Best way to force a varchar column to have no whitespace

SQL Apprentice Question
I have a column that I do not want any whitespace in whatsoever. I'm
wondering how do enforce this a DDL level instead of in 40 million
seat-of-the-pants after-the-fact computer programs accessing the
database.

Celko Answers
CONSTRAINT no_white_space
CHECK ( LEW(foo) = LEN (REPLACE (foo, ' ', '')))

You can then nest calls to REPLACE() for tabs,newlines, etc. easily.