A better way would be to use chaley's method, which I would've recommended if I knew it existed.
Code:
Search field: {template} (YES! This exists and it is awesome!)
Template: {isbn:|isbn:|}
Search for: (.*)
Replace with: isbn:\1
Destination field: identifiers
>> Mode: Replace field, Split Result="checked"
Identifier type: *
Template uses isbn # and prepends "isbn:" to it, then feeds that into the replace. No need for a custom column at all. Thanks chaley!!