Edit in JSFiddle

//宣告ng-app，並在app中掛載官方的ngMessage模組
var demoApp = angular.module("demoapp", ['ngMessages']);

//ng-app底下掛載ng-controller
demoApp.controller("demoCtrl", function($scope) {

});

<div ng-app="demoapp">
  <div ng-controller="demoCtrl">
    <h1>官方原生ng-message模組方法Demo</h1>
    <h3>限制input需為必填，且最大長度為5，最小長度為2</h3>
    <form name="demoForm">
      <div>
        輸入：
        <input type='text' name='userInput' ng-model='userInput' required ng-maxlength='5' ng-minlength='2'>
        <br>
        <div ng-messages="demoForm.userInput.$error" class='error' role="alert">
          <div ng-message="required">必填</div>
          <div ng-message="minlength">最少請輸入2個字</div>
          <div ng-message="maxlength">最多請輸入5個字</div>
        </div>
      </div>

      <div>
        <h3>input驗證錯誤內容：</h3>form.input.$error：{{demoForm.userInput.$error | json}}
      </div>
      <div>
        <h3>表單驗證結果：</h3>{{demoForm.$valid}}
      </div>
    </form>
  </div>
</div>

.error {
  color: red
}