diff --git a/acme.sh b/acme.sh
index 4a8ca763..740b21ea 100755
--- a/acme.sh
+++ b/acme.sh
@@ -438,12 +438,12 @@ _secure_debug3() {
 
 _upper_case() {
   # shellcheck disable=SC2018,SC2019
-  tr '[a-z]' '[A-Z]'
+  tr 'a-z' 'A-Z'
 }
 
 _lower_case() {
   # shellcheck disable=SC2018,SC2019
-  tr '[A-Z]' '[a-z]'
+  tr 'A-Z' 'a-z'
 }
 
 _startswith() {
@@ -1186,7 +1186,7 @@ _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" ]
 }
@@ -4247,7 +4247,7 @@ _match_issuer() {
 _isIPv4() {
   for seg in $(echo "$1" | tr '.' ' '); do
     _debug2 seg "$seg"
-    if [ "$(echo "$seg" | tr -d '[0-9]')" ]; then
+    if [ "$(echo "$seg" | tr -d '0-9')" ]; then
       #not all number
       return 1
     fi