diff --git a/libs/web/htdocs/luci-static/resources/cbi.js b/libs/web/htdocs/luci-static/resources/cbi.js
index 1cd49bc65d..1e751d47e6 100644
--- a/libs/web/htdocs/luci-static/resources/cbi.js
+++ b/libs/web/htdocs/luci-static/resources/cbi.js
@@ -218,6 +218,14 @@ var cbi_validators = {
 		if (!isNaN(max) && !isNaN(val))
 			return (val <= max);
 
+		return false;
+	},
+
+	'neg': function(v, args)
+	{
+		if (args[0] && typeof cbi_validators[args[0]] == "function")
+			return cbi_validators[args[0]](v.replace(/^\s*!\s*/, ''));
+
 		return false;
 	}
 };
diff --git a/libs/web/luasrc/cbi/datatypes.lua b/libs/web/luasrc/cbi/datatypes.lua
index 6640db6392..fc3048e37e 100644
--- a/libs/web/luasrc/cbi/datatypes.lua
+++ b/libs/web/luasrc/cbi/datatypes.lua
@@ -267,3 +267,11 @@ function max(val, max)
 
 	return false
 end
+
+function neg(val, what)
+	if what and type(_M[what]) == "function" then
+		return _M[what](val:gsub("^%s*!%s*", ""))
+	end
+
+	return false
+end