Every year or so someone reveals a new GREP code that works inside InDesign, so here is an update on the GREP capabilities since the majority of them were listed in InDesign CS4. It turns out there are more and newer GREP code tricks supported in InDesign since then within its Find/Change dialog box.
GREP code seems to have stayed mostly the same from CC 2019 down to CS5, with the exception of a few additions. InDesign CS4 supports many more GREP codes than InDesign CS3. I have added them to my handout sheet and it is available and downloadable here (CC 2019) and here (CC) and here (CS6) and here (CS5). I have also added a 1-column version of this GREP list for convenient viewing on hand-held devices like the Apple iPhone, available here (CC) and here (CS6) and here (CS5). (InDesign CS5 adds a few new GREP codes beyond CS4, but CS6 and CC seem to have stayed the same.)
Many of the additional codes are for obscure text situations; many for asian and middle eastern language typesetting situations, too. Some of the more interesting codes allow you to search for all spaces, or just some kinds of spaces; all punctuation, or just some kinds of punctuation.
There is a second page in my downloadable handout that gives explicit examples of GREP search scenarios, along with notes on how to set it up to accomplish your typesetting goals. So download it and enjoy the learning. Get yourself warmed-up to this new feature of InDesign. We use this handout in our Washington DC-area training classrooms all the time!