update fieldMatchesRegexByStringerValOrString for backward compatibility

util.go

@@ -298,8 +298,14 @@ func panicIf(err error) {
 // Checks if field value matches regex. If fl.Field can be cast to Stringer, it uses the Stringer interfaces
 // String() return value. Otherwise, it uses fl.Field's String() value.
 func fieldMatchesRegexByStringerValOrString(regex *regexp.Regexp, fl FieldLevel) bool {
-	if stringer, ok := fl.Field().Interface().(fmt.Stringer); ok {
-		return regex.MatchString(stringer.String())
+	switch fl.Field().Kind() {
+	case reflect.String:
+		return regex.MatchString(fl.Field().String())
+	default:
+		if stringer, ok := fl.Field().Interface().(fmt.Stringer); ok {
+			return regex.MatchString(stringer.String())
+		} else {
+			return regex.MatchString(fl.Field().String())
+		}
 	}
-	return regex.MatchString(fl.Field().String())
 }

validator_test.go

@@ -4113,7 +4113,14 @@ func (u uuidTestType) String() string {
 	return u.val
 }
 
+type uuidAlias string
+
+func (u uuidAlias) String() string {
+	return "This is a UUID " + string(u)
+}
+
 var _ fmt.Stringer = uuidTestType{}
+var _ fmt.Stringer = uuidAlias("")
 
 func TestUUIDValidation(t *testing.T) {
 	tests := []struct {
@@ -4170,6 +4177,12 @@ func TestUUIDValidation(t *testing.T) {
 	if err := validate.Struct(structWithInvalidUUID); err == nil {
 		t.Fatal("UUID failed Error expected but received nil")
 	}
+
+	// Test on Alias type with Stringer interface.
+	alias := uuidAlias("a987fbc9-4bed-3078-cf07-9141ba07c9f3")
+	if err := validate.Var(alias, "uuid"); err != nil {
+		t.Fatalf("UUID failed Error: %s", err)
+	}
 }
 
 func TestUUID5RFC4122Validation(t *testing.T) {