Yes - stop trying to do this in a single regular expression. While I am sure there will be a dozen correct and working solutions provided, I doubt any will be self-documenting. I also suspect anyone maintaining your code will take pause when they come across it.

Why not come up with a split_by_n() function where you can pass in a string and a length and get back what you want. Heck, you could then have optional parameters for what to do with empty strings, padding or not, etc.