diff --git a/src/ss-nat b/src/ss-nat
index 10b06fa8..f4b1ebdb 100755
--- a/src/ss-nat
+++ b/src/ss-nat
@@ -10,7 +10,7 @@
 TAG="SS_SPEC"          # iptables tag
 IPT="iptables -t nat"  # alias of iptables
 FWI=$(uci get firewall.shadowsocks.path 2>/dev/null)  # firewall include file
-REGEX="[^0-9.]"		   # used to check if hostnames have letters
+IP_REGEX="^([0-9]{1,3}\.){3}[0-9]{1,3}"		      # used to check if input is a valid IP
 
 usage() {
 	cat <<-EOF
@@ -127,7 +127,7 @@ tp_rule() {
 }
 
 get_wan_ip() {
-	cat <<-EOF | grep -E "^([0-9]{1,3}\.){3}[0-9]{1,3}"
+	cat <<-EOF | grep -E $IP_REGEX
 		$server
 		$SERVER
 		$WAN_BP_IP
@@ -173,7 +173,7 @@ while getopts ":s:l:S:L:i:I:e:a:b:w:ouUfh" arg; do
 		s)
 			command -v dig > /dev/null &&
 				server=$(dig +short $OPTARG) ||
-				if [[ $OPTARG =~ $REGEX ]]
+				if ! [[ $OPTARG =~ $IP_REGEX ]]
 				then
 					loger 3 "Hostname detected for server but no dig present to resolve!"
 					exit 1
@@ -187,7 +187,7 @@ while getopts ":s:l:S:L:i:I:e:a:b:w:ouUfh" arg; do
 		S)
 			command -v dig > /dev/null &&
 				SERVER=$(dig +short $OPTARG) ||
-				if [[ $OPTARG =~ $REGEX ]]
+				if ! [[ $OPTARG =~ $IP_REGEX ]]
 				then
 					loger 3 "Hostname detected for UDP server but no dig present to resolve!"
 					exit 1