Answered
This is the best resource I found for learning regular expressions: https://www.regular-expressions.info/tutorialcnt.html
It's not InDesign focused, but once you learn GREP it matters little if it's in InDesign or command line or whatever.
As far as metacharacters that can be used, I think @Marc Autret 's list is the most comprehensive one, but it's still not up to date: https://www.indiscripts.com/blog/public/data/idcs4-special-characters/en_InDesignCS4-CS5-SpecialChars.pdf
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