After a long, unsuccessful search I am starting to wonder if what I am looking for is possible, I would like a regular expression which requires that each letter chosen is mandatory but only once and in any order.
Example : ^[abc]{3}$
The result I expect would be that it matches only that :
abc, bac, cba, acb
While I get :
acc, abb, cca, aab
Do you see where I am going with this?
You may use a regex like this with a negative lookahead of the matched character in a back-reference:
^(?:([abc])(?!.*\1)){3}$
Here is another way.
^(?!.*([abc]).*\1)[abc]{3}$
The negative lookahead
(?!.*([abc]).*\1)
asserts that no character is repeated and
[abc]{3}
together with the two anchors asserts that the string has a length of three and is composed of the characters in the character class.