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I can see how I could do this in a loop to replace the different subfields. Is there another step I would need to take to guarantee that there is at most one of each type of subfield?

Well if you search for "C(\\d+)|O(\\w+)" it can only match one or the other can't it? Or am I missing something? I could be wrong, but I interpreted the OP's question to mean: is it

John Maddock wrote: [unknown] wrote (please try to attribute quotes) possible to have the regex generate an error if more than one sub-field of a specific type is found, i.e. the following would be invalid: ~C123~C345 The original poster can correct me if I've misinterpreted anything. -- Jim