So… what, like this?

iex(1)> Regex.split(~R/(?<!\\),/, "comma,separated\\,data,is,here")
["comma", "separated\\,data", "is", "here"]

Or what is it you are looking for?

If you want "comma,separated\\\\\\\\,data,is,here" to parse into ["comma", "separated\\\\\\\\", "data" ,"is" ,"here"] then you cannot do that easily with a proper regex. Something like nimble_parsec is what you want then.

(You ‘can’ do that with a normal regex but it will start getting very hairy-large and you will require a good chunk of external code from the regex to perform the final transformation, where a peg parser can do it all in one swoop.)