Comment by Semaphor
FWIW, I frequently want the string length. Not for anything complicated, but our authors have ranges of characters they are supposed to stay in. Luckily no one uses emojis or weird unicode symbols, so in practice there’s no problem getting the right number by simply ignoring all the complexities.
It's not unlikely that what you would ideally use here is the number of grapheme clusters. What is the length of "ë"? Either 1 or 2 codepoints depending on the encoding (combining [1] or single codepoint [2]), and either 1 byte (Latin-1), 2 bytes (UTF-8 single-codepoint) or 3 bytes (UTF-8 combining).
The metrics you care about are likely number of letters from a human perspective (1) or the number of bytes of storage (depends), possibly both.
[1]: https://tomsmeding.com/unicode#U+65%20U+308 [2]: https://tomsmeding.com/unicode#U+EB