Since you are looking to validate not only the presence of data, but also its format, the best thing that comes to mind is using an xsd. Create an xsd containing all the restrictions, patterns, allowed values (enumerations), and once you convert the flat file to an IS document using the ff dictionary/schema, validate the IS document against the xsd.
Since you would most likely want to validate the contents upfront, your xsd will correspond to the flat document corresponding to ff-schema.
There may be better ways - but from top of my head, this approach is reasonable and flexible (you can add/delete/modify conditions by simply changing the schema).
Hope this helps,
~Roh
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