From e7ddf524b056d2bc42ee6b2b5c3314e0dd5d95fb Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Tue, 7 Dec 2021 19:13:05 -0800
Subject: [PATCH] Show details when a regex fails to compile (GH-2678)

---
 CHANGES.md            | 6 ++++++
 src/black/__init__.py | 4 ++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index 9e13ef4..3724820 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,5 +1,11 @@
 # Change Log
 
+## Unreleased
+
+### _Black_
+
+- Improve error message for invalid regular expression (#2678)
+
 ## 21.12b0
 
 ### _Black_
diff --git a/src/black/__init__.py b/src/black/__init__.py
index 1923c06..e2376c4 100644
--- a/src/black/__init__.py
+++ b/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(
-- 
2.39.2