Regex to match string of unique characters

Question

Here is my regular expression:

((GO)( [A-Z])+)

I want every letter to appear at most once, unfortunately it isn't working properly, because this input:

GO A B C C

return true, but should return false.

Answer 1

You can use this regex:

^(GO(?: ([A-Z])(?!.*\2))+)$

RegEx Demo