I'm currently building a hash key string (collapsed from a map) where the values that are delimited by the special ASCII unit delimiter 31 (1F).
This nicely solves the problem of trying to guess what ASCII characters won't be used in the string values and I don't need to worry about escaping or quoting values etc.
However reading about the history of this is it appears to be a relic from the 1960s and I haven't seen many examples where strings are built and tokenised using this special character so it all seems too easy.
Are there any issues to using this delimiter in a modern application?
I'm currently doing this in a non-Unicode C++ application, however I'm interested to know how this applies generally in other languages such as Java, C# and with Unicode.
The lower 128 char map of ASCII is fully set in stone into the Unicode standard, this including characters 0->31. The only reason you don't see special ASCII chars in use in strings very often is simply because of human interfacing limitations: they do not visualize well (if at all) when displayed to screen or written to file, and you can't easily type them in from a keyboard either. They're also not allowed in un-escaped form within various popular 'human readable' file formats, such as XML.
For logical processing tasks within a program that do not need end-user interaction, however, they are perfectly suitable for whatever use you can find for them. Your particular use sounds novel and efficient and I think you should definitely run with it.