wpmu_validate_user_signup( string $user_name, string $user_email )

function wpmu_validate_user_signup($user_name, $user_email) {
	global $wpdb;

	$errors = new WP_Error();

	$orig_username = $user_name;
	$user_name = preg_replace( '/\s+/', '', sanitize_user( $user_name, true ) );

	if ( $user_name != $orig_username || preg_match( '/[^a-z0-9]/', $user_name ) ) {
		$errors->add( 'user_name', __( 'Usernames can only contain lowercase letters (a-z) and numbers.' ) );
		$user_name = $orig_username;
	}

	$user_email = sanitize_email( $user_email );

	if ( empty( $user_name ) )
	   	$errors->add('user_name', __( 'Please enter a username.' ) );

	$illegal_names = get_site_option( 'illegal_names' );
	if ( ! is_array( $illegal_names ) ) {
		$illegal_names = array(  'www', 'web', 'root', 'admin', 'main', 'invite', 'administrator' );
		add_site_option( 'illegal_names', $illegal_names );
	}
	if ( in_array( $user_name, $illegal_names ) ) {
		$errors->add( 'user_name',  __( 'Sorry, that username is not allowed.' ) );
	}

	/** This filter is documented in wp-includes/user.php */
	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );

	if ( in_array( strtolower( $user_name ), array_map( 'strtolower', $illegal_logins ) ) ) {
		$errors->add( 'user_name',  __( 'Sorry, that username is not allowed.' ) );
	}

	if ( ! is_email( $user_email ) ) {
		$errors->add( 'user_email', __( 'Please enter a valid email address.' ) );
	} elseif ( is_email_address_unsafe( $user_email ) ) {
		$errors->add( 'user_email', __( 'You cannot use that email address to signup. We are having problems with them blocking some of our email. Please use another email provider.' ) );
	}

	if ( strlen( $user_name ) < 4 )
		$errors->add('user_name',  __( 'Username must be at least 4 characters.' ) );

	if ( strlen( $user_name ) > 60 ) {
		$errors->add( 'user_name', __( 'Username may not be longer than 60 characters.' ) );
	}

	// all numeric?
	if ( preg_match( '/^[0-9]*$/', $user_name ) )
		$errors->add('user_name', __('Sorry, usernames must have letters too!'));

	$limited_email_domains = get_site_option( 'limited_email_domains' );
	if ( is_array( $limited_email_domains ) && ! empty( $limited_email_domains ) ) {
		$emaildomain = substr( $user_email, 1 + strpos( $user_email, '@' ) );
		if ( ! in_array( $emaildomain, $limited_email_domains ) ) {
			$errors->add('user_email', __('Sorry, that email address is not allowed!'));
		}
	}

	// Check if the username has been used already.
	if ( username_exists($user_name) )
		$errors->add( 'user_name', __( 'Sorry, that username already exists!' ) );

	// Check if the email address has been used already.
	if ( email_exists($user_email) )
		$errors->add( 'user_email', __( 'Sorry, that email address is already used!' ) );

	// Has someone already signed up for this username?
	$signup = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->signups WHERE user_login = %s", $user_name) );
	if ( $signup != null ) {
		$registered_at =  mysql2date('U', $signup->registered);
		$now = current_time( 'timestamp', true );
		$diff = $now - $registered_at;
		// If registered more than two days ago, cancel registration and let this signup go through.
		if ( $diff > 2 * DAY_IN_SECONDS )
			$wpdb->delete( $wpdb->signups, array( 'user_login' => $user_name ) );
		else
			$errors->add('user_name', __('That username is currently reserved but may be available in a couple of days.'));
	}

	$signup = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->signups WHERE user_email = %s", $user_email) );
	if ( $signup != null ) {
		$diff = current_time( 'timestamp', true ) - mysql2date('U', $signup->registered);
		// If registered more than two days ago, cancel registration and let this signup go through.
		if ( $diff > 2 * DAY_IN_SECONDS )
			$wpdb->delete( $wpdb->signups, array( 'user_email' => $user_email ) );
		else
			$errors->add('user_email', __('That email address has already been used. Please check your inbox for an activation email. It will become available in a couple of days if you do nothing.'));
	}

	$result = array('user_name' => $user_name, 'orig_username' => $orig_username, 'user_email' => $user_email, 'errors' => $errors);

	/**
	 * Filters the validated user registration details.
	 *
	 * This does not allow you to override the username or email of the user during
	 * registration. The values are solely used for validation and error handling.
	 *
	 * @since WP-MU (3.0.0)
	 *
	 * @param array $result {
	 *     The array of user name, email and the error messages.
	 *
	 *     @type string   $user_name     Sanitized and unique username.
	 *     @type string   $orig_username Original username.
	 *     @type string   $user_email    User email address.
	 *     @type WP_Error $errors        WP_Error object containing any errors found.
	 * }
	 */
	return apply_filters( 'wpmu_validate_user_signup', $result );
}