What should my validators look like?

I installed Validation API then clicked on the Validation API link in the Site building area. Then I went to add a validator. The Name and Type fields are straightforward enough, but when I get to Rule, I'm not really sure what to put in there. Do I start it with <?php on a PHP validator? Does the regex need to be preg_match('/string/', $string, $matches); or just '/string/' or even just /string/? Using validation_api_help() to add examples to admin/build/validation_api/validators/add would be a great help, I think.