New submission from David MacIver:

chr(304).lower() is a two character string - a lower case i followed by a 
combining chr(775) ('COMBINING DOT ABOVE').

The re module seems not to understand the combining character and a regex 
compiled with IGNORECASE will erroneously match a single lower case i without 
the required combining character. The attached file demonstrates this. I've 
tested this on Python 3.6.1 with my locale as ('en_GB', 'UTF-8') (I don't know 
whether that matters for reproducing this, but I know it can affect how 
lower/upper work so am including it for the sake of completeness).

The problem does not reproduce on Python 2.7.13 because on that case 
chr(304).lower() is 'i' without the combining character, so it fails earlier.

This is presumably related to #12728, but as that is closed as fixed while this 
still reproduces I don't believe it's a duplicate.

----------
components: Library (Lib)
files: casing.py
messages: 300219
nosy: David MacIver
priority: normal
severity: normal
status: open
title: re.IGNORECASE strips combining character from lower case of LATIN 
CAPITAL LETTER I WITH DOT ABOVE
versions: Python 3.6
Added file: http://bugs.python.org/file47080/casing.py

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<http://bugs.python.org/issue31193>
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