What version of Kohana are you using? For Kohana 3.3 it should be:
Validation::factory($_POST)
Also you can use $this->request->post()
instead of $_POST
Looks like the filter method was deprecated from Kohana 3.2 so you could do the following as suggested here: How do I call the trim function on a Kohana 3.2 validation object?
$post = array_map('trim', $this->request->post());
$post = Validation::factory($post)
->rule('txt_name', 'not_empty')
->rule('txt_name', 'regex', array('/^[a-z_.]++$/iD'));
Or alternatively, you could use http://kohanaframework.org/3.3/guide/orm/filters