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Below, the second regexp seems just as guilty as the first to me.
Python 2.5.1 (r251:54869, Apr 18 2007, 22:08:04) 
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> re.compile(r'((x|y)*)*')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/re.py",
line 180, in compile
 return _compile(pattern, flags)
 File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/re.py",
line 233, in _compile
 raise error, v # invalid expression
sre_constants.error: nothing to repeat
>>> re.compile(r'((x|y+)*)*')
<_sre.SRE_Pattern object at 0x18548>
I don't know if that error is to protect the sre engine from bad
patterns or just a courtesy to users. If the former, it could be a
serious bug.