This just doesn't make sense. Why should masking a field change an eight digit field to a ten character field containing eight digits and two hyphens??? Why should masking a field change a six digit field to a nine character field containing seven <x>s and two hyphens???

Your text says that a column with the text DOB (apparently in the 1st line of the field) identifies a field number and every that field should be changed on every line in the file to 9999-12-12. So, you are asking us to change the heading in that field from DOB to 9999-12-12??? Do you really want 9999-12-12, or should it be 9999-12-31???

Please show us some representative sample input data (with sanitized, but not masked, data) in CODE tags AND show the the corresponding output that should be produced by your script in CODE tags.

Note that you refer to DOB, dob, Date of Birth, Dateof Birth, and DATE-OF-BIRTH in the above quoted text; I assume that you realize that all five of these column headings are different and none of them will compare equal to any of the others!

Are we supposed to assume that all of the files you will be processing will have the same fields in the same order? Or is each file different?

You say that you have <comma> and <vertical-bar> separated files. Does a single file ever contain both delimiters? If you have a <comma> delimited file, will the data in that file ever contain a <vertical-bar> character as data (not as a field delimiter)?