[ python-Bugs-1284341 ] re nested conditional matching (?()) doesn't work

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Bugs item #1284341, was opened at 2005-09-07 18:36
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Category: Regular Expressions
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Erik Demaine (edemaine)
Assigned to: Gustavo Niemeyer (niemeyer)
Summary: re nested conditional matching (?()) doesn't work

Initial Comment:
Here is a simple regular expression that should match
\o, \{o}, {\o}, and \{o}}:  (This example arose as
a simplification of a general accent matcher for
LaTeX/BibTeX.)

r = re.compile(r'(\{)?\"(\{)?(.)(?(2)\})(?(1)\})')

However, it fails on two out of four of the desired
matches:

r.search(r'\o)  ## returns None (WRONG)
r.search(r\{o}').group()  ## returns '\"{o}"' (CORRECT)
r.search(r'{\o).group()  ## returns \"o} (WRONG)
r.search(r{\{o}}').group()  ## returns '{\"{o}}'
(CORRECT)

The third case is particularly bizarre.  Incidentally,
the behavior is different if '(.)' is replaced by '.'
(incorrect in different ways).

I have tested this on Python 2.4.1 on Windows and a CVS
version on Linux.  I do not believe it is a platform issue.

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