How add Custom Validation Rules when using Form Request Validation in Laravel 5

While the above answer is correct, in a lot of cases you might want to create a custom validation only for a certain form request. You can leverage laravel FormRequest and use dependency injection to extend the validation factory. I think this solution is much simpler than creating a service provider.

Here is how it can be done.

use Illuminate\Validation\Factory as ValidationFactory;

class UpdateMyUserRequest extends FormRequest {

    public function __construct(ValidationFactory $validationFactory)
    {

        $validationFactory->extend(
            'foo',
            function ($attribute, $value, $parameters) {
                return 'foo' === $value;
            },
            'Sorry, it failed foo validation!'
        );

    }

    public function rules()
    {
        return [
            'username' => 'foo',
        ];
    }
}