wp_trim_excerpt()

/**
 * Generates an excerpt from the content, if needed.
 *
 * The excerpt word amount will be 55 words and if the amount is greater than
 * that, then the string ' [...]' will be appended to the excerpt. If the string
 * is less than 55 words, then the content will be returned as is.
 *
 * The 55 word limit can be modified by plugins/themes using the excerpt_length filter
 * The ' [...]' string can be modified by plugins/themes using the excerpt_more filter
 *
 * @since 1.5.0
 *
 * @param string $text The excerpt. If set to empty an excerpt is generated.
 * @return string The excerpt.
 */
function wp_trim_excerpt($text) {
    $raw_excerpt = $text;
    if ( '' == $text ) {
        $text = get_the_content('');

        $text = strip_shortcodes( $text );

        $text = apply_filters('the_content', $text);
        $text = str_replace(']]>', ']]>', $text);
        $text = strip_tags($text);
        $excerpt_length = apply_filters('excerpt_length', 55);
        $excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
        $words = preg_split("/[\n\r\t ]+/", $text, $excerpt_length + 1, PREG_SPLIT_NO_EMPTY);
        if ( count($words) > $excerpt_length ) {
            array_pop($words);
            $text = implode(' ', $words);
            $text = $text . $excerpt_more;
        } else {
            $text = implode(' ', $words);
        }
    }
    return apply_filters('wp_trim_excerpt', $text, $raw_excerpt);
}