如何验证Laravel中的前三个字符？

<?php

namespace App\Rules;

use Illuminate\Contracts\Validation\Rule;

class StartsWith implements Rule
{
    protected $startsWith;

    public function __construct($startsWith)
    {
        $this->startsWith = $startsWith;
    }

    public function passes($attribute, $value)
    {
        return strpos($value, $this->startsWith) === 0;
    }

    public function message()
    {
        return 'The :attribute must start with ' . $this->startsWith;
    }
}

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Rules\StartsWith;

class YourController extends Controller
{
    public function validateString(Request $request)
    {
        $request->validate([
            'string' => ['required', new StartsWith('abc')],
        ]);
    }
}