Bugs item #1519638, was opened at 2006-07-09 14:34
Message generated for change (Comment added) made by mchaput
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Category: Regular Expressions
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Bobby Xiao (nneonneo)
Assigned to: Gustavo Niemeyer (niemeyer)
Summary: Unmatched Group issue
Initial Comment:
Using sre.sub[n], an "unmatched group" error can occur.
The test I used is this pattern:
sre.sub("foo(?:b(ar)|baz)","\\1","foobaz")
This will cause the following backtrace to occur:
Traceback (most recent call last):
File "", line 1, in ?
File "lib/python2.4/sre.py", line 142, in sub
return _compile(pattern, 0).sub(repl, string, count)
File "lib/python2.4/sre.py", line 260, in filter
return sre_parse.expand_template(template, match)
File "lib/python2.4/sre_parse.py", line 782, in expand_template
raise error, "unmatched group"
sre_constants.error: unmatched group
Python Version 2.4.3, Mac OS X (behaviour has been verified on
Windows 2.4.3 as well).
This behaviour, while by design, is unwanted because this type of
matching usually requests that a blank match be returned (i.e. the
example should return '')
The example that I was trying resembles the following:
sre.sub("User: (?:Registered User #(\d+)|Guest)","%USERID|\1%",data)
The intended behaviour is that the function returns "" when the user is
a guest and the user number if the user is a registered member.
However, when this function encounters a Guest, it raises an exception
and terminates, which is not what is wanted.
Perl and other regex engines behave as I have described, substituting
empty strings for unmatched groups. The code fix is relatively simple,
and would really help out for these types of things.
--
Comment By: Matt Chaput (mchaput)
Date: 2007-02-15 13:35
Message:
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The current behavior also makes the "sub" function useless when you need
to backreference a group that might not capture, since you have no chance
to deal with the exception.
--
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