Regex to check non-repetition of a set of characters

Question

Suppose I have the set of characters [ABC]. I\'m looking for a regex that would match any permutation of the superset except the empty set, i.e.

Answer 1

Try this : (UPDATED)

A[BC](?![ABC])|B[AC](?![ABC])|C[AB](?![ABC])|[ABC](?![ABC])|(ABC|ACB|BAC|BCA|CAB|CBA)(?![ABC])

Demo :

http://regexr.com?30pa6