I believe I've found a bug in the `re` module -- specifically, in the 3.7+ support for splitting on zero-width patterns. Compare Java's behavior...

    jshell> "1211".split("(?<=(\\d))(?!\\1)(?=\\d)");
    $1 ==> String[3] { "1", "2", "11" }

...with Python's:

    >>> re.split(r'(?<=(\d))(?!\1)(?=\d)', '1211')
    ['1', '1', '2', '2', '11']

(The pattern itself is pretty straightforward in design, but regex syntax can cloud things, so to be totally clear: it finds any point that follows a digit and precedes a *different* digit.)

* Tested on 3.7.1 win10 and 3.7.0 linux.