CodeIgniter Disallowed Key Characters

The problem is you are using characters not included in the standard Regex. Use this:

!preg_match("/^[a-z0-9\x{4e00}-\x{9fa5}\:\;\.\,\?\!\@\#\$%\^\*\"\~\'+=\\\ &_\/\.\[\]-\}\{]+$/iu", $str)

As per the comments (and personal experience) you should not modify they Input.php file — rather, you should create/use your own MY_Input.php as follows:

<?php

class MY_Input extends CI_Input {

    /**
     * Clean Keys
     *
     * This is a helper function. To prevent malicious users
     * from trying to exploit keys we make sure that keys are
     * only named with alpha-numeric text and a few other items.
     * 
     * Extended to allow: 
     *      - '.' (dot), 
     *      - '[' (open bracket),
     *      - ']' (close bracket)
     * 
     * @access  private
     * @param   string
     * @return  string
     */
    function _clean_input_keys($str) {
        // UPDATE: Now includes comprehensive Regex that can process escaped JSON
        if (!preg_match("/^[a-z0-9\:\;\.\,\?\!\@\#\$%\^\*\"\~\'+=\\\ &_\/\.\[\]-\}\{]+$/iu", $str)) {
            /**
             * Check for Development enviroment - Non-descriptive 
             * error so show me the string that caused the problem 
             */
            if (getenv('ENVIRONMENT') && getenv('ENVIRONMENT') == 'DEVELOPMENT') {
                var_dump($str);
            }
            exit('Disallowed Key Characters.');
        }

        // Clean UTF-8 if supported
        if (UTF8_ENABLED === TRUE) {
            $str = $this->uni->clean_string($str);
        }

        return $str;
    }

}

// /?/> /* Should never close php file - if you have a space after code, it can mess your life up */

---

++Chinese Character Support

// NOTE: \x{4e00}-\x{9fa5} = allow chinese characters
// NOTE: 'i' — case insensitive
// NOTE: 'u' — UTF-8 mode
if (!preg_match("/^[a-z0-9\x{4e00}-\x{9fa5}\:\;\.\,\?\!\@\#\$%\^\*\"\~\'+=\\\ &_\/\.\[\]-\}\{]+$/iu", $str) { ... }

// NOTE: When Chinese characters are provided in a URL, they are not 'really' there; the browser/OS
//   handles the copy/paste -> unicode conversion, eg:
//        一二三  -->  xn--4gqsa60b   
//   'punycode' converts these codes according to RFC 3492 and RFC 5891.
//   https://github.com/bestiejs/punycode.js ---  $ bower install punycode

Open libraries/Input.php (system/core/Input.php in CI version 2.0+) and locate function _clean_input_keys($str){, The whole block should look like so:

function _clean_input_keys($str)
{
    if ( ! preg_match("/^[a-z0-9:_\/-]+$/i", $str))
    {
        exit('Disallowed Key Characters.');
    }

    return $str;
}

Modify the PCRE sot that it allows the new chars.

Please not that the char thats missing is the .(dot) and you should always escape the .(dot) in Regular Expressions as they will otherwise allow any single char.

/^[a-z0-9:_\/-\.]+$/i

To use CodeIgniter with jQuery Ajax, use "Object" as data instead of Query string as below:

$.ajax({
    url: site_url + "ajax/signup",
    data: ({'email': email, 'password': password}), //<--- Use Object
    type: "post",
    success: function(response, textStatus, jqXHR){
        $('#sign-up').html(response);
    },
    error: function(jqXHR, textStatus, errorThrown){
        console.log("The following error occured: "+
                    textStatus, errorThrown);
    }
});