Issue #27528: Document and test warning messages must match at beginning

Doc/library/warnings.rst

@@ -141,14 +141,15 @@ the disposition of the match.  Each entry is a tuple of the form (*action*,
   |               | warnings, regardless of location             |
   +---------------+----------------------------------------------+
 
-* *message* is a string containing a regular expression that the warning message
-  must match (the match is compiled to always be case-insensitive).
+* *message* is a string containing a regular expression that the start of
+  the warning message must match.  The expression is compiled to always be
+  case-insensitive.
 
 * *category* is a class (a subclass of :exc:`Warning`) of which the warning
   category must be a subclass in order to match.
 
 * *module* is a string containing a regular expression that the module name must
-  match (the match is compiled to be case-sensitive).
+  match.  The expression is compiled to be case-sensitive.
 
 * *lineno* is an integer that the line number where the warning occurred must
   match, or ``0`` to match all line numbers.

Lib/test/test_warnings/__init__.py

@@ -261,6 +261,18 @@ class FilterTests(BaseTest):
             self.assertEqual(str(w[-1].message), text)
             self.assertTrue(w[-1].category is UserWarning)
 
+    def test_message_matching(self):
+        with original_warnings.catch_warnings(record=True,
+                module=self.module) as w:
+            self.module.simplefilter("ignore", UserWarning)
+            self.module.filterwarnings("error", "match", UserWarning)
+            self.assertRaises(UserWarning, self.module.warn, "match")
+            self.assertRaises(UserWarning, self.module.warn, "match prefix")
+            self.module.warn("suffix match")
+            self.assertEqual(w, [])
+            self.module.warn("something completely different")
+            self.assertEqual(w, [])
+
     def test_mutate_filter_list(self):
         class X:
             def match(self, a):