Science teacher with almost no regex knowledge needs help!

I am a science teacher who has developed a self-marking homework program. To mark questions requiring a sentence answer I have often used the RegEx below where A, B, C and D are required words in any position or order, and N is a forbidden word. 
/^(?!.*?(N))(?=.*?\b(A))(?=.*?\b(B))(?=.*?\b(C))(?=.*?\b(D)).{0,150}$/i
I need to develop a more sophisticated RegEx which links pairs of words so that the answer will fail if one of the other required words is positioned between the pair.
So if A and B are linked the following would be accepted:
A B C D		B A C D		D C A B		A and B C D	("and" is not a required word, so can be inserted)
But these would not
A C B D		B D A C		etc
Presumably you could also do 2 Pairs, A and B, C and D. A good example of using this would be a chemical equation:
NaOH + HCl -> NaCl + H2O 	where H2O + NaCl <- HCl + NaOH is allowed 
But NaOH + NaCl -> Hcl + H2O is not.