CodeIgniter 403 error instead of removing bad data - php

Question

CodeIgniter 2.1.4
After doing a little research about CodeIgniter's XSS protections, I decided to quickly and crudely test this by typing some random HTML into any input field on my CodeIgniter forms.
When I typed in <script>, the page is redirected to the server's default 403 error ("Forbidden") page. It's not even a CodeIgniter error page.
I'm very glad that any input data containing <script> is stopped, however, I'm not understanding why this is generating a 403 error page instead of a validation error, or at least pass the data with the offending parts stripped out.
I'm using htmlentities() to convert the < and > but this makes no difference.
It doesn't even matter if implement the form validation. The input data of <script> will generate a 403 error even without it.
Can anyone explain what's happening here and if I need to be worried out how this is being handled/redirected? To me it just seems like I should be getting some sort of CodeIgniter validation error or stripped down data rather than a 403 error.
Here is a concise version of one of my Controllers. (It's happening on all Controllers with data input fields.)
public function search($search_slug = NULL)
{
$this->load->library('form_validation');
$this->form_validation->set_rules('search-terms', 'Search Terms', 'xss_clean');
if ($this->input->post('search-terms') && ($this->form_validation->run() !== FALSE))
{
$search_slug = url_title(htmlentities($this->input->post('search-terms')), 'dash', FALSE);
}
if ($search_slug !== NULL)
{
$search_terms = preg_replace('/-/', ' ', $search_slug);
$query = // get my results from model;
if ($query['count'] > 0)
{
$data['results'] = $query['results'];
}
else
{
$data['results'] = '<h3>Sorry, nothing found.</h3>';
}
}
else
{
$data['results'] = '<h3>Please enter your search terms.</h3>';
}
$this->template->load('default', 'search', $data);
}

Answer 1

After reading the comments on my OP and digging into it deeper, this 403 redirect doesn't seem to have anything to with CodeIgniter after all.
Also, after installing ZenPhoto on the same account, I see the 403 redirect when entering <script> into the ZenPhoto search box too.
Since it's shared hosting, I cannot say, with 100% certainty, that this is the result of something in Apache protecting itself, but all evidence seems to point there.

Answer 2

This character is not allowed.
check accepted answer # SO - characters allowed in a url
Your request wasn't reaching index.php
Apart from that have a look at application/config/config.php
$config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-';
These are the characters your application allows when a request reaches it.