Add description to WP menu output, avoids preg_replace

gistfile1.php

<?php
/**
 * Add description to menu.
 *
 * Adjust description location as needed.
 *
 * @since 1.0.0
 * @param string $item_output 
 * @param object $item
 * @return string
 */
function ja_add_menu_description( $item_output, $item ) {

	if ( !empty( $item->post_content ) ) {
		$new_output  = '<span class="description">' . $item->post_content . '</span>' . $item->title;
		$item_output = str_replace( $item->title, $new_output, $item_output );
	}
	return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'ja_add_menu_description', 10, 2 );