Laravel conditional validation

ChartController.php

<?php

namespace App\Http\Controllers\Admin;

use Illuminate\Http\Request;
use Illuminate\Support\Carbon;
use App\Http\Controllers\Controller;
use Validator;
use Illuminate\Support\Fluent;

use App\Registration;

class ChartController extends Controller
{
    public function index(Request $request)
    {
        if ($request->isMethod('post')) {

            $validator = Validator::make(
                $request->all(),
                [
                    'periodicity' => 'required', // string: day|week|month
                    'filter' => 'required', // string: date|month number|week number
                ]
            );

            $validator->sometimes('filter', 'integer|between:1,52', function ($input) {
                return $input->periodicity === 'week';
            });

            $validator->sometimes('filter', 'integer|between:1,12', function ($input) {
                return $input->periodicity === 'month';
            });

            if ($validator->fails()) {
                return redirect()->back()->withErrors($validator)->withInput();
            }

        }

        return view('charts/admin/index');
    }
}