gh-130110: Support hyphens in RFC 2231 continuation parameter names

Lib/email/utils.py

@@ -399,7 +399,7 @@ def encode_rfc2231(s, charset=None, language=None):
     return "%s'%s'%s" % (charset, language, s)
 
 
-rfc2231_continuation = re.compile(r'^(?P<name>\w+)\*((?P<num>[0-9]+)\*?)?$',
+rfc2231_continuation = re.compile(r'^(?P<name>[\w-]+)\*((?P<num>[0-9]+)\*?)?$',
     re.ASCII)
 
 def decode_params(params):

Lib/test/test_email/test_email.py

@@ -400,6 +400,18 @@ def test_continuation_sorting_part_order(self):
         filename = msg.get_filename()
         self.assertEqual(filename, 'foo bar.txt')
 
+    def test_continuation_with_hyphenated_name(self):
+        # gh-130110: parameter names containing hyphens were not recognized
+        # as RFC 2231 continuations and were left undecoded.
+        msg = email.message_from_string(
+            "Content-Disposition: attachment; "
+            "file-name*0*=\"utf-8''start\"; "
+            "file-name*1*=\"-middle-\"; "
+            "file-name*2*=\"end\"\n"
+        )
+        value = msg.get_param('file-name', header='content-disposition')
+        self.assertEqual(value, ('utf-8', '', 'start-middle-end'))
+
     def test_sorting_no_continuations(self):
         msg = email.message_from_string(
             "Content-Disposition: attachment; "

Misc/NEWS.d/next/Library/2026-07-10-23-32-52.gh-issue-130110.R7xQz2.rst

@@ -0,0 +1,3 @@
+:meth:`email.utils.decode_params` (and the parameter parsing it backs) now
+correctly decodes :rfc:`2231` continuation parameter names that contain
+hyphens, such as ``file-name*0*``.