How do I use part of the existing filename in a rename?

I want to rename downloaded files that are named with the pattern "Document_XXX.pdf" (where XXX is a 12-digit reference) to "BankName_XXX.pdf". That is, I want to replace "Document" with "BankName" in the filename.

I can't see how to do this using the Hazel pattern feature – is this possible?

Create a custom attribute for the part of the filename you want to keep. Then in the Rename action, you can use that custom attribute in the pattern.

Thanks