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.

Can this be closed as a result of work done via #2636 or must it remain open?

Issue #2636 resulted in the regex module, which supports variable-length look-behinds.

I don't know how much work it would take even to put a limited fixed-length look-behind fix for this into the re module, so I'm afraid the issue must remain open.

The patch for issue9179 fixes this issue too.

New changeset fac649bf2d10 by Serhiy Storchaka in branch '2.7':
Issues #814253, #9179: Group references and conditional group references now
https://hg.python.org/cpython/rev/fac649bf2d10

New changeset 9fcf4008b626 by Serhiy Storchaka in branch '3.4':
Issues #814253, #9179: Group references and conditional group references now
https://hg.python.org/cpython/rev/9fcf4008b626

New changeset 60fccf0aad83 by Serhiy Storchaka in branch 'default':
Issues #814253, #9179: Group references and conditional group references now
https://hg.python.org/cpython/rev/60fccf0aad83

New changeset 0e2c7d774df3 by Serhiy Storchaka in branch '2.7':
Silence the failure of test_pyclbr after adding a property in sre_parse
https://hg.python.org/cpython/rev/0e2c7d774df3

New changeset 246c9570a757 by Serhiy Storchaka in branch '3.4':
Silence the failure of test_pyclbr after adding a property in sre_parse
https://hg.python.org/cpython/rev/246c9570a757

New changeset b2c17681404f by Serhiy Storchaka in branch 'default':
Silence the failure of test_pyclbr after adding a property in sre_parse
https://hg.python.org/cpython/rev/b2c17681404f

Now group references to groups with fixed width are supported in lookbehind assertions.

New changeset b78195af96f5 by Serhiy Storchaka in branch 'default':
Issues #814253, #9179: Group references and conditional group references now
https://hg.python.org/cpython/rev/b78195af96f5

New changeset 5387095b8675 by Serhiy Storchaka in branch '2.7':
Issues #814253, #9179: Warnings now are raised when group references and
https://hg.python.org/cpython/rev/5387095b8675

New changeset e295ad9be16d by Serhiy Storchaka in branch '3.4':
Issues #814253, #9179: Warnings now are raised when group references and
https://hg.python.org/cpython/rev/e295ad9be16d