Example of how to change the REG Code generated within Event Espresso 4+

tw_ee_add_year_prefix_to_reg_code.php

<?php //Do not include the opening PHP tag if you already have one

function tw_ee_add_year_prefix_to_reg_code($reg_code, $transaction, $ticket) {
    // Add a date prefix using 'C{year}-'
    $prefix = 'C' . date("y") . '-';
    //Return $reg_code
    return $prefix . $reg_code;
}
add_filter('FHEE__Create__regCode__new_reg_code', 'tw_ee_add_year_prefix_to_reg_code', 10, 3);