Codeigniter not_unique

I am new with codeigniter and I am working with the form_validation of it. I found a function called is_unique which looks if the value is not already in the table. But is there an opposite of this function available? That checks if the value IS once in the table?

No, there isn't one. There are 2 alternatives:

1. Make your own callback rule.
2. If you have those values pre-fetched, use is_array() (native PHP functions can be used as rules).