[issue9179] Lookback with group references incorrect (two issues?)

[andrew cooke]

New submission from andrew cooke <andrew at acooke.org>:

from re import compile

# these work as expected

assert compile('(a)b(?<=b)(c)').match('abc')
assert not compile('(a)b(?<=c)(c)').match('abc')
assert compile('(a)b(?=c)(c)').match('abc')
assert not compile('(a)b(?=b)(c)').match('abc')

# but when you add groups, you get bugs

assert not compile('(?:(a)|(x))b(?<=(?(2)x|c))c').match('abc') # matches!
assert not compile('(?:(a)|(x))b(?<=(?(2)b|x))c').match('abc')
assert compile('(?:(a)|(x))b(?<=(?(2)x|b))c').match('abc') # fails!
assert not compile('(?:(a)|(x))b(?<=(?(1)c|x))c').match('abc') # matches!
assert compile('(?:(a)|(x))b(?<=(?(1)b|x))c').match('abc') # fails!

# but lookahead works as expected

assert compile('(?:(a)|(x))b(?=(?(2)x|c))c').match('abc')
assert not compile('(?:(a)|(x))b(?=(?(2)c|x))c').match('abc')
assert compile('(?:(a)|(x))b(?=(?(2)x|c))c').match('abc')
assert not compile('(?:(a)|(x))b(?=(?(1)b|x))c').match('abc')
assert compile('(?:(a)|(x))b(?=(?(1)c|x))c').match('abc')

# these are similar but, in my opinion, shouldn't even compile
# (group used before defined)

assert not compile('(a)b(?<=(?(2)x|c))(c)').match('abc') # matches!
assert not compile('(a)b(?<=(?(2)b|x))(c)').match('abc')
assert not compile('(a)b(?<=(?(1)c|x))(c)').match('abc') # matches!
assert compile('(a)b(?<=(?(1)b|x))(c)').match('abc') # fails!

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')

# this is the error we should see above
try:
    compile('(a)\\2(b)')
    assert False, 'expected error'
except:
    pass

----------
components: Library (Lib)
messages: 109382
nosy: acooke
priority: normal
severity: normal
status: open
title: Lookback with group references incorrect (two issues?)
type: behavior
versions: Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue9179>
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