Clarify regex interpretation in str.contains (#44812)

Author: nealmcb

pandas/core/strings/accessor.py

@@ -1212,8 +1212,8 @@ def contains(self, pat, case=True, flags=0, na=None, regex=True):
         """
         if regex and re.compile(pat).groups:
             warnings.warn(
-                "This pattern has match groups. To actually get the "
-                "groups, use str.extract.",
+                "This pattern is interpreted as a regular expression, and has "
+                "match groups. To actually get the groups, use str.extract.",
                 UserWarning,
                 stacklevel=find_stack_level(),
             )

pandas/tests/strings/test_find_replace.py

@@ -919,7 +919,7 @@ def test_flags_kwarg(any_string_dtype):
     result = data.str.count(pat, flags=re.IGNORECASE)
     assert result[0] == 1
 
-    msg = "This pattern has match groups"
+    msg = "has match groups"
     with tm.assert_produces_warning(UserWarning, match=msg):
         result = data.str.contains(pat, flags=re.IGNORECASE)
     assert result[0]