diff --git a/etc/inc/util.inc b/etc/inc/util.inc
index 18f3905230..a9e533f27b 100644
--- a/etc/inc/util.inc
+++ b/etc/inc/util.inc
@@ -644,7 +644,7 @@ function is_domain($domain) {
 	if (!is_string($domain))
 		return false;
 
-	if (preg_match('/^(?:(?:[a-z0-9]|[a-z0-9][a-z0-9\-]*[a-z0-9])\.)*(?:[a-z0-9]|[a-z0-9][a-z0-9\-]*[a-z0-9])$/i', $domain))
+	if (preg_match('/^(?:(?:[a-z_0-9]|[a-z_0-9][a-z_0-9\-]*[a-z_0-9])\.)*(?:[a-z_0-9]|[a-z_0-9][a-z_0-9\-]*[a-z_0-9])$/i', $domain))
 		return true;
 	else
 		return false;