I honestly have no idea what I was doing wrong, maybe just needed a break from programming, but I've got it working now.
GettinSwifty
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It's been a few weeks since I've worked on this issue. Your link references https://developer.apple.com/documentation/foundation/nsstring/1412937-stringbyreplacingoccurrencesofst which seems like it would work but I don't know how to implement that function.