Add not (!) to invert validation rules

validate.go

@@ -206,10 +206,15 @@ func getTagFuncs(i int, ft reflect.StructField, kind reflect.Kind, tags []string
 			continue
 		}
 
-		check, val := getTagFunc(tag, value, kind)
+		var not bool
+		if strings.HasPrefix(tag, `!`) {
+			not = true
+		}
+
+		check, val := getTagFunc(strings.TrimPrefix(tag, `!`), value, kind)
 
 		f := func(rv reflect.Value) ([]ValidationError, bool) {
-			if check(rv, val) {
+			if check(rv, val) == !not {
 				return nil, true
 			}
 

validate_test.go

@@ -76,3 +76,24 @@ func TestValidationErrorField(t *testing.T) {
 		t.Fatal(`Expected errors to be A.B.C[0].D and A.B.C[2].D; got`, errs[0].Field, `and`, errs[1].Field)
 	}
 }
+
+func TestNot(t *testing.T) {
+	type s struct {
+		A string `validate:"!len=3"`
+		B int    `validate:"!eq=3"`
+	}
+
+	var pass1 = s{`ab`, 2}
+	var pass2 = s{`abcd`, 4}
+
+	var fail = s{`abc`, 3}
+
+	check(t, pass1, 0)
+	check(t, pass2, 0)
+	check(t, fail, 2)
+
+	errs := Validate(fail)
+	if errs[0].Check != `!len` || errs[1].Check != `!eq` {
+		t.Errorf(`Checknames missing !, got "%s" and "%s"`, errs[0].Check, errs[1].Check)
+	}
+}