Regex to match (same-length) permutations of AAB

Question

Hello stackoverflow community!

I want to match all (same-length) string permutations of AAB, so I want to match:

AAB
BAA
ABA

but not:

ABB
AB
AABA

I have already found many sources mentioning lookarounds and backreferences at similar questions such as [1] or [2] but I am struggling with repeated characters such as "AA".

I have tried:

^(?=[AAB]{3}$)(?!.*(.).*\1).*$
^([AAB])(?!\1)([AAB])(?!\1|\2)([AAB])(?!\1|\2|\3)$

Do you have any ideas on that? Thanks in advance!

I have a feeling this may be an [XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). Regex isn't really the best way to do something like this. Unless you want something as simple as `^(AAB|BAA|ABA)$`. — R Nar, Aug 08 '16 at 15:23

score 1 · Accepted Answer · answered Aug 08 '16 at 15:27

1

Here's a pattern that matches the three permutations:

^(?=.*B)(?=(.*A){2})...$

This pattern requires that:

the length of the string is 3 (...)
there is one B (the (?=.*B) lookahead)
there are two As (the (?=(.*A){2}) lookahead)

But wouldn't the much simpler ^(AAB|ABA|BAA)$ also work?

answered Aug 08 '16 at 15:27

Cristian Lupascu

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Awesome, thanks! Yes, the `^(AAB|ABA|BAA)$` would have worked out for the simple example, but ultimately I want to generalize it for words with >3 characters. Now I am able to do so. – d053420 Aug 08 '16 at 15:47
@d053420 Great! Glad I could help. – Cristian Lupascu Aug 08 '16 at 16:02

Regex to match (same-length) permutations of AAB

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