diff --git a/sphinx/transforms/post_transforms/images.py b/sphinx/transforms/post_transforms/images.py
index e220df0a8..3b4009c18 100644
--- a/sphinx/transforms/post_transforms/images.py
+++ b/sphinx/transforms/post_transforms/images.py
@@ -61,7 +61,7 @@ class ImageDownloader(BaseImageConverter):
             if basename == '' or len(basename) > MAX_FILENAME_LEN:
                 filename, ext = os.path.splitext(node['uri'])
                 basename = sha1(filename.encode(), usedforsecurity=False).hexdigest() + ext
-            basename = re.sub(CRITICAL_PATH_CHAR_RE, "_", basename)
+            basename = CRITICAL_PATH_CHAR_RE.sub("_", basename)
 
             dirname = node['uri'].replace('://', '/').translate({ord("?"): "/",
                                                                  ord("&"): "/"})
@@ -143,7 +143,7 @@ class DataURIExtractor(BaseImageConverter):
 
 def get_filename_for(filename: str, mimetype: str) -> str:
     basename = os.path.basename(filename)
-    basename = re.sub(CRITICAL_PATH_CHAR_RE, "_", basename)
+    basename = CRITICAL_PATH_CHAR_RE.sub("_", basename)
     return os.path.splitext(basename)[0] + (get_image_extension(mimetype) or '')