compare the output of

$ python -c "open('/tmp/tst','w').write(100*'x\n');import re;print len(re.split('\n(?=x)', open('/tmp/tst').read()))"
100

with

$ python -c "open('/tmp/tst','w').write(100*'x\n');import re;print len(re.split('\n(?=x)', open('/tmp/tst').read(), re.MULTILINE))"
9

re.split = split(pattern, string, maxsplit=0, flags=0)
    Split the source string by the occurrences of the pattern,
    returning a list containing the resulting substrings.  If
    capturing parentheses are used in pattern, then the text of all
    groups in the pattern are also returned as part of the resulting
    list.  If maxsplit is nonzero, at most maxsplit splits occur,
    and the remainder of the string is returned as the final element
    of the list.

maxsplit=0 in your fist example and maxsplit=8 (re.MULTILINE is 8) in your second example. This is not a bug, this is a wrong understanding.

There are actually 2 issues here:

1. The third argument is 'maxsplit', the fourth is 'flags'.

2. It never splits on a zero-width match. See issue 3262.

See also #11957.

Dang!  Thanks, and sorry for wasting everyone's time on this.