[Python-bugs-list] [ python-Bugs-814253 ] Grouprefs in lookbehind assertions

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Bugs item #814253, was opened at 2003-09-28 19:31
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Category: Regular Expressions
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Greg Chapman (glchapman)
Assigned to: Fredrik Lundh (effbot)
Summary: Grouprefs in lookbehind assertions

Initial Comment:
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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