Note that Unicode did add a "uppercase-ish" ß, as it does appear in German, but only in context of an all caps word e.g on a sign board, so captilazation of a whole word to SS and that new all caps ß are both correct (not sure if UNICODE changed the capitalization rules or just added that strange all caps ß)

For backwards compatibility reasons, the capitalization rules can't be changed for existing characters. So normalizing by naive case-folding now requires at least three steps:

    "ẞ".to_lower() → "ß"
    "ß".to_upper() → "SS"
    "SS".to_lower() → "ss"

(there's a standard for how to compare strings case-insensitively that doesn't involve repeated case-folding, but it's much more complex)

Unicode has a dedicated case folding[0][1] (as opposed to upper/lower case mapping) algorithm to cover these cases.

[0] https://www.unicode.org/reports/tr21/tr21-3.html#Caseless%20... [1] http://userguide.icu-project.org/transforms/casemappings#TOC...

Wow, I did not know about this weird edge case, interesting, thanks!