Wordpress - How to end the excerpt with a sentence rather than a word?

This requires PHP 5.3+ (WP requires PHP 5.2.4+)

add_filter('get_the_excerpt', 'end_with_sentence');

function end_with_sentence($excerpt) {
  $allowed_end = array('.', '!', '?', '...');
  $exc = explode( ' ', $excerpt );
  $found = false;
  $last = '';
  while ( ! $found && ! empty($exc) ) { 
    $last = array_pop($exc);
    $end = strrev( $last );
    $found = in_array( $end{0}, $allowed_end );
  }
  return (! empty($exc)) ? $excerpt : rtrim(implode(' ', $exc) . ' ' .$last);
}

Edit

After @kaiser comment I try to run this on save/update to prevent page slow down on showing a lot of posts. This should be tested

add_filter('wp_insert_post_data', 'end_with_sentence_on_save', 20, 2);

function end_with_sentence_on_save($data, $postarr) {
  if ( ! empty( $data['post_content'] ) && $data['post_status'] != 'inherit' && $data['post_status'] != 'trash' ) {
    $text = strip_shortcodes( $data['post_content'] );
    $text = apply_filters('the_content', $text );
    $text = str_replace(']]>', ']]>', $text );
    $excerpt_length = apply_filters('excerpt_length', 55);
    $data['post_excerpt'] = wp_trim_words($text, $excerpt_length, '');
  } else {
    return $data;
  }
  $allowed_end = array('.', '!', '?', '...');
  $exc = explode(' ', $data['post_excerpt']);
  $found = false;
  $last = '';
  while ( ! $found && ! empty($exc) ) { 
    $last = array_pop($exc);
    $end = strrev( $last );
    $found = in_array( $end{0}, $allowed_end );
  }
  if (! empty($exc)) $data['post_excerpt'] = rtrim(implode(' ', $exc) . ' ' .$last);
  return $data; 
}

Here is my version, keeping all html tags and also trimming the content after the last word in of a sentence

if ( ! function_exists( 'pietergoosen_custom_wp_trim_excerpt' ) ) : 

    function pietergoosen_custom_wp_trim_excerpt($pietergoosen_excerpt) {
    global $post;
    $raw_excerpt = $pietergoosen_excerpt;
        if ( '' == $pietergoosen_excerpt ) {

            $pietergoosen_excerpt = get_the_content('');
            $pietergoosen_excerpt = strip_shortcodes( $pietergoosen_excerpt );
            $pietergoosen_excerpt = apply_filters('the_content', $pietergoosen_excerpt);
            $pietergoosen_excerpt = str_replace(']]>', ']]>', $pietergoosen_excerpt);

            //Set the excerpt word count and only break after sentence is complete.
                $excerpt_word_count = 75;
                $excerpt_length = apply_filters('excerpt_length', $excerpt_word_count); 
                $tokens = array();
                $excerptOutput = '';
                $count = 0;

                // Divide the string into tokens; HTML tags, or words, followed by any whitespace
                preg_match_all('/(<[^>]+>|[^<>\s]+)\s*/u', $pietergoosen_excerpt, $tokens);

                foreach ($tokens[0] as $token) { 

                    if ($count >= $excerpt_word_count && preg_match('/[\?\.\!]\s*$/uS', $token)) { 
                    // Limit reached, continue until  ? . or ! occur at the end
                        $excerptOutput .= trim($token);
                        break;
                    }

                    // Add words to complete sentence
                    $count++;

                    // Append what's left of the token
                    $excerptOutput .= $token;
                }

            $pietergoosen_excerpt = trim(force_balance_tags($excerptOutput));

                $excerpt_end = ' <a href="'. esc_url( get_permalink() ) . '">' . '&nbsp;&raquo;&nbsp;' . sprintf(__( 'Read more about: %s &nbsp;&raquo;', 'pietergoosen' ), get_the_title()) . '</a>'; 
                $excerpt_more = apply_filters('excerpt_more', ' ' . $excerpt_end); 

                //$pos = strrpos($pietergoosen_excerpt, '</');
                //if ($pos !== false)
                // Inside last HTML tag
                //$pietergoosen_excerpt = substr_replace($pietergoosen_excerpt, $excerpt_end, $pos, 0);
                //else
                // After the content
                $pietergoosen_excerpt .= $excerpt_end;

            return $pietergoosen_excerpt;   

        }
        return apply_filters('pietergoosen_custom_wp_trim_excerpt', $pietergoosen_excerpt, $raw_excerpt);
    }

endif; 

remove_filter('get_the_excerpt', 'wp_trim_excerpt');
add_filter('get_the_excerpt', 'pietergoosen_custom_wp_trim_excerpt');