How to replace string on HTML output?

If WP Super Cache caching disabled, I can replace without problem.

But when caching enabled, no replace happen.

Below is my functions:

if (!is_admin()) {
        function letsdoreplace($buffer) {
            $toreplace = array(
                "giraffe",                                                                 // Regular CSS
                "rabbit",
            );
            $withtext = array(
                "giraffe and friends",                                                                    // Regular CSS
                "elephant and rabbit",
            );
            return (str_replace($toreplace, $withtext, $buffer));
        }
        ob_start("letsdoreplace");
};