For now re.split doesn't split with zero-width regex. There are a number of issues for this (issue852532, issue988761, issue3262, issue22817). This is definitely a bug, but fixing this bug will likely break existing code which use regular expressions which can match zero-width (e.g. re.split('(:*)', 'ab')).

I propose to deprecate splitting on possible zero-width regular expressions. This expressions either not work at all as expected (r'\b' never split) or can be rewritten to not match empty string ('(:*)' to '(:+)').

In next release (3.6) we can convert deprecation warning to the exception, an then after transitional period change behavior to more correct handling zero-width matches without breaking backward compatibility.