I found a function online which appears similar to what you are looking for and tested it by creating two span classes instead of the single span from their example.

Then I played around with CSS for the two classes. You can test it, too if you want.

function replace_content($content) {
$content = str_replace('acmeproduct', 
           '<span class="acme-style">acme</span>
            <span class="product-style">product</span>'
           ,$content);
return $content;
}
add_filter('the_content','replace_content');

span.acme-style {
   display: inline;
   font-style: italic;
   text-transform: lowercase;
}

span.product-style {
   display: inline;
   font-style: normal;
   text-transform: capitalize;
}