Show details when a regex fails to compile (GH-2678)

CHANGES.md

@@ -1,5 +1,11 @@
 # Change Log
 
+## Unreleased
+
+### _Black_
+
+- Improve error message for invalid regular expression (#2678)
+
 ## 21.12b0
 
 ### _Black_

src/black/__init__.py

@@ -177,8 +177,8 @@ def validate_regex(
 ) -> Optional[Pattern[str]]:
     try:
         return re_compile_maybe_verbose(value) if value is not None else None
-    except re.error:
-        raise click.BadParameter("Not a valid regular expression") from None
+    except re.error as e:
+        raise click.BadParameter(f"Not a valid regular expression: {e}") from None
 
 
 @click.command(