gh-133306: Support \z as a synonym for \Z in regular expressions (GH-133314)

\Z was an error inherited from PCRE 0.95. It was fixed in PCRE 2.0.
In other engines, \Z means not “anchor at string end”, but
“anchor before optional newline at string end”.

\z means “anchor at string end” in most RE engines.

Doc/howto/regex.rst

@@ -738,9 +738,12 @@ given location, they can obviously be matched an infinite number of times.
    different: ``\A`` still matches only at the beginning of the string, but ``^``
    may match at any location inside the string that follows a newline character.
 
-``\Z``
+``\z``
    Matches only at the end of the string.
 
+``\Z``
+   The same as ``\z``.  For compatibility with old Python versions.
+
 ``\b``
    Word boundary.  This is a zero-width assertion that matches only at the
    beginning or end of a word.  A word is defined as a sequence of alphanumeric

Doc/library/re.rst

@@ -266,7 +266,7 @@ The special characters are:
      not a word boundary as outside a set, and numeric escapes
      such as ``\1`` are always octal escapes, not group references.
      Special sequences which do not match a single character such as ``\A``
-     and ``\Z`` are not allowed.
+     and ``\z`` are not allowed.
 
    .. index:: single: ^ (caret); in regular expressions
 
@@ -661,10 +661,16 @@ character ``'$'``.
    matches characters which are neither alphanumeric in the current locale
    nor the underscore.
 
-.. index:: single: \Z; in regular expressions
+.. index:: single: \z; in regular expressions
+           single: \Z; in regular expressions
+
+``\z``
+   Matches only at the end of the string.
+
+   .. versionadded:: next
 
 ``\Z``
-   Matches only at the end of the string.
+   The same as ``\z``.  For compatibility with old Python versions.
 
 .. index::
    single: \a; in regular expressions

Doc/whatsnew/3.14.rst

@@ -624,6 +624,11 @@ Other language changes
   ASCII :class:`bytes` and :term:`bytes-like objects <bytes-like object>`.
   (Contributed by Daniel Pope in :gh:`129349`.)
 
+* Support ``\z`` as a synonym for ``\Z`` in :mod:`regular expressions <re>`.
+  It is interpreted unambiguously in many other regular expression engines,
+  unlike ``\Z``, which has subtly different behavior.
+  (Contributed by Serhiy Storchaka in :gh:`133306`.)
+
 * ``\B`` in :mod:`regular expression <re>` now matches empty input string.
   Now it is always the opposite of ``\b``.
   (Contributed by Serhiy Storchaka in :gh:`124130`.)

Lib/re/__init__.py

@@ -61,7 +61,7 @@ below.  If the ordinary character is not on the list, then the
 resulting RE will match the second character.
     \number  Matches the contents of the group of the same number.
     \A       Matches only at the start of the string.
-    \Z       Matches only at the end of the string.
+    \z       Matches only at the end of the string.
     \b       Matches the empty string, but only at the start or end of a word.
     \B       Matches the empty string, but not at the start or end of a word.
     \d       Matches any decimal digit; equivalent to the set [0-9] in

Lib/re/_parser.py

@@ -49,7 +49,8 @@ CATEGORIES = {
     r"\S": (IN, [(CATEGORY, CATEGORY_NOT_SPACE)]),
     r"\w": (IN, [(CATEGORY, CATEGORY_WORD)]),
     r"\W": (IN, [(CATEGORY, CATEGORY_NOT_WORD)]),
-    r"\Z": (AT, AT_END_STRING), # end of string
+    r"\z": (AT, AT_END_STRING), # end of string
+    r"\Z": (AT, AT_END_STRING), # end of string (obsolete)
 }
 
 FLAGS = {

Lib/test/re_tests.py

@@ -531,7 +531,7 @@ xyzabc
     (r'a[ ]*?\ (\d+).*', 'a   10', SUCCEED, 'found', 'a   10'),
     (r'a[ ]*?\ (\d+).*', 'a    10', SUCCEED, 'found', 'a    10'),
     # bug 127259: \Z shouldn't depend on multiline mode
-    (r'(?ms).*?x\s*\Z(.*)','xx\nx\n', SUCCEED, 'g1', ''),
+    (r'(?ms).*?x\s*\z(.*)','xx\nx\n', SUCCEED, 'g1', ''),
     # bug 128899: uppercase literals under the ignorecase flag
     (r'(?i)M+', 'MMM', SUCCEED, 'found', 'MMM'),
     (r'(?i)m+', 'MMM', SUCCEED, 'found', 'MMM'),

