var re = new RegExp('^(?:localhost|(?:[a-z0-9]+(?:[-\\w]*[a-z0-9])?(?:\\.[a-z0-9][-\\w]*[a-z0-9])*)*\\.[a-z]{2,})$', 'i')

console.log('should true:')
console.log(re.test('localhost'));
console.log(re.test('domian.hello'));
console.log(re.test('domi-an.hello'));
console.log(re.test('dom.ian.hello'));
console.log(re.test('dom.i-an.hello'));
console.log(re.test('1.dom.i-an.hello'));
console.log(re.test('12306.cn'));
console.log(re.test('12306.com.cn'));

console.log('should false:')
console.log(re.test('dom.-ian.hello'));
console.log(re.test('dom.ian-.hello'));
console.log(re.test('dom.i+an.hello'));
console.log(re.test('dom.ian..hello'));
console.log(re.test('hello'));

var re = new RegExp('^(?:[a-z0-9]+(?:\\-[\\w]+)*\\.)*[a-z]{2,}$', 'i')

console.log('should false:')
console.log(re.test('hello')); // true