gh-153177: Deprecate re.LOCALE

The flag only works with bytes patterns in 8-bit locales and makes
matching much slower.  Emit a DeprecationWarning when a pattern uses
the flag or the inline flag (?L).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/deprecations/pending-removal-in-3.18.rst

@@ -11,6 +11,12 @@ Pending removal in Python 3.18
     C implementation, has been deprecated since Python 3.13.
     (Contributed by Serhiy Storchaka in :gh:`89902`.)
 
+* :mod:`re`:
+
+  * The :const:`~re.LOCALE` flag and the inline flag ``(?L)``.
+    Decode the input and match str patterns instead.
+    (Contributed by Serhiy Storchaka in :gh:`153177`.)
+
 * Deprecations defined by :pep:`829`:
 
   * ``import`` lines in :file:`{name}.pth` files are silently ignored.

Doc/library/re.rst

@@ -881,6 +881,10 @@ Flags
       no longer depend on the locale at compile time.
       Only the locale at matching time affects the result of matching.
 
+   .. deprecated:: next
+      The flag and the inline flag ``(?L)`` are deprecated.
+      Decode the input and match str patterns instead.
+
 
 .. data:: M
           MULTILINE

Doc/whatsnew/3.16.rst

@@ -575,6 +575,12 @@ New deprecations
     3.9, now issues a deprecation warning on use. This property is slated for
     removal in 3.21. Use ``ast.Tuple.elts`` instead.
 
+* :mod:`re`:
+
+  * The :const:`~re.LOCALE` flag and the inline flag ``(?L)`` are
+    deprecated.  Decode the input and match str patterns instead.
+    (Contributed by Serhiy Storchaka in :gh:`153177`.)
+
 * :mod:`struct`:
 
   * Soft-deprecated since Python 3.15, using ``'F'`` and ``'D'`` type codes are now

Lib/re/_compiler.py

@@ -396,6 +396,12 @@ def compile(p, flags=0):
     else:
         pattern = None
 
+    if p.state.flags & SRE_FLAG_LOCALE or p.state.locale_used:
+        import warnings
+        warnings.warn("re.LOCALE and the inline flag (?L) are deprecated",
+                      DeprecationWarning,
+                      stacklevel=4)
+
     code = _code(p, flags)
 
     if flags & SRE_FLAG_DEBUG:

Lib/re/_parser.py

@@ -77,6 +77,7 @@ class State:
     # keeps track of state for parsing
     def __init__(self):
         self.flags = 0
+        self.locale_used = False
         self.groupdict = {}
         self.groupwidths = [None]  # group 0
         self.lookbehindgroups = None
@@ -1097,6 +1098,8 @@ def _parse_flags(source, state, char):
         raise source.error("bad inline flags: cannot turn off global flag", 1)
     if add_flags & del_flags:
         raise source.error("bad inline flags: flag turned on and off", 1)
+    if add_flags & SRE_FLAG_LOCALE:
+        state.locale_used = True
     return add_flags, del_flags
 
 def fix_flags(src, flags):

Lib/test/test_re.py

@@ -0,0 +1,3 @@
+Deprecate the :const:`re.LOCALE` flag and the inline flag ``(?L)``.
+They only work with bytes patterns in 8-bit locales and make matching
+much slower.  Decode the input and match str patterns instead.