improve error message

Author: mhoff

logprep/filter/expression/filter_expression.py

@@ -340,7 +340,9 @@ def _normalize_regex(regex: str) -> str:
             end_token = "$"
         match = RegExFilterExpression.match_parts_pattern.match(regex)
         if match is None:  # pragma: no cover
-            raise FilterExpressionError("regex is designed to always match")
+            raise FilterExpressionError(
+                f"Internal error: regex is designed to always match: '{regex!r}'"
+            )
         flag, _, pattern = match.groups()
         flag = "" if flag is None else flag
         pattern = "" if pattern is None else pattern