diff --git a/tor-proxy/entrypoint.sh b/tor-proxy/entrypoint.sh
index f21afb4..8497ec3 100644
--- a/tor-proxy/entrypoint.sh
+++ b/tor-proxy/entrypoint.sh
@@ -4,14 +4,13 @@ set -e
 validate_alnum() {
     local val="$1"
     local name="$2"
-    if ! echo "$val" | grep -qE '^[a-zA-Z0-9._-]+$'; then
+    case "$val" in
+        *$'\n'*) echo "ERROR: $name contains newlines"; exit 1 ;;
+    esac
+    if ! printf '%s' "$val" | grep -qE '^[a-zA-Z0-9._-]+$'; then
         echo "ERROR: $name contains invalid characters: $val"
         exit 1
     fi
-    if echo "$val" | grep -q $'\n'; then
-        echo "ERROR: $name contains newlines: $val"
-        exit 1
-    fi
 }
 
 validate_alnum "$SSH_HOST_IP" "SSH_HOST_IP"