Laravel 5 : Where can I find the source code of the validation rules?

I am using Laravel 5.2 for my project. Now I need a validation rule namely "after_or_equal" which was introduced in 5.4. So my aim is to clone (or get inspired by it) this rule as it does not exist in 5.2 (I want to implement it as described in the doc).

However I searched Laravel's git repository for this rule but could only find its corresponding error message in English.

So where can I find how they wrote the rule for "after_or_equal" ?

Any help appreciated :-)

via Chebli Mohamed