There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
What resource did you use to master it? As every time I have to use regex I want to cry.
regex101.com has a convenient searchable cheat sheet for all the somewhat odd but powerful functions like negative lookbehind/lookahead with a brief explanation of each, a regex pattern input with checkable boxes that helps you get down single replacements vs global replacements, a large input that lets you dump text to test against the pattern, an explanation on the right of what each symbol is trying to match, and the left side lets you switch between the different flavors to see some of the variants between languages/standards. I still have a lot to learn before I’ll consider it mastered, but I have enough common stuff memorized now that it works great for me!