Validating Forms - php

Question

I have a form that I submit with jQuery ajax and have it being sent to a controller function called submit to validate and do any other tasks I need to with the form data. I'm trying to find out why my form validation library isn't showing an error when the username doesn't contain only lowercase letters and numbers.
$this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean|strtolower');
POST Value after form submission:
username TestingUSER
EDIT:
As far as I know it gets to the php server side properly.
PHP:
public function submit()
{
$output_status = 'Notice';
$output_title = 'Not Processed';
$output_message = 'The request was unprocessed!';
$this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean|strtolower');
$this->form_validation->set_rules('password', 'Password', 'trim|required|xss_clean');
$this->form_validation->set_rules('remember', 'Remember Me', 'trim|xss_clean|integer');
if ($this->form_validation->run() == TRUE)
{
}
else
{
$output_status = 'Error';
$output_title = 'Form Not Validated';
$output_message = validation_errors();
}
echo json_encode(array('output_status' => $output_status, 'output_title' => $output_title, 'output_message' => $output_message));
}
EDIT 2:
Based off of Sheikh answer. I am getting a response back that says "Unable to access an error message corresponding to your field name." It does say Form Not Validated for the title so the message isn't working.
public function check_username($str)
{
if (preg_match('#[0-9]#', $str) && preg_match('#[a-z]#', $str))
{
return TRUE;
}
$this->form_validation->set_message('username', 'This is not have an accepted value!');
return FALSE;
}
EDIT 3:
What I'm wanting to do is have it report back that there there are validation errors but not the specific errors in the pnotify response. However I do want it to display the specific errors under the form elements.
jQuery Code:
http://pastebin.com/1KehMJkh
Login Form:
http://pastebin.com/EfpBfbfN

Answer 1

I think you can use a callback function in your controller
public function check_username($str)
{
if (preg_match('#[a-z0-9]#', $str)) {
return TRUE;
}
$this->form_validation->set_message('check_username', 'This is not have an accepted value!');
return FALSE;
}
Validation rules for username
$this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean|callback_check_username');
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