add ExceptChars validator

chars.go

@@ -20,3 +20,15 @@ func Chars(allow string) func(string) error {
 		return nil
 	}
 }
+
+// ExceptChars validates whether a string does not contain disallowed characters.
+func ExceptChars(disallow string) func(string) error {
+	return func(value string) error {
+		for _, r := range disallow {
+			if strings.ContainsRune(value, r) {
+				return ErrDisallowedChars
+			}
+		}
+		return nil
+	}
+}

chars_test.go

@@ -17,6 +17,7 @@ func TestChars(t *testing.T) {
 	testCases := []TestCase{
 		{Input: "abcd1234", C: hexRange},
 		{Input: "abcd 1234", C: hexRange, Err: ErrDisallowedChars},
+		{Input: "ghijklmno", C: hexRange, Err: ErrDisallowedChars},
 	}
 
 	for n, tc := range testCases {
@@ -30,3 +31,30 @@ func TestChars(t *testing.T) {
 		}
 	}
 }
+
+func TestExceptChars(t *testing.T) {
+	type TestCase struct {
+		Input string
+		C     string
+		Err   error
+	}
+
+	hexRange := "0123456789abcdef"
+
+	testCases := []TestCase{
+		{Input: "abcd1234", C: hexRange, Err: ErrDisallowedChars},
+		{Input: "abcd 1234", C: hexRange, Err: ErrDisallowedChars},
+		{Input: "ghijklmno", C: hexRange},
+	}
+
+	for n, tc := range testCases {
+		t.Logf("(%d) Testing %q against %q", n, tc.Input, tc.C)
+
+		f := ExceptChars(tc.C)
+		err := f(tc.Input)
+
+		if !errors.Is(err, tc.Err) {
+			t.Errorf("Expected error %v, got %v", tc.Err, err)
+		}
+	}
+}