Bug report

The pure-Python zoneinfo parser accepts a POSIX TZ Mm.w.d transition rule whose third separator is not ., while the C accelerator (and POSIX) rejects it.

Reproduced on main (3.16.0a0), from_file footer path through _parse_tz_str -> _parse_dst_start_end:

python: 3.16.0a0
tzstr : EST5EDT,M3.2X0,M11.1.0

pure  zoneinfo._zoneinfo.ZoneInfo.from_file: ZoneInfo(key='EST5EDT,M3.2X0,M11.1.0') -> ACCEPTED
C     _zoneinfo.ZoneInfo.from_file: ValueError: Malformed transition rule in TZ string: b'EST5EDT,M3.2X0,M11.1.0'

The pure parser also accepts M3.2!0, M3.2-0, M3.2:0 and a bad separator in the second rule (M11.1X0); the C accelerator rejects all of them.

Cause

Lib/zoneinfo/_zoneinfo.py:710:

m = re.fullmatch(r"M(\d{1,2})\.(\d).(\d)", date, re.ASCII)

The first separator is escaped (\.) but the third is a bare ., which in a regex matches any single character. So M3.2X0 matches with week 2, day 0, and the separator X consumed by the wildcard.

Which side is wrong

The pure parser is wrong. POSIX (IEEE Std 1003.1-2024, Issue 8, Base Definitions, 8.3 "Other Environment Variables", the TZ Mm.w.d rule) specifies the form Mm.w.d with literal . separators between month, week, and day. The C accelerator already enforces both periods at Modules/_zoneinfo.c:1862 and Modules/_zoneinfo.c:1868:

if (*ptr++ != '.') {
    return -1;
}

so the pure path should match.

Fix

Escape the third separator so it is a literal .:

m = re.fullmatch(r"M(\d{1,2})\.(\d)\.(\d)", date, re.ASCII)

Non-breaking: every well-formed Mm.w.d rule and all bundled IANA zones still parse; only the previously-accepted malformed separators now raise, matching the C accelerator.

Related

Distinct from #152212 / PR #152213, which fix the separate missing-STD-offset parity gap. This is the Mm.w.d separator regex.

Linked PRs

• gh-152246: Fix pure-Python zoneinfo accepting invalid seperators in POSIX TZ (Mm.w.d) rules #152247
• [3.15] gh-152246: Fix pure-Python zoneinfo accepting invalid seperators in POSIX TZ rules (GH-152247) #152265
• [3.14] gh-152246: Fix pure-Python zoneinfo accepting invalid seperators in POSIX TZ rules (GH-152247) #152266
• [3.13] gh-152246: Fix pure-Python zoneinfo accepting invalid seperators in POSIX TZ rules (GH-152247) #152267