Highlight Search Results

I found this function on the net. I have placed this in my custom functions file separate from functions.php.

function highlight_search_term($text){
    if(is_search()){
		$keys = implode('|', explode(' ', get_search_query()));
		$text = preg_replace('/(' . $keys .')/iu', '<span class="search-term">\1</span>', $text);
	}
    return $text;
}
add_filter('the_excerpt', 'highlight_search_term');
add_filter('the_title', 'highlight_search_term');

It works fine except it alters the read more link. Hovering over this link shows:

http://www.mysite.com/%3span%20classs=

When it is removed the link is unaffected. What is wrong with it? I have used the following CSS with it:

.search-excerpt {
	background: yellow;
}