[ python-Bugs-214033 ] re incompatibility in sre

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Bugs item #214033, was opened at 2000-09-11 08:24
Message generated for change (Settings changed) made by tmick
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Category: Extension Modules
Group: None
Status: Open
>Resolution: Accepted
Priority: 5
Submitted By: Martin v. Löwis (loewis)
Assigned to: Fredrik Lundh (effbot)
Summary: re incompatibility in sre

Initial Comment:
[submitted by Adam Sampson]

Under Python 1.5.2, I had a script containing the following line:

m = re.match(r"[a-z0-9]*://[^/]+/.*\.([^.#\?/]*)([#\?]?.*)?", url)

Under 1.6, this fails with:

[...]
  File "/usr/local/lib/python1.6/sre.py", line 44, in match                                              
    return _compile(pattern, flags).match(string)                                                        
  File "/usr/local/lib/python1.6/sre.py", line 102, in _compile                                          
    raise error, v # invalid expression                                                                  
sre_constants.error: nothing to repeat

I can narrow it down to:

>>> m = re.match(r"(x?)?", url)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/local/lib/python1.6/sre.py", line 44, in match
    return _compile(pattern, flags).match(string)
  File "/usr/local/lib/python1.6/sre.py", line 102, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

whereas:

>>> m = re.match(r"(x?.)?", url)

works fine. Is this correct behaviour for SRE, or am I just being stupid?
"(x?)?" looks like a perfectly reasonable Perl-style regexp to me (and Perl
too)...


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Comment By: Trent Mick (tmick)
Date: 2006-04-10 23:11

Message:
Logged In: YES 
user_id=34892

I've run into another incarnation of this (it breaks in
Python 2.3.5 and Python 2.4.3):

>>> import sre
  >>> sre.compile("(a*)?")
  Traceback (most recent call last):
    File "<stdin>", line 1, in ?
    File "C:\Python24\Lib\sre.py", line 180, in compile
      return _compile(pattern, flags)
    File "C:\Python24\Lib\sre.py", line 227, in _compile
      raise error, v # invalid expression
  sre_constants.error: nothing to repeat

Now granted that the '?' here is redundant for the '*'
quantifier on 'a', but compiling this regex works with
Python 2.3's "pre" and it works in Perl.

The actual use case I've hit here is trying to compile all
the regex's in Fedora Core 5's SELinux config files
(/etc/selinux/targeted/contexts/files/file_contexts*). The
first such regex that broke was:
  '/usr/share/selinux-policy([^/]*)?/html(/.*)?'


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Comment By: Martin v. Löwis (loewis)
Date: 2000-10-01 18:13

Message:
Yes, it is still broken in 2.0b2.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2000-10-01 04:33

Message:
Martin, is this still broken in 2.0? Fredrik, any idea?

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