Does BNF or ABNF support negation. That is exclude certain members of the set? I did not see any such negation operator in its syntax.
For example, suppose S is the set of all alphanumeric strings that are not equal to "foo"
What is the BNF for S?
Context free grammars are not closed under "difference" or "complements". So while you might decide to add an operator "subtract" to your BNF, the result will not be a context free grammar even if it has a simple way to express it. Consequence: people don't allow such operators in BNF grammars used to express context-free grammars.