<form name="signUp" ng-submit="submitForm()">

<div class="form-group">

<label >用户名：</label>

<input type="text" name="username" 

ng-model="username" 

ng-class="{ 'error' : signUp.username.$invalid && 

signUp.username.$touched }"

ng-minlength=4

required 

class="form-control">

</div>

</form>