From the PR title, it sounds to me like this only changes the Ruby implementation, but that is clearly not the case. Can you elaborate a bit?

When I added the ReDoS queries for Ruby, I started from the existing Python parser and the JS/Python ReDoSUtil.qll, but I found the only forms of character class they handled were simple escapes like \d and \w. I had to extend them to support POSIX character classes like [[:digit:]] and the similar \p{} construct.

I haven't reviewed @erik-krogh's changes closely yet, but it looks he's generalised the concept of a character class so that the code can be shared with Ruby without losing support for those constructs. Is that right?

Edit: if that is correct, then this also paves the way for supporting the \N{name} syntax added in Python 3.8.

I haven't reviewed @erik-krogh's changes closely yet, but it looks he's generalised the concept of a character class so that the code can be shared with Ruby without losing support for those constructs. Is that right?

That is exactly right.
I introduced a new predicate isEscapeClass(RegExpTerm term, string clazz), which is used instead of RegExpCharacterClassEscape.

The new predicate generalizes escape classes, and "normalizes" them to \d, \s, \w.
Each language must implement this new predicate.
The JS/Python implementations are currently trivial (and identical), but the Ruby implementation maps e.g. [[:digit:]] to \d.