Function offset 

pub fn offset(
    s: string,
    n: number,
    i: option<number>,
) -> option<(number, number)>

Returns the position (in bytes) where the encoding of the n-th character of s (counting from byte position i) starts, and — since Lua 5.5 — also the position of the final byte of that character. A negative n gets characters before position i. utf8.offset(s, -1) gets the last character of the string.

Examples

print(utf8.offset("héllo", 2))     --> 2   3   (é spans bytes 2-3)
print(utf8.offset("héllo", 3))     --> 4   4   (l is a single byte)
print(utf8.offset("héllo", -1))    --> 6   6   (the last char)