Field mapping ui for serialized data

I need to provide an interface for users to define field mapping for some associated uploaded data. This mapping will be used as a template to bulk import data.

An example of the mapping is:

[Field A] - Data A, [Field B] - Data B

However the data is not comma delimited, it’s serialized as such:

000006784FIRSTLASTNAMEADDRESS

Each segment of the serialized number has a dedicated character count and meaning, however the system doesn’t know and cannot parse separations like a comma delimited file. Here’s where the user comes in. Our team wants to provide an interface for the user to manually divide the serialized segments and associate them to fields.

Has anyone any ideas how to handle or seen other interfaces that deal with serialized mappings. Is there a usable interface for selecting where the segments begin and end?