function improved_trim_excerpt($text) {
global $post;
if ( '' == $text ) {
$text = get_the_content('');
$text = apply_filters('the_content', $text);
$text = str_replace(']]>', ']]>', $text);
$text = preg_replace('@<script[^>]*?>.*?</script>@si', '', $text);
$text = strip_tags($text, '<p>');
$excerpt_length = 80;
$words = explode(' ', $text, $excerpt_length + 1);
if (count($words)> $excerpt_length) {
array_pop($words);
array_push($words, '[...]');
$text = implode(' ', $words);
}
}
return $text;
}

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

http://www.milienzo.com/2007/09/02/improving-wordpress-the_excerpt-template-tag/

Improving WordPress’ the_excerpt() template tag

template, wordpress, replace, text

PHP