<?php
$pattern = '/^[А-Я]{2}\d{6}/u';
echo 'Мала кирилиця: '.preg_match($pattern, 'кс845696').PHP_EOL
; echo 'Велика кирилиця: '.preg_match($pattern, 'КС845696').PHP_EOL
;
PD9waHAKJHBhdHRlcm4gPSAnL15b0JAt0K9dezJ9XGR7Nn0vdSc7CmVjaG8gJ9Cc0LDQu9CwINC60LjRgNC40LvQuNGG0Y86ICcucHJlZ19tYXRjaCgkcGF0dGVybiwgJ9C60YE4NDU2OTYnKS5QSFBfRU9MOwplY2hvICfQktC10LvQuNC60LAg0LrQuNGA0LjQu9C40YbRjzogJy5wcmVnX21hdGNoKCRwYXR0ZXJuLCAn0JrQoTg0NTY5NicpLlBIUF9FT0w7Cgo=