===================================================================
RCS file: /cvs/mandoc/regress/regress.pl,v
retrieving revision 1.9
retrieving revision 1.10
diff -u -p -r1.9 -r1.10
--- mandoc/regress/regress.pl	2018/12/16 00:17:04	1.9
+++ mandoc/regress/regress.pl	2019/03/10 09:23:33	1.10
@@ -1,6 +1,6 @@
 #!/usr/bin/env perl
 #
-# $Id: regress.pl,v 1.9 2018/12/16 00:17:04 schwarze Exp $
+# $Id: regress.pl,v 1.10 2019/03/10 09:23:33 schwarze Exp $
 #
 # Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.org>
 #
@@ -27,6 +27,7 @@ use IPC::Open3 qw(open3);
 # Define this at one place such that it can easily be changed
 # if diff(1) does not support the -a option.
 my @diff = qw(diff -au);
+system @diff, '/dev/null', '/dev/null' and @diff = qw(diff -u);
 
 # --- utility functions ------------------------------------------------