final TextField fieldInput = new TextField();
fieldInput.setValue("first");
fieldInput.setAllowBlank(false);
fieldInput.addValidator(new MaxLengthValidator(20));
final RegExp regex = RegExp.compile("\\s");
fieldInput.addValidator(new Validator<String>() {
public List<EditorError> validate(Editor<String> editor, String value) {
if (regex.test(value)) {
List<EditorError> errors = new ArrayList<EditorError>();
errors.add(new DefaultEditorError(editor, "Field name cannot contain spaces.", ""));
return errors;