Forgive my ignorance in Grep but...

Don't apologize, GREP is all but an easy subject (it actually becomes easier with a lot of practice). What matters is to keep learning 😉

Also don't hesitate in taking advantage of the GREP F/C dialog within InDesign and the shortcuts the dialog offers (see @ button):

Then you can copy-paste supplied code as a starter.

FWIW

Loic

You want to go from 18.55

or 18.5

to 18

???

\d+\K\.\d{1,2}