The split pattern behaves mostly like a regular expression, with the differences well documented in split. There is no such thing as a "d" meta-character-class. But there is \d. Your example of /d{5}/ would work if you just used proper metasymbol escaping: /\d{5}/.

And any quantifier that is valid for regular expressions would be valid for the pattern used in split. If you wish, you might use +, {0,5}, {5,}, and so on.