[nnf]

I read a lot on https://www.regular-expressions.info and experimented on https://rubular.com since I was also learning Ruby at the time. https://regexr.com is another good tool that breaks down your regex and matches.

One of the things I remember being difficult at the beginning was the subtle differences between implementations, like `^` meaning "beginning of line" in Ruby (and others) but meaning "beginning of string" in JavaScript (and others).

If you're just starting out, it'd be helpful to read about how a regex engine evaluates an expression against a string so that you can understand the "order of operations" and how repeating elements are matched.