In Codeigniter 3
I use alpha to validate form for only letters
$this->form_validation->set_rules('firstname', 'FirstName', 'trim|required|alpha',
array('required' => 'please refill', 'alpha' => 'don't want integer')
As one of the solution you can use the regex_match rule and specify allowed characters.
Or you can override the
alpha rule and implement it with help of preg_match function. Check the Extending Native Libraries section of the CodeIgniter docs.
Also you can try to play with locales. The
alpha rule is implemented using the ctype_alpha function. According to the docs, by default, it uses the standard C locale(
[A-Za-z]). You can try to change locale using the setlocale function.