Regex for no duplicate characters from a limited character pool

Question

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.

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

Thanks!

Answer 1

That's easy to do with a negative lookahead assertion:

^(?!.*(.).*\1)[ABC]+$

matches exactly as you described.

Test it live on regex101.com.

Explanation:

^      # Start of the string
(?!    # Assert that it's impossible to match...
 .*    # Any number of characters (including zero)
 (.)   # followed by one character (remember this one in group 1)
 .*    # that's followed by any number of characters
 \1    # and the same character as before
)      # End of lookahead
[ABC]+ # Match one or more characters from this list
$      # until the end of the string