Regex to match 1234, 1324, 2341 (all permutations of {1,2,3,4})

Question

I am implementing the following problem in ruby.

Here's the pattern that I want :

1234, 1324, 1432, 1423, 2341 and so on

i.e. the digits in the four digit number should be between [1-4] and should also be non-repetitive.

to make you understand in a simple manner I take a two digit pattern and the solution should be : 12, 21

i.e. the digits should be either 1 or 2 and should be non-repetitive.

To make sure that they are non-repetitive I want to use $1 for the condition for my second digit but its not working.

Please help me out and thanks in advance.

Answer 1

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

• regular-expressions.info/Lookarounds and Backreferences

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Answer 2

Just for a giggle, here's another option:

^(?:1()|2()|3()|4()){4}\1\2\3\4$

As each unique character is consumed, the capturing group following it captures an empty string. The backreferences also try to match empty strings, so if one of them doesn't succeed, it can only mean the associated group didn't participate in the match. And that will only happen if string contains at least one duplicate.

This behavior of empty capturing groups and backreferences is not officially supported in any regex flavor, so caveat emptor. But it works in most of them, including Ruby.