diff --git a/storage/filesystem.go b/storage/filesystem.go
index 52a4974..6f3a189 100644
--- a/storage/filesystem.go
+++ b/storage/filesystem.go
@@ -21,7 +21,10 @@ type filesystemBackend struct {
 	path string
 }
 
-var nilBackend carddav.Backend = (*filesystemBackend)(nil)
+var (
+	nilBackend         carddav.Backend = (*filesystemBackend)(nil)
+	validFilenameRegex                 = regexp.MustCompile(`^/[A-Za-z0-9_-]+(.[a-zA-Z]+)?$`)
+)
 
 func NewFilesystem(path string) (carddav.Backend, error) {
 	info, err := os.Stat(path)
@@ -65,8 +68,7 @@ func (b *filesystemBackend) safePath(ctx context.Context, path string) (string,
 	}
 	// We are mapping to local filesystem path, so be conservative about what to accept
 	// TODO this changes once multiple addess books are supported
-	var valid = regexp.MustCompile(`^/[A-Za-z0-9_-]+(.[a-zA-Z]+)?$`)
-	if !valid.MatchString(path) {
+	if !validFilenameRegex.MatchString(path) {
 		return "", fmt.Errorf("invalid request path")
 	}
 	return filepath.Join(basePath, path), nil