I was trying to get a pattern like this to work:

   pat = re.compile(r'(?<=(...)\1)abc')
   pat.match('jkljklabc', 6)

Unfortunately, that doesn't work.  The problem is that 
sre_parse.Subpattern.getwidth() ignores GROUPREFs 
when calculating the width, so the subpattern in the 
assertion is deemed to have length of 3 (I was hoping 
that sre could detect that the group 1 had a fixed 
length, so the reference to it would also have a fixed 
length).

I've since discovered that both Perl and PerlRE cannot 
handle the above pattern, but they both generate 
exceptions indicating that the assertion has a variable 
length pattern.  I think it would be a good idea if sre 
generated an exception as well (rather than silently 
ignoring GROUPREFs).

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Attached is a patch which gives GROUPREFs an arbitrary 
variable width, so that they raise an exception if used in a 
lookbehind assertion.  Obviously, it would be better if 
GROUPREFs returned the length of the group to which they 
refer, but I don't see any obvious way for getwidth() to get 
that information (perhaps I missed something?).

As part of issue #2636 group references now work in lookbehinds.

However, your example:

    (?<=(...)\1)abc

will fail but:

    (?<=\1(...))abc

will succeed.

Why? Well, in lookbehinds it searches backwards. In the first regex it
sees the group reference before the capture, whereas in the second it
sees the group reference after the capture. (Hope that's clear! :-))

I've deliberately changed the stage to patch review and the version to 3.2 to highlight the fact that a lot of work will be needed to get the new regex engine into the standard library.  Feel free to change these as is seen fit.