diff --git a/acme.sh b/acme.sh
index 3d65612f..ef37f163 100755
--- a/acme.sh
+++ b/acme.sh
@@ -1196,7 +1196,7 @@ _createkey() {
 _is_idn() {
   _is_idn_d="$1"
   _debug2 _is_idn_d "$_is_idn_d"
-  _idn_temp=$(printf "%s" "$_is_idn_d" | tr -d '0-9' | tr -d 'a-z' | tr -d 'A-Z' | tr -d '*.,-_')
+  _idn_temp=$(printf "%s" "$_is_idn_d" | tr -d [0-9] | tr -d [a-z] | tr -d [A-Z] | tr -d '*.,-_')
   _debug2 _idn_temp "$_idn_temp"
   [ "$_idn_temp" ]
 }