Fix to_offset fallback and add reproduction test

pandas/_libs/tslibs/offsets.pyx

@@ -6537,17 +6537,20 @@ cpdef to_offset(freq, bint is_period=False):
                 else:
                     result = result + offset
         except (ValueError, TypeError) as err:
-            raise_invalid_freq(
-                freq=freq,
-                extra_message=f"Failed to parse with error message: {repr(err)}"
-            )
+            # --- FALLBACK---
+            try:
+                td = Timedelta(freq)
+                result = delta_to_tick(td)
+            except (ValueError, TypeError):
+                # If Timedelta parsing also fails, raise the original error
+                raise_invalid_freq(
+                    freq=freq,
+                    extra_message=f"Failed to parse with error message: {repr(err)}"
+                )
 
-        # TODO(3.0?) once deprecation of "d" is enforced, the check for it here
-        #  can be removed
         if (
                 isinstance(result, Hour)
                 and result.n % 24 == 0
-                and ("d" in freq or "D" in freq)
         ):
             # Since Day is no longer a Tick, delta_to_tick returns Hour above,
             #  so we convert back here.

pandas/tests/tseries/offsets/test_offsets_fallback.py

@@ -0,0 +1,38 @@
+import pytest
+
+from pandas import Timedelta
+
+from pandas.tseries.frequencies import to_offset
+
+
+def test_to_offset_timedelta_string_fallback():
+    # This is the case you found: str(Timedelta)
+    # Previously, this might have failed in the regex loop
+    freq_str = "0 days 01:30:00"
+    result = to_offset(freq_str)
+
+    expected = to_offset("1h30min")
+    assert result == expected
+
+
+@pytest.mark.parametrize(
+    "freq",
+    [
+        "1 days 02:00:00",
+        "00:00:00.001",
+        "1h 30min",  # spaces between components
+    ],
+)
+def test_to_offset_consistent_with_timedelta(freq):
+    # Ensure our fallback produces the same result as direct Timedelta conversion
+    result = to_offset(freq)
+    expected_td = Timedelta(freq)
+
+    # Convert result to total seconds to compare magnitude
+    assert result.nanos == expected_td.value
+
+
+def test_to_offset_invalid_still_raises():
+    # Ensure that if BOTH fail, we still get the Invalid Frequency error
+    with pytest.raises(ValueError, match="Invalid frequency"):
+        to_offset("not_a_time_at_all")