Make Java regex '[abc]+' match once per letter

Question

String[] words = {"a","ab","ac","abc","aac","aa"};
for(String str:words)   {
    if(str.matches("[abc]+"))  {
        System.out.println(str);
    }
}

This code print out

a,ab,ac,abc,aac,aa

This is almost what I want except I do not want a letter be matched twice. I want to change the regex [abc]+ so that the match will only happen once. The aac and aa should not printed because aa is matched twice. Can I do this?

Answer 1

Using negative lookahead you can use this regex:

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

RegEx Demo

If you want to allow any character not just [abc] then use:

^(?:(.)(?!.*\1))+$

• (.) matches and groups any character
• (?!.*\1) is negative lookahead that asserts for each character captured, that the same character does not exist ahead of the current position.
• (?:...)+ groups matching character and lookahead in a non-capturing group which is repeated 1 or more times to match whole string.