user_registration_control.module

<?php

/**
 * This module provides a simple way to provide advanced control over
 * what user e-mail domains are allowed for registration.
 *
 * The development of this module is sponsored by Astonish Design.
 *
 * @author Brandon Smith <brandon@astonishdesign.com>
 */

/**
 * Implements hook_help().
 * @return help documentation
 */
function user_registration_control_help($path, $arg) {
  if ($path == 'admin/help#user_registration_control') {
    /**
     * @todo Add documentation.
     */
    $output = t('<p><strong><em>User Registration Control</em> provides a simple way to implement advanced control
       over what user e-mail domains are allowed for registration.</strong></p>
       <p>This module functions by applying additional required validation to the e-mail field in the
       form of a regular expression.  It can be used for a variety of reasons, including (but not limited to)
       corporate or academic internet sites, and social media networks.</p>
       <p>There are currently two levels of control available:
       <ul><li>Basic (single domain restriction)</li><li>Advanced (Custom Regular Expression)</li></ul></p>
       <h3>Basic Options</h3>
       <p>Basic settings allow registration restriction to users with a valid e-mail address for a specific
       domain (or subdomain) suffix.  For example, by specifying example.com in the settings, only
       visitors with an @example.com e-mail address will be allowed to register.  Similarly, if
       subdomain.example.com is specified, only visitors with an @subdomain.example.com address can register.</p>
       <h3>Advanced Options</h3>
       <p>Advanced settings allow complete control over the entire address to be validated using a
       custom regular expression (PCRE syntax).  By using this option, the administrator has registration
       control over both the prefix and suffix, as well as multiple variants, in e-mail address validation.</p>
       <h3>Error Message</h3>
       <p>An optional setting allows the display of a custom error message in the case of a registration attempt
       with an e-mail address that is not allowed.</p>');

    return $output;
  }
}

/**
 * Implements hook_menu().
 * @return array $items
 */
function user_registration_control_menu() {
  $items = array();
  $items['admin/config/people/user_registration_control'] = array(
    'title' => 'User Registration Control',
    'description' => 'Configure e-mail restriction settings for user registration.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('user_registration_control_admin_settings'),
    'file' => 'user_registration_control.admin.inc',
    'access arguments' => array('administer site configuration'),
    'weight' => 0,
  );

  return $items;
}

/**
 * Implements hook_form_user_register_form_alter().
 * @param array $form
 * @param type $form_state
 */
function user_registration_control_form_user_register_form_alter(&$form, &$form_state) {
  $form['#validate'][] = 'user_registration_control_user_register_validate';
}

/**
 * Implements hook_user_register_validate().
 * @param type $form
 * @param type $form_state
 */
function user_registration_control_user_register_validate($form, &$form_state) {
  $config_level = variable_get('registration_email_configuration_level');
  if ($config_level == 'basic' || $config_level == 'advanced') {
    if ($config_level == 'basic') {
      $domain = str_replace('.', '\.', variable_get('registration_email_allowed_domain'));
      $expression = '/('.$domain.')$/';
    } else if ($config_level == 'advanced') {
      $expression = variable_get('registration_email_advanced_expression');
    }
    $valid_email = false;
    $submitted_email = $form_state['values']['mail'];

    // Check if valid e-mail address.
    if (preg_match($expression, $submitted_email)) {
      $valid_email = true;
    }

    // If the email address is not valid, set error message.
    if (!$valid_email) {
      form_set_error('mail', t('The e-mail address %email is not valid for this site.  %message', array('%email' => $form_state['values']['mail'], '%message' => variable_get('registration_email_error_message'))));
    }
  }
}