Another Regexp Question

[andrew cooke]

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.

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? (

Thanks,
Andrew

 

[MRAB]

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? (

Nothing. It's a bug. :-(

 

[andrew cooke]

Nothing. It's a bug. :-(

Sweet )

Thanks - do you want me to raise an issue or will you?

Cheers,
Andrew

 

[MRAB]

Sweet )

Thanks - do you want me to raise an issue or will you?

You found it. You can have the pleasure.

 

[andrew cooke]

http://bugs.python.org/issue9179

You found it. You can have the pleasure.