Search and replace by GREP - keeping original letters

Hi!
I have looked around in the community and found a few topics, which I must admit I did not understand. (I am far from "code wizard"...)
What I want to do is replace one or more characters etc, but keep others. I believe this can be done in GREP, but my attempts have been unsuccessful.
Example:
Search for "(any lowercase letter) (any uppercase letter)" - with a space in the middle.
Change to "(the found lowercase letter)./r(the found uppercase letter)" - thus insert a full stop and a new paragraph instead of the space, but keeping the two letters surrounding it.

a R
becomes
a.
R

Grateful for instructions. 🙏