[ python-Bugs-1647489 ] zero-length match confuses re.finditer()

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Bugs item #1647489, was opened at 2007-01-29 14:35
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Category: Regular Expressions
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Jacques Frechet (jfrechet)
Assigned to: Gustavo Niemeyer (niemeyer)
Summary: zero-length match confuses re.finditer()

Initial Comment:
Hi!

re.finditer() seems to incorrectly increment the current position immediately after matching a zero-length substring.  For example:

>>> [m.groups() for m in re.finditer(r'(^z*)|(\w+)', 'abc')]
[('', None), (None, 'bc')]

What happened to the 'a'?  I expected this result:

[('', None), (None, 'abc')]

Perl agrees with me:

% perl -le 'print defined($1)?"\"$1\"":"undef",",",defined($2)?"\"$2\"":"undef" while "abc" =~ /(z*)|(\w+)/g' 
"",undef
undef,"abc"
"",undef

Similarly, if I remove the ^:

>>> [m.groups() for m in re.finditer(r'(z*)|(\w+)', 'abc')]
[('', None), ('', None), ('', None), ('', None)]

Now all of the letters have fallen through the cracks!  I expected this result:

[('', None), (None, 'abc'), ('', None)]

Again, perl agrees:

% perl -le 'print defined($1)?"\"$1\"":"undef",",",defined($2)?"\"$2\"":"undef" while "abc" =~ /(z*)|(\w+)/g' 
"",undef
undef,"abc"
"",undef

If this bug has already been reported, I apologize -- I wasn't able to find it here.  I haven't looked at the code for the re module, but this seems like the sort of bug that might have been accidentally introduced in order to try to prevent the same zero-length match from being returned forever.

Thanks,
Jacques

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