Fix exception cause in regular_languages/validation.py

Author: cool-RR

prompt_toolkit/contrib/regular_languages/validation.py

@@ -53,7 +53,7 @@ def validate(self, document: Document) -> None:
                         raise ValidationError(
                             cursor_position=v.start + e.cursor_position,
                             message=e.message,
-                        )
+                        ) from e
         else:
             raise ValidationError(
                 cursor_position=len(document.text), message="Invalid command"