REGR: freq "m" (as alias of deprecated "M") raises an error (#56849)

* dont raise if users use (undocumented) "m" for "month end", warn to use "ME" instead

* fixup test

* lint

Author: MarcoGorelli

pandas/_libs/tslibs/offsets.pyx

@@ -4860,15 +4860,15 @@ cpdef to_offset(freq, bint is_period=False):
 
             tups = zip(split[0::4], split[1::4], split[2::4])
             for n, (sep, stride, name) in enumerate(tups):
-                if is_period is False and name in c_OFFSET_DEPR_FREQSTR:
+                if is_period is False and name.upper() in c_OFFSET_DEPR_FREQSTR:
                     warnings.warn(
                         f"\'{name}\' is deprecated and will be removed "
                         f"in a future version, please use "
-                        f"\'{c_OFFSET_DEPR_FREQSTR.get(name)}\' instead.",
+                        f"\'{c_OFFSET_DEPR_FREQSTR.get(name.upper())}\' instead.",
                         FutureWarning,
                         stacklevel=find_stack_level(),
                     )
-                    name = c_OFFSET_DEPR_FREQSTR[name]
+                    name = c_OFFSET_DEPR_FREQSTR[name.upper()]
                 if is_period is True and name in c_REVERSE_OFFSET_DEPR_FREQSTR:
                     if name.startswith("Y"):
                         raise ValueError(

pandas/tests/indexes/datetimes/test_date_range.py

@@ -818,6 +818,17 @@ def test_frequencies_A_deprecated_Y_renamed(self, freq, freq_depr):
             result = date_range("1/1/2000", periods=2, freq=freq_depr)
         tm.assert_index_equal(result, expected)
 
+    def test_to_offset_with_lowercase_deprecated_freq(self) -> None:
+        # https://github.com/pandas-dev/pandas/issues/56847
+        msg = (
+            "'m' is deprecated and will be removed in a future version, please use "
+            "'ME' instead."
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = date_range("2010-01-01", periods=2, freq="m")
+        expected = DatetimeIndex(["2010-01-31", "2010-02-28"], freq="ME")
+        tm.assert_index_equal(result, expected)
+
     def test_date_range_bday(self):
         sdate = datetime(1999, 12, 25)
         idx = date_range(start=sdate, freq="1B", periods=20)

pandas/tests/tslibs/test_to_offset.py

@@ -44,6 +44,7 @@ def test_to_offset_negative(freqstr, expected):
     assert result.n == expected
 
 
+@pytest.mark.filterwarnings("ignore:.*'m' is deprecated.*:FutureWarning")
 @pytest.mark.parametrize(
     "freqstr",
     [