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

Here's my version:

\b(?=[ABC]{1,3})([ABC]{1})(?:(?!\1)([ABC]{1})(?:(?!\1)(?!\2)[ABC]{1})?)?\b

Logic:

• \b: look for a word boundary
• (?=[ABC]{1,3}): lookahead to see if there is a string of length = 3 with values of only A, B, C
• ([ABC]{1}): match the first character then optionally
• (?!\1)([ABC]{1}): check if the next character is not the same as previously matched - if it's not, match it and optionally
• (?!\1)(?!\2)[ABC]{1}: check if the next character is not the same as previously matched char 1 or 2 - if it's not, match the character

I tested it against this input, so it seems quite reliable:

AABCC BBCC AB BC AC CB CA BA A B C ABC ACB BAC BCA CAB CBA AAA ABB AAA BBC AA

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EDIT:

As you mentioned the character set can be larger I would follow the PS advice in your question and do this the following way:

• introduce chars array which will hold each character in the allowed set (split the string into chars)

• get an array of inputStrings (split the input string on whitespace or whatever else required)

• for each string in inputStrings {

• check if the string.length <= inputStrings.length
• tryMatch each character in the list against the current input and save the number of matches found in a matches list
• check if the matches list contains any entries and then if all entries == 1 or 0 }