angular_jade_form.html

<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"/>
        <ng-messages for="registration.username.$error" 
         ng-show="registration.username.$dirty && registration.username.$invalid" role="alert">
            <ng-message when="required">Username is required</ng-message>
            <ng-message when="minlength">Username must be at least 3 characters long</ng-message>
        </ng-messages>
    </div>
</form>