Adding custom validation in form altersfor Drupal 8 , 9 , 10 , and 11

In the world of Drupal form development, ensuring the accuracy and integrity of user-provided data is of utmost importance. By employing custom validation through form alter hooks, developers can enforce specific rules and constraints to control the quality of the data submitted in forms.

Understanding Custom Validation

Custom validation in Drupal forms allows you to add unique checks and rules that cannot be covered by default validations. This can include ensuring the uniqueness of user input, matching a specific format, or even interacting with external systems for validation purposes.

Advantages of Custom Validation

• Improved Data Integrity: Guarantees that the data collected through forms meets the expected format and constraints.
• Enhanced User Feedback: Provides immediate feedback, reducing erroneous submissions and guiding users in data entry.
• Flexibility: Allows forms to adapt to complex business logic that is specific to your application's needs.

Implementing Custom Validation

To demonstrate how custom validation can be added, we will modify the user registration form to validate that a provided phone number matches a specified format using a custom validation function.

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_user_register_form_alter(array &$form, \Drupal\Core\Form\FormStateInterface $form_state) {
    // Add a custom validation handler.
    $form['#validate'][] = 'mymodule_user_register_form_validate';
}

/**
 * Custom validation handler for the user registration form.
 *
 * @param array $form
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 */
function mymodule_user_register_form_validate(array $form, \Drupal\Core\Form\FormStateInterface $form_state) {
    // Retrieve the phone number field value.
    $phone_number = $form_state->getValue('phone_number');

    // Define a regex pattern for phone number validation.
    $pattern = '/^\+\d{1,3}-\d{3}-\d{3}-\d{4}$/';

    // Validate phone number against the pattern.
    if (!preg_match($pattern, $phone_number)) {
        // Set an error if the phone number is not valid.
        $form_state->setErrorByName('phone_number', t('The phone number must be in the format +123-456-789-1011.'));
    }
}

Explaining the Example

This example achieves the following:

• mymodule_form_user_register_form_alter() adds a custom validation handler to the user registration form by attaching mymodule_user_register_form_validate to the #validate array.
• mymodule_user_register_form_validate() defines a regular expression pattern to validate the phone number. If the provided number does not match, an error message is set using $form_state->setErrorByName().
• This results in a required phone format, guiding users toward correct input and ensuring reliable data collection.

Best Practices for Custom Validation

• Clear Messages: Make validation messages precise and user-friendly to aid user correction.
• Efficient Validation: Avoid overly complex logic in validation functions for optimum performance.
• Reusability: Consider extracting complex rules to helper functions or services if used across multiple forms.

Conclusion

Adding custom validation to forms in Drupal enhances the input accuracy and supports precise data management. By setting clear and appropriate constraints, you can greatly improve the interaction between your application and its users.

What's Next?

For our next lesson, we will explore how to add custom submit handlers via form alters. This powerful technique helps you implement bespoke submission logic, enabling complex integrations and data processing workflows within your Drupal site. Stay tuned!