title: Regex for no duplicate characters from a limited character pool
body: <p>Is there a way to write a regex to match a string that only contains certain characters, and never repeats those characters? I already wrote some code using a set to implement this, but would like to know if there's a regex way to do it.</p>
  
  <p>So for example, if I only wanted a string that contains [A,B,C], and I want to match to a string that never duplicates any of those characters, eg A, B, C, AB, AC, B, BC, ABC, and so on, but never matches AA, BB, CC, etc</p>
  
  <p>Thanks!</p>