Slicing by byte-offset is pretty unhelpful, given how many Unicode characters occupy more than one byte. In an encoding like UTF-16, that's "all of them" but even in UTF-8 it's still "most of them".
Slicing by UTF-16 code-unit is still pretty unhelpful, since a lot of Unicode characters (such as emoji) do not fit in 16 bits, and are encoded as "surrogate pairs". If you happen to slice a surrogate pair in half, you've made a mess.
Slicing by code-points (the numbers allocated by the Unicode consortium) is better, but not great. A shape like the "é" in "café" could be written as U+0065 LATIN SMALL LETTER E followed by U+0301 COMBINING ACUTE ACCENT. Those are separate code-points, but if you slice between them you'll wind up with "cafe" and an isolated acute accent that will stick to whatever it's next to, like this:́
When combining characters stick to a base character, the result is called a "grapheme cluster". Slicing by grapheme clusters is the best option, but it's expensive since you need a bunch of data from the Unicode database to find the edges of each cluster - it depends on the properties assigned to each character.
Comments
It depends what your string is a string of.
Slicing by byte-offset is pretty unhelpful, given how many Unicode characters occupy more than one byte. In an encoding like UTF-16, that's "all of them" but even in UTF-8 it's still "most of them".
Slicing by UTF-16 code-unit is still pretty unhelpful, since a lot of Unicode characters (such as emoji) do not fit in 16 bits, and are encoded as "surrogate pairs". If you happen to slice a surrogate pair in half, you've made a mess.
Slicing by code-points (the numbers allocated by the Unicode consortium) is better, but not great. A shape like the "é" in "café" could be written as U+0065 LATIN SMALL LETTER E followed by U+0301 COMBINING ACUTE ACCENT. Those are separate code-points, but if you slice between them you'll wind up with "cafe" and an isolated acute accent that will stick to whatever it's next to, like this:́
When combining characters stick to a base character, the result is called a "grapheme cluster". Slicing by grapheme clusters is the best option, but it's expensive since you need a bunch of data from the Unicode database to find the edges of each cluster - it depends on the properties assigned to each character.
Doesn't splitting by grapheme cluster also depends on which version of the unicode standard you use, since new standards come with new combinations?