Great regex tutorial: https://www.princeton.edu/~mlovett/reference/Regular-Express...

Good regex book: https://www.amazon.com/gp/product/0596528124/

Good regex website: https://www.regular-expressions.info/

Interesting regex links: https://github.com/aloisdg/awesome-regex

https://swtch.com/~rsc/regexp/regexp1.html

https://swtch.com/~rsc/regexp/regexp2.html

And actual implementations based on these articles: https://github.com/google/re2 and https://github.com/rust-lang/regex