New submission from Leif Arne Storset: A non-ASCII string does not match a regular expression case-insensitively unless the UNICODE flag is set. This seems reasonable, but the documentation seems to imply that this is not the case. The example: import re # Does not match re.compile(u"неоднозначность", re.IGNORECASE) \ .findall(u"Неоднозначность") # Matches re.compile(u"неоднозначность", re.IGNORECASE | re.UNICODE) \ .findall(u"Неоднозначность") (In Python 3, it does not match if re.ASCII is given.) The documentation (2.7) says: re.UNICODE Make \w, \W, \b, \B, \d, \D, \s and \S dependent on the Unicode character properties database. (https://docs.python.org/2/library/re.html#re.UNICODE) My regex does not use any of those escapes, yet the regex changes behavior with the UNICODE flag. This leads to confusion when the regex doesn't match. The documentation is very specific about the behavior that changes with the flag, implying that behavior not mentioned is unaffected. Of course, it's easy to guess the correct (hopefully) solution. Still, I suggest changing the documentation to mention that re.IGNORECASE is affected. Looking at the source code, there seems to be further consequences (it mentions "Unicode locale") which may also warrant a mention. If you do want to avoid specifics, however, even a hand-wavy reference to something like "match according to Unicode" would help, because it implies that not only the escapes change behavior. In Python 3, there is a counterpart to the 2.7 problem: re.ASCII makes our Cyrillic string not match. Again, this behavior makes intuitive sense, but the documentation seems to indicate something different: re.ASCII Make \w, \W, \b, \B, \d, \D, \s and \S perform ASCII-only matching instead of full Unicode matching. This is only meaningful for Unicode patterns, and is ignored for byte patterns. … re.IGNORECASE Perform case-insensitive matching; expressions like [A-Z] will match lowercase letters, too. This is not affected by the current locale and works for Unicode characters as expected. re.ASCII does appear to affect re.IGNORECASE. Since this is the non-default case, however, I'm not sure it's worth calling it out. I'd be happy even if only the 2.7 docs change. ---------- assignee: docs@python components: Documentation messages: 248829 nosy: Leif Arne Storset, docs@python priority: normal severity: normal status: open title: It is undocumented that re.UNICODE affects re.IGNORECASE versions: Python 2.7, Python 3.4 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue24896> _______________________________________