People searching “Laravel custom validation rule” want one check that required and max cannot express. The Vue page does not learn a new error shape. It still reads form.errors.slug from the CRUD guide.
TL;DR
- A custom rule is a class with validate().
- The Vue page still prints form.errors for that field.
- Do not throw.
- Do not reach into the database from this class for a unique check.
The fix#
shell
php artisan make:rule SlugRulephp
use Closure;
use Illuminate\Contracts\Validation\ValidationRule;
class SlugRule implements ValidationRule
{
public function validate(string $attribute, mixed $value, Closure $fail): void
{
if (! is_string($value) || preg_match('/^[a-z0-9]+(?:-[a-z0-9]+)*$/', $value) !== 1) {
$fail('The slug may contain lowercase letters, numbers, and hyphens.');
}
}
}php
$validated = $request->validate([
'slug' => ['required', 'string', 'max:80', new SlugRule],
]);Call $fail() with a sentence. Do not throw. Throwing skips the error bag and the Vue page gets a 500 instead of form.errors.slug.
What to avoid#
Gotcha
Do not reach into the database from this class for a unique check. Rule::unique() already does that, and a query inside validate() is easy to forget when the field is optional.
FAQ#
How do I write a custom Laravel validation rule?
A custom rule is a class with validate(). The Vue page still prints form.errors for that field.
What goes wrong with Laravel custom validation rule?
Many developers want one check that required and max cannot express. Do not throw.
