Lookaheads generate content and negative lookaheads have no effect

Lookaheads are only assertions that the following character(s) match, but don't actually consume characters. It seems randexp generates random content for them, however:

```
^(?=a).$ // can only match a single "a" but generates two characters ("a" and a random one)
```

Negative lookaheads should remove possible matches ahead, but randexp ignores them:

```
^(?!a)[a-b]$ // can only match "b" but also generates "a"
```