How about this:

    ( x ) / b / a

It could be part of:

    var x = (x) / b / a;

Or

    if (x) /b/a.test(z)

X could be an arbitrary complex expression. Looking at one preceding token is not enough to disambiguate the slash.

I suspect the only real solution is to intertwine lexing and parsing, so the parser asks for the next token with a flag indicating context. But this constrains what parsing algorithm can be used.