CodeIgniter uses index.php
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/system.*
RewriteRule ^(.*)$ index.php?/$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)$ index.php?/$1 [L]

$config['index_page'] = "";

public function index()
    {
        $this->load->helper(array('form', 'url'));
        $this->load->model('Registration_Model');
        $this->load->library('form_validation');
        $this->form_validation->set_error_delimiters('<div class="error">', '</div>');

        $this->form_validation->set_rules('username', 'Username', 'trim|required|min_length[5]|max_length[12]|callback_username_check|xss_clean');
        $this->form_validation->set_rules('password', 'Password', 'trim|required|min_length[6]|max_length[12]|matches[passconf]');
        $this->form_validation->set_rules('passconf', 'Password Confirmation', 'trim|required');
        $this->form_validation->set_rules('email', 'Email', 'trim|required|valid_email|callback_email_check');

        if ($this->form_validation->run() == FALSE)
        {
            $this->load->view('registration');
        }
        else
        {
            //input data into database here
            $username = strtolower($_POST['username']);
            $password = md5($_POST['password']);
            $email = strtolower($_POST['email']);

            $data = array
                    (
                        'username' => $username,
                        'password' => $password,
                        'email' => $email
                    );

            $this->Registration_Model->addUser($data);
            $this->load->view('registrationsuccess');
        }
    }

RewriteEngine on

RewriteCond $1 !^(index.php|robots.txt|images|css|js|user_guide)
RewriteRule ^(.*)$ /index.php/$1 [L]