In MARC modification templates if you define a copy of a control field into another control field. The regexp part does not work. Because of : https://git.koha-community.org/Koha-community/Koha/src/commit/83f8ba4eeed5c3b484161e40505171f5d31c14c0/Koha/SimpleMARC.pm#L638
"control field" means field with tag number < 10 Like 003