"Not" operator in validation rule - laravel

Question

I have a custom validation rule, is_admin, that checks if a user is an administrator.
Does Laravel have an "opposite" operator (like how ! works in PHP), such that I can do something like not:is_admin, which would check that the user isn't an admin:
$rules = array(
'user_id' => 'required|numeric|not:is_admin'
);
$validator = Validator::make(Input::all(), $rules);
if ($validator->fails())
{
// return error
}
else
{
// continue
}
Thanks.

Answer 1

Yes we can by using conditional statements on rules array.
$rules is an array that we pass to validation class or define in Request class.
Example #1:
public function rules{
//here we return an array of rules like shown below.
return [
'field_a' => 'required',
'field_b' => 'required',
];
//we can add any operator by a little change.
save validation rules array in variable like shown below.
$rules = [
'field_a' => 'required',
'field_b' => 'required',
];
//now we can add any rule in $rules array using common ways of writing conditional statements.
//For example field_c is required only when field_a is present and field_b is not
if(isset($this->field_a) && !isset($this->field_b)){
$rules['field_c' => 'required'];
}
//we can apply any kind of conditional statement and add or remove validation rules on the basis of our business logic.
}
Example#2
public function rules(){
$rules = [];
if ($this->attributes->has('some-key')) {
$rules['other-key'] = 'required|unique|etc';
}
if ($this->attributes->get('some-key') == 'some-value') {
$rules['my-key'] = 'in:a,b,c';
}
if ($this->attributes->get('some-key') == 'some-value') {
$this->attributes->set('key', 'value');
}
return $rules;
}

Answer 2

Yes, you can validate it through required_if:field,value.
You can check more details at http://laravel.com/docs/5.0/validation#rule-required-if
Or you can use not_in:foo,bar.
You can check more details at http://laravel.com/docs/5.0/validation#rule-not-in