Adding Custom Form Validation

            

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/**
* Implements hook_form_alter() to add custom validation.
*/
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {
    if ($form_id == 'user_register_form') {
        // Add custom validation callback.
        $form['#validate'][] = 'my_module_custom_validate';
    }
}
// Now the validation function should be implemented.
function my_module_custom_validate(&$form, FormStateInterface $form_state, $form_id) {
 // Validation code. 
}