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I was trying to get a pattern like this to work:
 pat = re.compile(r'(?<=(...)1円)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).