Required field only if another field has a value, must be empty otherwise

Here is a question I asked on Stackoverflow about a validation rule. It seems it does not exists.

My question is about Laravel validation rules.

I have two inputs a and b. a is a select input with three possible values: x, y and z. I want to write this rule:

b must have a value only if a values is x. And b must be empty otherwise.

Is there a way to write such a rule? I tried required_with, required_without but it seems it can not cover my case.

In other words, if the previous explanation was not clear enough:

If a == x, b must have a value.
If a != x, b must be empty.

Original Post: https://stackoverflow.com/questions/48420718/required-field-only-if-another-field-has-a-value-must-be-empty-otherwise

Is this kind of rule interesting for "Laravel Validation Rules"? It seems this collection of rule is about "things" (like credit card), not generic rules: so maybe such a rule does not have its place here. What do you think? Should I submit a PR?

laravel-validation-rules / readme

Required field only if another field has a value, must be empty otherwise #7