Backreferences in lookbehind

Looks like your suspicion is correct that backreferences generally can't be used in Java lookbehinds. The workaround you proposed makes the finite length of the lookbehind explicit and looks very clever to me.

I was intrigued to find out what Python does with this regex. Python only supports fixed-length lookbehind, not finite-length like Java, but this regex is fixed length. I couldn't use re.split() directly because Python's re.split() never splits on an empty match, but I think I found a bug in re.sub():

>>> r=re.compile("(?<=(.)\\1)")
>>> a=re.sub(r,"|", "Bazooka killed the poor aardvark (yummy!)")
>>> a
'Bazo|oka kil|led the po|or a|ardvark (yum|my!)'

The lookbehind matches between the two duplicate characters!