The key to using GREP in InDesign is being able to define patterns. So, instead of looking for specific text (like the number 3 or the letter H), you can tell GREP to find any digit or any letter. To find unknown values like these, GREP uses something called wildcards.
In this video, I explain how they work in a GREP query and look closer at a few of them. I also discuss some GREP queries you already have built right into in InDesign.