This one will match your criteria
if REGEX_Match([TEST],'.*[A-D].*') then "True" else "False" endif
This is an interesting case. It appears that regex_match and regex_replace act in slightly different ways. Here's the test I set up
First formula is the original. The second, a replace using the same regex as the first. The 3rd one expands the expression to match the entire field.
And here are the results
If appears that Regex_Match has to match the entire field in order to return true, whereas Regex_replace acts on any part of the field.