In a classic two-for-Tuesday move, another string based and built-in function solved question has come in.

Data validation of a string.sql

--
-- Author: Scott Forstie
-- Date  : July 14, 2020
-- 
-- Question: How can I use SQL to validate that a string only contains an approved list of characters?
-- Answer  : The TRANSLATE() built-in function.
--
-- TRANSLATE replaces the allowed characters with blanks.
-- Then, RTRIM throws all the blanks away.
-- Then, if the resulting LENGTH is zero, you can conclude that the string only contained allowed characters.
--
create or replace variable toystore.sometext varchar(1000) for sbcs data;

set toystore.sometext = 'asldkfjj23949';

-- length = 0 means that only allowed characters were used
values length(rtrim(TRANSLATE(toystore.sometext , 
                              ' ', 
                              'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890(+-*/%=_&"''()<>,.:;?) ' )));
  
  
set toystore.sometext = 'asldkfj{}j23949';
-- length > 0 means that only disallowed characters were used
values length(rtrim(TRANSLATE(toystore.sometext , 
                              ' ', 
                              'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890(+-*/%=_&"''()<>,.:;?) ' )));