Absence operator is broken

```
p /(?~ab)/.match("abc")
p /(?~abc)/.match("abc")
p /(?~ab|abc)/.match("abc")
=>
#<MatchData "a">
#<MatchData "ab">
#<MatchData "a">
```

But
```
p /(?~abc|ab)/.match("abc")
=>
#<MatchData "ab">
```
