Under re.IGNORECASE | re.LOCALE, [bc] matches b'B', but so does [^bc].

import re

W = re.IGNORECASE | re.LOCALE
print(re.fullmatch(rb'[bc]', b'B', W))
print(re.fullmatch(rb'[^bc]', b'B', W))

<re.Match object; span=(0, 1), match=b'B'>
<re.Match object; span=(0, 1), match=b'B'>

Expected: the second line is None, which is what 3.6 and earlier print.

charset_loc_ignore() in Modules/_sre/sre_lib.h tests the whole set once per locale case and returns true if either test matches. For a set carrying NEGATE the negation has to be applied outside that disjunction: b'B' matches because 'B' itself is not in {b, c}, though its lowercase is.

A one-member set escapes this, since it compiles to NOT_LITERAL_LOC_IGNORE and compares against both cases correctly, so [^b] is fine and two members are needed to see it.

IN_LOC_IGNORE and charset_loc_ignore() were added in 3.7 by bpo-30215 (898ff03), which moved locale case folding from compile time to match time. 3.2–3.6 are correct, 3.7 through main are affected.

The set-difference fusion new in 3.16 hits the same limitation from another direction: it rewrites A(?<![B]) into a single NEGATE-bearing set, which then compiles to IN_LOC_IGNORE. See gh-155985 and GH-155993, which avoids the fusion under these flags.

Linked PRs

• gh-156444: Fix a negated character set with IGNORECASE and LOCALE #156445
• [3.15] gh-156444: Fix a negated character set with IGNORECASE and LOCALE (GH-156445) #156456
• [3.14] gh-156444: Fix a negated character set with IGNORECASE and LOCALE (GH-156445) #156457
• [3.13] gh-156444: Fix a negated character set with IGNORECASE and LOCALE (GH-156445) #156458