<form name="registration">
    <div class="form-group">
        <label for="username">Username</label>
        <input type="text" class="form-control" required ng-minlength="3"
         id="username" name="username" ng-model="user.username"/>
    </div>
    <div ng-messages="registration.username.$error"
     ng-show="registration.username.$dirty && registration.username.$invalid" role="alert">
        <div ng-message="required">Username is required</div>
        <div ng-message="minlength">Username must be at least 3 characters long</div>
    </div>
</form>