ConfigFormExampleConfigForm.php

<?php
   
/**
 * @file
 * Contains \Drupal\configform_example\Form\ConfigFormExampleConfigForm.
 */
   
namespace Drupal\configform_example\Form;
   
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
   
class ConfigFormExampleConfigForm extends ConfigFormBase {
     
  /**
   * {@inheritdoc}.
   */
  public function getFormId() {
    return 'configform_example_form';
  }
     
  /**
   * {@inheritdoc}.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildForm($form, $form_state);
    $config = $this->config('configform_example.settings');
    $form['email'] = [
      '#type' => 'email',
      '#title' => $this->t('Your .com email address.'),
      '#default_value' => $config->get('email_address'),
    ];
    return $form;
  }
     
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    if (strpos($form_state->getValue('email'), '.com') === FALSE ) {
      $form_state->setErrorByName('email', $this->t('This is not a .com email address.'));
    }
  }
     
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $config = $this->config('configform_example.settings');
    $config->set('email_address', $form_state->getValue('email'));
    $config->save();
    return parent::submitForm($form, $form_state);
  }
 
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return ['configform_example.settings'];
  }
     
}