I found a solution for this, apparently we need to use Validator::extend() to make an alias for our Validation Class.

in AppServiceProvider or another provider you have

 Validator::extend('month_year', function ($attribute, $value, $parameters, $validator) {
            $rule = new MonthYearValue;
            // Optionally inject data / validator if needed:
            $rule->setData($validator->getData());
            $rule->setValidator($validator);

            $failed = false;
            $rule->validate($attribute, $value, function ($attr, $message = null) use (&$failed, $validator) {
                $failed = true;
                // Add message; we can translate or store code
                $validator->errors()->add($attr, $message ?? 'Invalid month/year.');
            });

            return ! $failed;
        }, 'Format bulan dan tahun tidak valid. Gunakan MM/YYYY.');

with that, now we can use the alias in the validation method.

$request->validate([
'data.formalEducations.*.start_month_and_year' => ['required', 'month_year'],
],[
'data.formalEducations.*.start_month_and_year.month_year' => 'Now this custom error message will displayed if exist',
]

I found a solution for this, apparently we need to use Validator::extend() to make an alias for our Validation Class.

in AppServiceProvider or another provider you have

 Validator::extend('month_year', function ($attribute, $value, $parameters, $validator) {
            $rule = new MonthYearValue;
            // Optionally inject data / validator if needed:
            $rule->setData($validator->getData());
            $rule->setValidator($validator);

            $failed = false;
            $rule->validate($attribute, $value, function ($attr, $message = null) use (&$failed, $validator) {
                $failed = true;
                // Add message; we can translate or store code
                $validator->errors()->add($attr, $message ?? 'Invalid month/year.');
            });

            return ! $failed;
        }, 'Format bulan dan tahun tidak valid. Gunakan MM/YYYY.');

with that, now we can use the alias in the validation method.

$request->validate([
'data.formalEducations.*.start_month_and_year' => ['required', 'month_year'],
],[
'data.formalEducations.*.start_month_and_year.month_year' => 'Now this custom error message will displayed if exist',
]

well I don't think thats necessary because this is not related to what I meant. but to clarify the example invalid data is when user input 29/2020 because it should be in format MM/YYYY so the valid data is like 12/2020 or 08/2024 etc

I create a custom validation using Class that implements ValidationRule. it works perfectly, the problem is I can't use custom error message just like other laravel native rule.

<?php

namespace App\Rules;

use Closure;
use Illuminate\Contracts\Validation\ValidationRule;
use Illuminate\Validation\Validator;
use Illuminate\Contracts\Validation\ValidatorAwareRule;
use Illuminate\Contracts\Validation\DataAwareRule;

class MonthYearValue implements ValidationRule, ValidatorAwareRule,DataAwareRule
{
    /**
     * The validator instance.
     *
     * @var \Illuminate\Validation\Validator
     */
    protected $validator;

      /**
     * All of the data under validation.
     *
     * @var array<string, mixed>
     */
    protected $data = [];


    /**
     * Set the current validator.
     */
    public function setValidator(Validator $validator): static
    {
        $this->validator = $validator;

        return $this;
    }

    public function setData(array $data): static
    {
         $this->data = $data;
        return $this;
    }

    public function validate(string $attribute, mixed $value, Closure $fail): void
    {

        if (! is_string($value)) {
            $fail("Format $attribute tidak valid.");
            
            return;
        }
        
        $value = trim($value);
        
        if (! preg_match('/^(0[1-9]|1[0-2])\/\d{4}$/', $value)) {
            $fail('validation.month_year')->translate([
                'attribute' => $attribute,
            ]);
        }
    }
    
}

In example above, I can display the message via validation translate file

<?php

return [
    'month_year' => 'The :attribute must be a valid month/year (MM/YYYY).',

];

But, what exactly my goal is to use custom error message if provided.

Here is what work for now,

'data.formalEducations.*.start_month_and_year.App\Rules\MonthYearValue' => 'Bulan dan tahun mulai tidak valid. Gunakan format MM/YYYY.',

that work because I specify namespace to my Class, but then I can't display correct message based on condition. for example I want it to display based on some rule

'data.formalEducations.*.start_month_and_year.App\Rules\MonthYearValue.invalid' => 'Only show this message when invalid.',

for now, I don't have idea how to check that invalid to show correct error message value.

@jegramos

'rules.Illuminate\Validation\Rules\Enum' => 'Valid role values are regular, admin, super-admin.'