diff --git a/lib/sources.nix b/lib/sources.nix
index 0ec4c020e545..63b3749d19ec 100644
--- a/lib/sources.nix
+++ b/lib/sources.nix
@@ -15,9 +15,10 @@ rec {
   cleanSourceFilter = name: type: let baseName = baseNameOf (toString name); in ! (
     # Filter out Subversion and CVS directories.
     (type == "directory" && (baseName == ".git" || baseName == ".svn" || baseName == "CVS" || baseName == ".hg")) ||
-    # Filter out backup files.
+    # Filter out editor backup / swap files.
     lib.hasSuffix "~" baseName ||
-    builtins.match "^.*\.sw[a-z]$" baseName != null ||
+    builtins.match "^\\.sw[a-z]$" baseName != null ||
+    builtins.match "^\\..*\\.sw[a-z]$" baseName != null ||
 
     # Filter out generates files.
     lib.hasSuffix ".o" baseName ||