From what I understand you want to compare a text file that contains bsb numbers to another flat file. I would recommend that you extract the bsb numbers from the text using a "find" functions in a Normalizer component(allow you to create one or more output records from each single input record). Then use a FlatFilereader or UniversalDataReader to extract the bsb number from the flat file. Afterwards, compare both streams using a DataIntersection component which will allow you to determine which bsb numbers are present in both the text and flat file, and which bsb numbers are in the text but not in the flat file.
I have taken the liberty of attaching a example project where this use case is implemented.