Negative lookahead seems to not be working

[cgutierrezpa]

So I want to generate a random expression that does not contain a certain combination of characters. For example, let's imagine that I want to generate 2-character words with the letters 'a' and 'b' such that I can generate:

'aa'
'ab'
'ba'
'bb'

Then I want the generator to avoid generating one of those words, for instance 'aa', for which I created the following Regular Expression:

/((?!aa)[a-b]){2}/

However, a randexp based on this Regular Expression will be generating all of the words listed above, including 'aa' which is supposed to be skipped.

Isn't this considered by the algorithm? Is there something wrong with my Regular Expression? Or is there a bug in the algorithm?

Thanks in advance.

[fent]

Currently, negative lookaheads are ignored.

Dupe of #18