[Python-bugs-list] [ python-Bugs-576079 ] Inconsistent behaviour in re grouping

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Bugs item #576079, was opened at 2002-07-01 10:22
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Category: Regular Expressions
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Pedro Rodriguez (pedro_rodriguez)
Assigned to: Fredrik Lundh (effbot)
Summary: Inconsistent behaviour in re grouping

Initial Comment:
The following expression (?P<name>.*) and
(?P<name>(.*)) don't behave in the same way. 

When the matching fails, the first group will be None,
but the last one will contain an empty string.

The problem occurs with python 2.1.1 and 2.2. (and
latest CVS for 2.3)

Python 1.5.2 OTH works fine.


(example file attached)

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Comment By: Greg Chapman (glchapman)
Date: 2002-07-07 08:09

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I believe this is another example of the bug at which Patch 
527371 was aimed.  With that patch applied to the 2.2.1 
_sre.c, I get this:

>>> pat2 = re.compile(r"(((.*))x)?(y)")
>>> print pat2.match('y').groups()
(None, None, None, 'y')

I see that the patch is marked as accepted, but it does not 
yet appear to have made it into _sre.c even in CVS (at least 
it's not in version 2.80).  Perhaps this is an oversight?


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Comment By: Tim Peters (tim_one)
Date: 2002-07-01 10:36

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Here's a simpler example:

import re
pat1 = re.compile(r"((.*)x)?(y)")
pat2 = re.compile(r"(((.*))x)?(y)")
print pat1.match('y').groups()
print pat2.match('y').groups()

That prints

(None, None, 'y')
(None, '', None, 'y')

If (y) in the regexps is changed to plain y:

pat1 = re.compile(r"((.*)x)?y")
pat2 = re.compile(r"(((.*))x)?y")
print pat1.match('y').groups()
print pat2.match('y').groups()

the output changes to (the expected):

(None, None)
(None, None, None)

So it's not *just* the extra level of parens -- whether there's 
a capturing group "to the right" also affects the outcome.

FWIW, I agree it's a buglet.

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