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