codeigniter 1.7 on PHP5.3

[eluser]nikkofu[/eluser]
and this is MY_URI.php, it works fine at PHP5.2.3-5


<?php

class MY_URI extends CI_URI {
function _filter_uri($str)
{
if ($str != '' AND $this->config->item('permitted_uri_chars') != '')
{
//echo $str . '<br>';//here is my debug code
$str = urlencode($str);
if ( ! preg_match("|^[".preg_quote($this->config->item('permitted_uri_chars'))."]+$|i", $str))
{
exit('The URI you submitted has disallowed characters.');
}
$str = urldecode($str);
}
return $str;
}

}

when i goes to http://domain/welcome/index with PHP5.2.3-5, the original debug line shows:
welcome<br>index<br>

and i goes to http://domain/welcom/index again in PHP5.3.0, the new debug line shows:
welcome<br>

it breaks the URI and lost the "index"

please help!

thanks again!