Python flavor would probably be different than PCRE, which is probably different than JS flavor.
Even worse is that it might be too late to standardize all the regex flavors because there is already so much written in different regex flavors that it just costs too much for them to become obsolete in the future.
This is really demotivating.
1) Learn PCRE regex. 2) Try regex golf or cross words to learn PCRE regex. 3) Take the quiz on regex101.
Once you're done with all 3:
Learn the minor/major differences in the other languages. There aren't many. For example this named capture group:
(?<somename>someregex)
Would look like this in a different language:
(?P<somename>someregex)
There's some differences about what language can and cannot do like recursion because someone thought it was a great idea to make javascript awful at regex, but that's besides the point. Regex is totally worth learning.
Clear your afternoon, and just learn it. Seriously, it takes a couple of hours at best and then - BOOM - you're done for the rest of your life.
If that were so easy then I don't think much of these cheatsheets would exist.