gh-152246: Reject a POSIX TZ Mm.w.d rule with a non-period separator in pure-Python zoneinfo

Author: tonghuaroot

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -1184,6 +1184,11 @@ def test_invalid_tzstr(self):
             # Invalid weekday
             "AAA4BBB,M1.1.7/2,M2.1.1/2",
             "AAA4BBB,M1.1.1/2,M2.1.7/2",
+            # Invalid Mm.w.d separator (must be a literal '.')
+            "AAA4BBB,M3.2X0,M11.1.0",
+            "AAA4BBB,M3.2.0,M11.1X0",
+            "AAA4BBB,M3.2-0,M11.1.0/3",
+            "AAA4BBB,M3.2.0/2,M11.1:0",
             # Invalid numeric offset
             "AAA4BBB,-1/2,20/2",
             "AAA4BBB,1/2,-1/2",

Lib/zoneinfo/_zoneinfo.py

@@ -707,7 +707,7 @@ def _parse_dst_start_end(dststr):
     type = date[:1]
     if type == "M":
         n_is_julian = False
-        m = re.fullmatch(r"M(\d{1,2})\.(\d).(\d)", date, re.ASCII)
+        m = re.fullmatch(r"M(\d{1,2})\.(\d)\.(\d)", date, re.ASCII)
         if m is None:
             raise ValueError(f"Invalid dst start/end date: {dststr}")
         date_offset = tuple(map(int, m.groups()))

Misc/NEWS.d/next/Library/2026-06-26-12-50-26.gh-issue-152246.MfXMd1.rst

@@ -0,0 +1,6 @@
+Fixed a parity bug in the pure-Python :mod:`zoneinfo` implementation, which
+accepted a POSIX TZ ``Mm.w.d`` transition rule whose third separator was not a
+period (for example ``M3.2X0``). Only the first period was escaped in the
+regular expression, so the third one matched any character. Such a string is
+invalid per POSIX and is already rejected by the C accelerator. Patch by
+tonghuaroot.