About validation of one field based on other field in laravel

I have problem with validating one field with respect to other field in laravel.first field => ['required','unique','regex:([A-Z]{3}[P|C|H|F|A|T|B|L|J|G][This must be first character of second field][0-9]{4}[A-Z]{1})'],second field => "string.

via Chebli Mohamed