GREP in InDesign is a powerful way to find patterns of text, then do something to that found text. To create a GREP expression that forms the basis of that search pattern, you’ll need to master many regular expressions (the RE in GREP). Don’t worry if you can’t remember them all, though. In InDesign, there is a little menu next to GREP input fields. Click on the little “@” symbol to the right of the field and you’ll find all sorts of GREP search options.
And, if you just want a handy reference, be sure to download my free GREP cheat sheet!