<style>

.erro{

background: red;

}

</style>

<form ng-submit="submainForm" name="signUpForm" >

<div class="form-group" >

<label>用户名：</label>

<input type="text"  name="ngText" ng-module="ngText" 

ng-class="{'erro':signUpForm.ngText.$invalid}"

class="form-control"

ng-minlengtn="4"

ng-maxlength="8"

/>

</div>

</form>