How to add special character validation on specific field in form registration process

Question

I want to add a special character validation on a specfic field. Here I have my code but it does not work and there is no error.

function mymodule_form_alter(&$form, &$form_state, &$form_id) { 
   if ($form_id == 'registration_node_form') {
        $form['#validate'][] = 'mymodule_form_validate';        
   }
}

function mymodule_form_validate($form, $form_state){
    if(isset ($form['field_employer_name']['und'][0]['value']['#default_value'])){
        $special_character_list = '[\'^£$%&*()}{@#~?><>,|=_+¬-]'; 
        if (preg_match($special_character_list, $form_state['field_employer_name']['und'][0]['value']['#default_value'])== 0) {
            form_set_error('field_employer_name', t('The specified form name contains one or more illegal characters. Spaces or any other special characters except dash (-) and underscore (_) are not allowed.'));
        }
    }
}

Please help. Thank You.

Answer 1

Try using Field Validation Module it simplifies your work!!

If you want to code it then check this post It solves the same issue

Try changing your code:

 $special_character_list = '\'^£$%&*()}{@#~?><>,|=_+¬\-\[\]';
  if(preg_match('/[' . $special_character_list . ']/',$form_state['field_employer_name']['und'][0]['value']['#default_value']) !== 0)