Lib/test/test_re.py

@@ -619,6 +619,7 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.fullmatch(r"a.*?b", "axxb").span(), (0, 4))
         self.assertIsNone(re.fullmatch(r"a+", "ab"))
         self.assertIsNone(re.fullmatch(r"abc$", "abc\n"))
+        self.assertIsNone(re.fullmatch(r"abc\z", "abc\n"))
         self.assertIsNone(re.fullmatch(r"abc\Z", "abc\n"))
         self.assertIsNone(re.fullmatch(r"(?m)abc$", "abc\n"))
         self.assertEqual(re.fullmatch(r"ab(?=c)cd", "abcd").span(), (0, 4))
@@ -802,6 +803,8 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.search(r"\B(b.)\B",
                                    "abc bcd bc abxd", re.ASCII).group(1), "bx")
         self.assertEqual(re.search(r"^abc$", "\nabc\n", re.M).group(0), "abc")
+        self.assertEqual(re.search(r"^\Aabc\z$", "abc", re.M).group(0), "abc")
+        self.assertIsNone(re.search(r"^\Aabc\z$", "\nabc\n", re.M))
         self.assertEqual(re.search(r"^\Aabc\Z$", "abc", re.M).group(0), "abc")
         self.assertIsNone(re.search(r"^\Aabc\Z$", "\nabc\n", re.M))
         self.assertEqual(re.search(br"\b(b.)\b",
@@ -813,6 +816,8 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.search(br"\B(b.)\B",
                                    b"abc bcd bc abxd", re.LOCALE).group(1), b"bx")
         self.assertEqual(re.search(br"^abc$", b"\nabc\n", re.M).group(0), b"abc")
+        self.assertEqual(re.search(br"^\Aabc\z$", b"abc", re.M).group(0), b"abc")
+        self.assertIsNone(re.search(br"^\Aabc\z$", b"\nabc\n", re.M))
         self.assertEqual(re.search(br"^\Aabc\Z$", b"abc", re.M).group(0), b"abc")
         self.assertIsNone(re.search(br"^\Aabc\Z$", b"\nabc\n", re.M))
         self.assertEqual(re.search(r"\d\D\w\W\s\S",
@@ -836,7 +841,7 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.match(r"[\^a]+", 'a^').group(), 'a^')
         self.assertIsNone(re.match(r"[\^a]+", 'b'))
         re.purge()  # for warnings
-        for c in 'ceghijklmopqyzCEFGHIJKLMNOPQRTVXY':
+        for c in 'ceghijklmopqyCEFGHIJKLMNOPQRTVXY':
             with self.subTest(c):
                 self.assertRaises(re.PatternError, re.compile, '\\%c' % c)
         for c in 'ceghijklmopqyzABCEFGHIJKLMNOPQRTVXYZ':
@@ -2608,8 +2613,8 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.findall(r'(?>(?:ab){1,3})', 'ababc'), ['abab'])
 
     def test_bug_gh91616(self):
-        self.assertTrue(re.fullmatch(r'(?s:(?>.*?\.).*)\Z', "a.txt")) # reproducer
+        self.assertTrue(re.fullmatch(r'(?s:(?>.*?\.).*)\z', "a.txt")) # reproducer
-        self.assertTrue(re.fullmatch(r'(?s:(?=(?P<g0>.*?\.))(?P=g0).*)\Z', "a.txt"))
+        self.assertTrue(re.fullmatch(r'(?s:(?=(?P<g0>.*?\.))(?P=g0).*)\z', "a.txt"))
 
     def test_bug_gh100061(self):
         # gh-100061

Misc/NEWS.d/next/Library/2025-05-02-21-35-03.gh-issue-133306.-vBye5.rst

@@ -0,0 +1 @@
+Support ``\z`` as a synonym for ``\Z`` in :mod:`regular expressions <re>`.