Laravel中表单验证可以用FormRequest来进行，但是这个默认是返回视图文件的，如果想要把验证结果以JSON格式返回，可以使用如下代码（failedValidation函数）：

<?php

namespace App\Http\Requests;

use Illuminate\Contracts\Validation\Validator;
use Illuminate\Foundation\Http\FormRequest;
use Illuminate\Validation\ValidationException;
use Symfony\Component\HttpFoundation\Response;

class TagStoreRequest 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<string, mixed>
     */
    public function rules(): array
    {
        return [
            'tag' => 'required|unique:tags',
        ];
    }

    public function messages(): array
    {
        return [
            'tag.required'=> '标签名称不能为空！',
            'rag.unique' => '该标签已存在！',
        ];
    }

    protected function failedValidation(Validator $validator)
    {
        throw new ValidationException(
            $validator,
            response()->json(
                [
                    'code'=>-1,
                    'msg'=> $validator->errors()->first()
                ],
                Response::HTTP_UNPROCESSABLE_ENTITY
            )
        );       
    }
}