Ugh Unicode has been the bane of my existence trying to write a text format spec. I started by trying to forbid certain characters to keep files editable and avoid Unicode rendering exploits (like hiding text, or making structured text behave differently than it looks), but in the end it became so much like herding cats that I had to just settle on https://github.com/kstenerud/concise-encoding/blob/master/ct...

Basically allow everything except some separators, most control chars, and some lookalike characters (which have to be updated as more characters are added to Unicode). It's not as clean as I'd like, but it's at least manageable this way.