Regex to match all permutations of {1,2,3,4} without repetition

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You can use this (see on rubular.com):

^(?=[1-4]{4}$)(?!.*(.).*\1).*$

The first assertion ensures that it’s ^[1-4]{4}$, the second assertion is a negative lookahead that ensures that you can’t match .*(.).*\1, i.e. a repeated character. The first assertion is “cheaper”, so you want to do that first.

References

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