Micro-Plugin to improve auto excerpts generated from the_content. Can be placed in plugins, integrated in other projects, or called with an include statement in functions.php

better_auto_excerpts.php

<?php if(!defined('ABSPATH')) { die(); } // Include in all php files, to prevent direct execution
/**
 * Plugin Name: Better Auto Excerpts
 * Plugin URI:  
 * Description: replaces block-level elements with whitespace, to improve the_excerpt
 * Version:     1.0.0
 * Author:      Burlington Bytes
 * Author URI:  https://www.burlingtonbytes.com
 * Text Domain: better-auto-excerpts
 * License:     GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */
if( !class_exists('BBytesBetterExcerpts') ) {
	class BBytesBetterExcerpts {
		private static $_this;

		public static function Instance() {
			static $instance = null;
			if ($instance === null) {
				$instance = new self();
			}
			return $instance;
		}

		private function __construct() {
			// ~PHP_INT_MAX (aka PHP_INT_MIN) as priority ensures our filter runs first
			add_filter( 'wp_trim_words', array( $this, 'wp_trim_words' ), ~PHP_INT_MAX, 4 );
		}
		// Public Functions
		public function wp_trim_words( $filtered_text, $num_words = 55, $more = null, $text ) {
			if ( null === $more ) {
				$more = __( '&hellip;' );
			}
			
			$original_text = $text;
			// THIS IS THE ONLY NOVEL LINE OF CODE IN THIS FUNCTION
			$text = $this->strip_all_tags( $text );
			
			if ( strpos( _x( 'words', 'Word count type. Do not translate!' ), 'characters' ) === 0 && preg_match( '/^utf\-?8$/i', get_option( 'blog_charset' ) ) ) {
				$text = trim( preg_replace( "/[\n\r\t ]+/", ' ', $text ), ' ' );
				preg_match_all( '/./u', $text, $words_array );
				$words_array = array_slice( $words_array[0], 0, $num_words + 1 );
				$sep = '';
			} else {
				$words_array = preg_split( "/[\n\r\t ]+/", $text, $num_words + 1, PREG_SPLIT_NO_EMPTY );
				$sep = ' ';
			}
			
			if ( count( $words_array ) > $num_words ) {
				array_pop( $words_array );
				$text = implode( $sep, $words_array );
				$text = $text . $more;
			} else {
				$text = implode( $sep, $words_array );
			}
			return $text;
		}
		
		// Private Functions
		private function strip_all_tags( $string, $remove_breaks = false ) {
			$string = preg_replace( '@<(script|style)[^>]*?>.*?</\\1>@si', '', $string );
			// THIS IS THE ONLY NOVEL LINE OF CODE IN THIS FUNCTION
			$string = $this->prepend_space_to_block_elements( $string );
			$string = strip_tags($string);
			
			if ( $remove_breaks )
				$string = preg_replace('/[\r\n\t ]+/', ' ', $string);

			return trim( $string );
		}
		// This is the actual meat and potatos
		private function prepend_space_to_block_elements( $string ) {
			$block_elements = array(
				"address","article","aside","blockquote",
				"br", "canvas","dd","div","dl","fieldset",
				"figcaption","figure","footer","form",
				"h[1-6]","header","hgroup","hr","li","main",
				"nav","noscript","ol","output","p","pre",
				"section","table","tfoot","ul","video"
			);
			$block_elements = apply_filters( 'bbytes_block_elements', $block_elements );
			$block_elements = implode( "|", $block_elements );
			$pattern = "/(?:\s*<\/?(?:" . $block_elements . ")[^>]*>\s*)+/i";
			$string = preg_replace( $pattern, "\n \n$0", $string );
			
			return $string;
		}
  }
  BBytesBetterExcerpts::Instance();
}