controler function question

[eluser]quasiperfect[/eluser]
that's what i want but if i use _ before the function name then the function doesn't get called

example normal use but the problem with enyone can call the controller/field_check url

Code:

$this->form_validation->set_rules('field', 'Field name', 'callback_field_check');
function field_check($str){
blah blah
}

like u said the function doesn't get called

Code:

$this->form_validation->set_rules('field', 'Field name', 'callback_field_check');
function _field_check($str){
blah blah
}

if i modify the code like this it works but we have to modify everything with a _ and for a big site that's not something i would love to do any other solution available ?

Code:

$this->form_validation->set_rules('field', 'Field name', 'callback__field_check');
function _field_check($str){
blah blah
$this->form_validation->set_message('_field_check', 'error mesage');
}