The Python re library is broken in its approach to case-insensitive matches. It erroneously attempts to compare lowercase mappings. This is wrong. You must compare the Unicode casefolds, not the Unicode casemaps. Otherwise you get wrong answers. I include a small test case that illustrates this bug. The bug exists on both 2.7 and 3.2, and on both wide builds and narrow builds. For comparison, I also show results using Matthew Barnett's regex library, which gets all 5 tests correct where re gets all 5 tests wrong.

A sample run is:

FAIL: re pattern Ι is not the same as string ͅ
PASS: regex pattern Ι is indeed the same as string ͅ
FAIL: re pattern Μ is not the same as string µ
PASS: regex pattern Μ is indeed the same as string µ
FAIL: re pattern ſ is not the same as string s
PASS: regex pattern ſ is indeed the same as string s
FAIL: re pattern ΣΤΙΓΜΑΣ is not the same as string στιγμας
PASS: regex pattern ΣΤΙΓΜΑΣ is indeed the same as string στιγμας
FAIL: re pattern POST is not the same as string poſt
PASS: regex pattern POST is indeed the same as string poſt

re lib passed 0 of 5 tests
regex lib passed 5 of 5 tests

I am not sure that everyone will agree that this is a bug, rather than a feature request, or that if a bug, that it should be changed in existing releases and possibly break running code. The doc just says, somewhat vaguely, that IGNORECASE "works for Unicode characters as expected". I have added others as nosy for their opinions.

The test file should have omitted the gratuitous and distracting warnings, especially the one that effectively scolds Windows users for running Windows. With those omitted, the test cases given would form the basis for an added TestCase.

Terry J. Reedy <tjreedy@udel.edu> added the comment:

I am not sure that everyone will agree that this is a bug, rather than a fe=
ature request, or that if a bug, that it should be changed in existing rele=
ases and possibly break running code. The doc just says, somewhat vaguely, =
that IGNORECASE "works for Unicode characters as expected". I have added ot=
hers as nosy for their opinions.

Working as expected for Unicode characters means it must the Unicode's
rules for casefolding. Otherwise you don't have Unicode at all; you just
have ISO 10646. Unicode is not merely a larger character repertoire; again,
that is merely ISO 10646. Unicode is all about the rules for processing this
larger repertoire. This is a very common mistake, so common that it is in the
Unicode FAQ:

Q: What is the relation between ISO/IEC 10646 and Unicode?

A: In 1991, the ISO Working Group responsible for ISO/IEC 10646 (JTC
   1/SC 2/WG 2) and the Unicode Consortium decided to create one
   universal standard for coding multilingual text. Since then, the
   ISO 10646 Working Group (SC 2/WG 2) and the Unicode Consortium
   have worked together very closely to extend the standard and to
   keep their respective versions synchronized. [EH]

Q: So are they the same thing?

A: No. Although the character codes and encoding forms are
   synchronized between Unicode and ISO/IEC 10646, the Unicode
   Standard imposes additional constraints on implementations to
   ensure that they treat characters uniformly across platforms and
   applications. To this end, it supplies an extensive set of
   functional character specifications, character data, algorithms
   and substantial background material that is *not* in ISO/IEC 10646.

http://unicode.org/faq/unicode_iso.html

Part of those functional character specifications can be found in the three
casefolding fields of the file UnicodeData.txt and also in two auxiliary
files of the Unicode distribution, CaseFolding.txt and SpecialCasing.txt.
The Unicode Character Database is not optional. If you do not use it, you
do not have Unicode; instead you merely have ISO 10646, which is of zero
practical use to anyone compared with Unicode. I'm sure that Python would
not want to be stuck having something of no use to anyone when everyone
else actually supports Unicode.

One is not allowed to make up one's own rules that run counter to Unicode's
and still make the claim that one is working on Unicode, since that is in
fact not what one is doing. Based on all that, Python does not do case
insensitive matching on Unicode, a condition contrary to its documented
claims. That clearly makes it a bug that needs fixing rather than a
feature request to be summarily ignored.

The test file should have omitted the gratuitous and distracting warnings, =
especially the one that effectively scolds Windows users for running Window=
s. With those omitted, the test cases given would form the basis for an add=
ed TestCase.

I have absolutely no idea what on earth you could possibly be referring to.
Honestly. I ran my tests on both releases (2.7 and 3.2), on both builds
(wide and narrow), and on both platforms (Unix and Mac). The warnings are
in there so I can make sure I have everything set up correctly to run the
tests, and will understand why I get more failures than expected in the event
that things are not set up appropriately.

Let me make perfectly clear that I have never in my life come anywhere near a
Microsoft system, let alone touched one, and that I furthermore never shall.
I have not the foggiest notion what in the world you are complaining about.
If the problem is that you are for some reason unable to create a Python with
full Unicode support under Microsoft, that is hardly my fault. Render unto
Caesar that which is Caesar's: complain to Microsoft about Microsoft's bugs,
not to me, as I am wholly blameless of their problems.

If you don't like my test cases, you know where to find vi.

I supposed I could always send you the program that writes these programs
for me, but as I knew you won't like it, I withheld it. You already have
all that you need to see exactly where the bugs are and how to fix them.

--tom

This bug could do with a little less attitude. That said, I think it is a bug and should be fixed, at the very least for Python 3.3. As always, it is a matter of much debate to what extent bugs can be fixed in previous Python versions (specifically, 2.7 and 3.2) without breaking more code than it fixes, and I don't want to jump the gun on that issue. Let's first see what it takes to fix this for 3.3.

Here is preliminary patch which fixes case-insensitive regular expression matching of unicode strings. It is incomplete, it needs applying patches from bpo-17381, which fixes other aspects of case-insensitive matching.

One bug is left for Turkish letters. This matching is not transitive. Three pairs of letters should match: ı ~ I ~ i ~ İ. All other combinations should not match (ı !~ i, I !~ İ, ı !~ İ). This patch doesn't fixes this bug.

Here are complete patch and script used to generate equivalence table.

Could anyone please make a review?

The script is updated so that it now is compatible with 2.7. There are some differences in equivalence table between 2.7 and 3.4 (e.g. 'ΐ' (U+0390) is not equivalent to 'ΐ' (U+1FD3) in 2.7).

New changeset 4caa695af94c by Serhiy Storchaka in branch '2.7':
Issue bpo-12728: Different Unicode characters having the same uppercase but
https://hg.python.org/cpython/rev/4caa695af94c

New changeset 47b3084dd6aa by Serhiy Storchaka in branch '3.4':
Issue bpo-12728: Different Unicode characters having the same uppercase but
https://hg.python.org/cpython/rev/47b3084dd6aa

New changeset 09ec09cfe539 by Serhiy Storchaka in branch 'default':
Issue bpo-12728: Different Unicode characters having the same uppercase but
https://hg.python.org/cpython/rev/09ec09cfe539

This solution (with hardcoded table of equivalent lowercases) is temporary. In future re engine will be changed to support correct caseless matching of different lowercase forms internally.