Is there a way to only apply the formatting to the letters "H2" for it to read H₂?

Answer

Apply the character style (subscript) to any number 2 preceded by ?< an uppercase character \u

(?<=\u)2

or, apply the character style (subscript) to any number 2 preceded by ?< an uppercase H

(?<=H)2