Replace underscore (_) with dashes (-) in URL

[eluser]livewirerules[/eluser]
i have been going through the website and googling for a working solution to replace all the _ with - for the urls. Below is a solution which i found, but doesn't work.

Code:

<?php
class MY_Router extends CI_Router
{
    function _set_request($segments = array()) {
        parent::_set_request(str_replace('-', '_', $segments));
    }
}
?>

i added to the core folder but nothing happens to the url. Can someone suggest be a good solution where i can replace the underscores. i don't want to use the routing feature because i have over 300 urls.

Any help will be appreciated.