respect-validation/docs/custom-validators.md

Custom validators

You can also create and use your own validators. To do this, you will need to create a validator and an exception to go with the validator.

To create a validator, you need to create a class that implements the Validator interface and is within the Validators namespace. It is convenient to just extend the Simple or Standard class. When the validator is called the logic inside the validate method will be executed. Here's how the class should look:

namespace My\Validation\Validators;

use Respect\Validation\Message\Template;
use Respect\Validation\Validators\Core\Simple;

#[Template(
    '{{subject}} is something',
    '{{subject}} is not something',
)]
final class Something extends Simple
{
    protected function isValid(mixed $input): bool
    {
        // Do something here with the $input and return a boolean value
    }
}

The '{{subject}} is not something message would be used then you call the validator with the not().

All classes in Validation are created by the Factory class. If you want Validation to execute your validator (or validators) in the chain, you must overwrite the default Factory.

Factory::setDefaultInstance(
    (new Factory())
        ->withNamespace('My\\Validation\\Validators')
);
v::something(); // Try to load "My\Validation\Validators\Something" if any