On register.php usercake, i found ctype_alnum not working as well to validate form. someone who fills in only letter or only number will pass the form. where the correct is, they must fill combination NUMBER and Letter only.

so, i use this regex:

/^([a-z]+[0-9]+|[0-9]+[a-z]+)$/i

so final working code is:

 if(!preg_match('/^([a-z]+[0-9]+|[0-9]+[a-z]+)$/i',$username));
 {
 $errors[] = lang("USERNAME_KUDU_ANGKA_JEUNG_HURUP");
 }

and now, add to language file on models/languages/en.php this line

"USERNAME_KUDU_ANGKA_JEUNG_HURUP"   => "username harus ANGKA dan HURUF (must contain number AND Letter)",