# Validators

****

Custom validators are registered with the `validators` extension. The validators extension accepts an array of validators where the key is the name of the validator and the value a callback function that receives the value as parameter and returns a boolean. You can find the list of built-in validators in the <a href="https://getkirby.com/docs/reference/system/validators">Validators reference</a>.

```php "/site/plugins/your-plugin/index.php"
Kirby::plugin('your/plugin', [
  'validators' => [
    'isGreen' => function ($value) {
      return V::in($value, ['green','lightgreen', 'darkgreen', 'mediumgreen']);
    }
  ]
]);
```

In your templates, you can now check if the value is in the given array of green values:

```php "/site/templates/default.php"
$input = 'lightgreen';
if (V::isGreen($input)) {
  echo 'Yay, valid!';
} else {
  echo 'Oh no, not valid';
}
```