It isn't quite that easy to emulate lookahead using intersection (AND). For (?=a)b you're right, but that's because `a` and `b` both only ever match strings of

It isn't quite that easy to emulate lookahead using intersection (AND). For (?=a)b you're right, but that's because `a` and `b` both only ever match strings of length one. `(?=foo)f`, for example, will match the first character of the string "foobar". But `foo&f` is the empty language: no string is both "foo" and "f"; they have different lengths! The trouble with lookahead and lookbehind is that they aren't even describable in terms of the "language" which the regular expression corresponds to; rather, they modify how the pattern matches in the context of an overall string. So they don't quite use the same formalism as intersection, union, and negation of regular languages.