raise error for string indices in dataframes passed to is_regular

Author: lily-tomkovic-DWR

vtools/data/timeseries.py

@@ -551,19 +551,7 @@ def is_regular(ts, raise_exception=False):
 
     # If index is string-like, try datetime first, then numeric.
     if pd.api.types.is_string_dtype(idx.dtype):
-        dt_idx = pd.to_datetime(idx, errors="coerce")
-        if not dt_idx.isna().any():
-            idx = pd.DatetimeIndex(dt_idx)
-        else:
-            num = pd.to_numeric(idx, errors="coerce")
-            if np.isnan(num).any():
-                msg = (
-                    "Index is a string that can't convert to datetime or numeric - can't assess regularity."
-                )
-                if raise_exception:
-                    raise ValueError(msg)
-                return False
-            idx = pd.Index(num)
+        raise ValueError("String indices are not supported for regularity checks.")
 
     # Handle numeric indices (int, float)
     if isinstance(idx, (pd.Index, pd.RangeIndex)) and pd.api.types.is_numeric_dtype(idx.dtype):