WordPress: Filter to wrap teaser content with a div so we can style the teaser content

gistfile1.php

/**
 * If a page has the more-tag added then wrap the content before the more-tag in a div with the class
 * .entry-content-teaser
 * this makes it easy for us to style the teaser, like making in bold.
 * we hook onto prio 9 instead of 10 so it run before the content has got the tags added
 */
add_filter("the_content", "teasify_the_content", 9);
function teasify_the_content($content) {
	
	if (is_page()) {

		if ( preg_match('!<span id="more(.*?)?"></span>!', $content, $matches) ) {

			// more-tag-exists, so wrap that in div
			$arr_content = explode($matches[0], $content, 2); // [0] = teaser, [1] = content
			$content = "<div class='entry-content-teaser'>" . $arr_content[0] . "</div>" . $matches[0] . $arr_content[1];
			
		} else {
			// no more-tag, content should just remain content
		}				

	}
	
	return $content;
}