Problem Statement

Display a substring:

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:

2 constant Pos
3 constant Len
: Str ( -- c-addr u )  s" abcdefgh" ;

Str Pos /string drop Len type    \ cde
Str Pos /string type             \ cdefgh
Str 1- type                      \ abcdefg
Str char d scan drop Len type    \ def
Str s" de" search 2drop Len type \ def