function bp_core_validate_user_signup( $user_name, $user_email ) {

	// Make sure illegal names include BuddyPress slugs and values.
	bp_core_flush_illegal_names();

	// WordPress Multisite has its own validation. Use it, so that we
	// properly mirror restrictions on username, etc.
	if ( function_exists( 'wpmu_validate_user_signup' ) ) {
		$result = wpmu_validate_user_signup( $user_name, $user_email );

	// When not running Multisite, we perform our own validation. What
	// follows reproduces much of the logic of wpmu_validate_user_signup(),
	// minus the multisite-specific restrictions on user_login.
	} else {
		$errors = new WP_Error();

		/**
		 * Filters the username before being validated.
		 *
		 * @since 1.5.5
		 *
		 * @param string $user_name Username to validate.
		 */
		$user_name = apply_filters( 'pre_user_login', $user_name );

		// User name can't be empty.
		if ( empty( $user_name ) ) {
			$errors->add( 'user_name', __( 'Please enter a username', 'buddypress' ) );
		}

		// User name can't be on the blacklist.
		$illegal_names = get_site_option( 'illegal_names' );
		if ( in_array( $user_name, (array) $illegal_names ) ) {
			$errors->add( 'user_name', __( 'That username is not allowed', 'buddypress' ) );
		}

		// User name must pass WP's validity check.
		if ( ! validate_username( $user_name ) ) {
			$errors->add( 'user_name', __( 'Usernames can contain only letters, numbers, ., -, and @', 'buddypress' ) );
		}

		// Minimum of 4 characters.
		if ( strlen( $user_name ) < 4 ) {
			$errors->add( 'user_name',  __( 'Username must be at least 4 characters', 'buddypress' ) );
		}

		// No underscores. @todo Why not?
		if ( false !== strpos( ' ' . $user_name, '_' ) ) {
			$errors->add( 'user_name', __( 'Sorry, usernames may not contain the character "_"!', 'buddypress' ) );
		}

		// No usernames that are all numeric. @todo Why?
		$match = array();
		preg_match( '/[0-9]*/', $user_name, $match );
		if ( $match[0] == $user_name ) {
			$errors->add( 'user_name', __( 'Sorry, usernames must have letters too!', 'buddypress' ) );
		}

		// Check into signups.
		$signups = BP_Signup::get( array(
			'user_login' => $user_name,
		) );

		$signup = isset( $signups['signups'] ) && ! empty( $signups['signups'][0] ) ? $signups['signups'][0] : false;

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

		// Validate the email address and process the validation results into
		// error messages.
		$validate_email = bp_core_validate_email_address( $user_email );
		bp_core_add_validation_error_messages( $errors, $validate_email );

		// Assemble the return array.
		$result = array(
			'user_name'  => $user_name,
			'user_email' => $user_email,
			'errors'     => $errors,
		);

		// Apply WPMU legacy filter.
		$result = apply_filters( 'wpmu_validate_user_signup', $result );
	}

	/**
	 * Filters the result of the user signup validation.
	 *
	 * @since 1.2.2
	 *
	 * @param array $result Results of user validation including errors, if any.
	 */
	return apply_filters( 'bp_core_validate_user_signup', $result );
}