Problem Statement

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If the program uses UTF-8 or UTF-16,   it must work on any valid Unicode code point, whether in the   Basic Multilingual Plane   or above it. The program must reference logical characters (code points),   not 8-bit code units for UTF-8 or 16-bit code units for UTF-16. Programs for other encodings (such as 8-bit ASCII, or EUC-JP) are not required to handle all Unicode characters.

Let's start with the solution:

s = "abcdefgh"
String.slice(s, 2, 3)           #=> "cde"
String.slice(s, 1..3)           #=> "bcd"
String.slice(s, -3, 2)          #=> "fg"
String.slice(s, 3..-1)          #=> "defgh"

# UTF-8
s = "αβγδεζηθ"
String.slice(s, 2, 3)           #=> "γδε"
String.slice(s, 1..3)           #=> "βγδ"
String.slice(s, -3, 2)          #=> "ζη"
String.slice(s, 3..-1)          #=> "δεζηθ"