<form name="signUpForm" ng-submit="submitForm()">
   <div class="form-group">
       <label>用户名：</label>
       <input name="username"
              ng-model="username"
              ng-class="{'error': signUpForm.username.$invalid && signUpForm.username.$touched }"
              ng-minlength="3"
              required
              class="form-control input">
   </div>
   <div ng-if="signUpForm.username.$invalid && signUpForm.username.$touched">您的输入有误请检查</div>
</form>