How do I check that my input doesn't have incorrect characters?

In this program, we ask people to input a three letter codon. This input must only have the letters a,c,g,u in it. How do I check that the rest of the letters are not in it?

Answer 1

[Walter Roberson]

all(ismember(str, 'acgu'))

or

isempty( setdiff(str, 'acqu') )

Answer 2

[Azzi Abdelmalek]

test=0
while test==0
  str=input('enter dtring','s')
  q=regexp(str,'[acgu]')
  test=numel(q)==numel(str)
end