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.
Emacs regexps are unfortunately their own weird beast - they handle parentheses differently than other regexp engines, because Emacs assumes that you'll be running regexps on Lisp code a lot and want to easily match parentheses. The best documentation on that syntax is (confusingly) in the Elisp reference manual: https://www.gnu.org/software/emacs/manual/html_node/elisp/Sy....
\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b
(->> "\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\\b"
pcre-to-elisp xr)
(seq word-boundary
(one-or-more
(any "0-9A-Z" "%+._-"))
"@"
(one-or-more
(any "0-9A-Z" ".-"))
not-newline
(repeat 2 4
(any "A-Z"))
word-boundary)1. Stick to using the lowest common denominator like you did for case insensitivity.
2. If that becomes too cumbersome, then consider whether regex is the right tool for the job. Maybe you can use e.g Python/your favorite language with a known regex standard.
3. If there are no other tools and you're stuck with whatever flavor of regex one particular thing supports, only then invest time in learning the details. There is probably a book out there with the details even if there's no webpage.
Then pray you never get to step 3 :)
It's not so bad going between JS, Ruby and Elixir regex (possibly due to my use of a smaller set of features), but VIM regex disappoint me time after time.
But as someone who actually knows [some flavours of] regex fairly well, what I would really like, is a reference that covers all the subtle differences between the various regex engines, along with community-managed documentation (perhaps wiki pages) of which applications & API versions use which flavour of regex.
For example, the other day I wanted to run a find on my NAS. I needed to use a regex, but the Busybox version of find doesn't support the iregex option, so all expressions are case-sensitive. With some googling, I was able to find out that the default regex type is Emacs, but I wasn't able to find either a good reference for exactly what Emacs regex does and doesn't support, nor any information about how to set the "i" flag. In the end I had to manually convert every character into a class (like [aA] for "a") which was tedious, but quicker than trying to find a better solution or resorting to grep.
A related, annoyingly common pattern is that the documentation for `find` states that `--regex` specifies a regex, but it does not state which flavour of regex. The documentation for certain versions of `find`, which support alternative engines, note that the default is Emacs. From this I was able to infer (perhaps wrongly) that the Busybox `find` uses Emacs-flavoured regex, but ultimate I still had to resort to some trial-and-error. This problem is all too common in API documentation.