<?php

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;

class StoreFolderRequest extends FormRequest
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
            'name' => 'required|alpha_num',
            'parent_id' => 'exists:folders,id',
        ];
    }
}

<?php

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;

class StoreFolderRequest extends FormRequest
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
            'name' => 'required|alpha',
            'parent_id' => 'exists:folders,id',
        ];
    }
}

--- 
+++ 
@@ -24,7 +24,7 @@
     public function rules()
     {
         return [
-            'name' => 'required|alpha',
+            'name' => 'required|alpha_num',
             'parent_id' => 'exists:folders,id',
         ];
     }
Allow folders with alpha-numerical name (no dash, no space)
