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

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