diff --git a/CHANGES.rst b/CHANGES.rst
index 90faecc17..35a0ff428 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,8 @@ Version 8.1.7
 
 Unreleased
 
+-   Fix issue with regex flags in shell completion. :issue:`2581`
+
 
 Version 8.1.6
 -------------
diff --git a/src/click/shell_completion.py b/src/click/shell_completion.py
index 5de124702..9091f787e 100644
--- a/src/click/shell_completion.py
+++ b/src/click/shell_completion.py
@@ -230,7 +230,7 @@ def func_name(self) -> str:
         """The name of the shell function defined by the completion
         script.
         """
-        safe_name = re.sub(r"\W*", "", self.prog_name.replace("-", "_"), re.ASCII)
+        safe_name = re.sub(r"\W*", "", self.prog_name.replace("-", "_"), flags=re.ASCII)
         return f"_{safe_name}_completion"
 
     def source_vars(self) -> t.Dict[str, t.Any]: