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. ``` ABC ACB BAC BCA CAB CBA AB BC AC CB CA BA A B C ``` The regex should (obviously) not match the empty string. p.s. An alternative way to express the same objective is "match any non-empty string containing each character in the set at most once". update: The set `[ABC]` is just an example, for the real set may also be bigger. With this question I was hoping to find a "general" solution rather than a particular one for `[ABC]`.

Accepted Answer

Thanks to your answers (especially anubhava's and codaddict's) I was able to find this solution, that I think is pretty elegant because it allows to type the set only once:

```
\b(([ABC])(?!.*\2))+\b
```

the `\b` are needed to match full words; omitting them will also find subwords respecting the required property. To match a full string, you'd obviously do:

```
^(([ABC])(?!.*\2))+$
```

Regex to check non-repetition of a set of characters

Solution

Problem