[ python-Bugs-1212411 ] Incorrect result for regular expression - "|(hello)|(world)"

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Bugs item #1212411, was opened at 2005-06-01 01:13
Message generated for change (Comment added) made by tim_one
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Category: Regular Expressions
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Vijay Kumar (karamana)
Assigned to: Gustavo Niemeyer (niemeyer)
Summary: Incorrect result for regular expression - "|(hello)|(world)"

Initial Comment:
The regular expression "|hello|world" incorrectly gives a 
match, owing to the starting '|'.  Below is a sample 
program which highlights this.  The correct result 
behavior is to return None:

If the leading '|' is removed then the result is correct.

-----
import re
m = re.search("|hello|world","This is a simple sentence")
print m

m2 = re.search("hello|world","This is a simple sentence")
print m2

---- output ---
<_sre.SRE_Match object at 0x00B71F70>
None
----------
The first one is incorrect.  Should have returned a None.


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>Comment By: Tim Peters (tim_one)
Date: 2005-06-01 01:19

Message:
Logged In: YES 
user_id=31435

I expect you'll find that, e.g., Perl does the same thing:  
a "missing" alternative is treated as an empty string, and an 
empty string always matches.  What basis do you have for 
claiming it should not match (beyond just repeating that it 
should not <wink>)?

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