andrew cooke wrote:
As ever, I guess it's most likely I've misunderstood something, but in
Python 2.6 lookback seems to actually be lookahead. All the following
tests pass:
from re import compile
assert compile('(a)b(?<=(?(2)x|c))(c)').match('abc')
assert not compile('(a)b(?<=(?(2)b|x))(c)').match('abc')
assert compile('(a)b(?<=(?(1)c|x))(c)').match('abc')
assert compile('(a)b(?=(?(2)x|c))(c)').match('abc')
assert not compile('(a)b(?=(?(2)b|x))(c)').match('abc')
assert compile('(a)b(?=(?(1)c|x))(c)').match('abc')
But it seems to me that the first block should fail, because they
check the match *before* the point in question.
Both the first and third should fail, but they pass.
Note that without group references these work as I would expected:
assert compile('(a)b(?<=b)(c)').match('abc')
assert not compile('(a)b(?<=c)(c)').match('abc')
assert not compile('(a)b(?=b)(c)').match('abc')
assert compile('(a)b(?=c)(c)').match('abc')
in which lookback does indeed lookback (note the asymmetry, while the
first examples were symmetrical).
What am I missing this time? :o(
Nothing. It's a bug. :-(
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