Referring to bug 22571: I just tested the original case in Koha 20.05.00, but wasn't successful: After creating a MARC modification template in Koha menu "Tools > Batch record modification" with the following action: Add new field 041$a with value ger if 008 matches RegEx m/^...................................ger..$/ I also tried the following action (omitting ^ and $): Add new field 041$a with value ger if 008 matches RegEx m/...................................ger../ The action is run over a given record and Koha says that the bibliographic record has successfully been modified - but in fact nothing has changed, thus no new MARC 041$a with content "ger" is created.
It works for me on master using: https://snipboard.io/PHw2sz.jpg
It seems I was doing something wrong. I tried once again and the bibliographic record has successfully been modified. Thanks, Jonathan! So this bug is obsolete and can be closed.