As painless does not support `\n`, as per https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-literals.html
you have to get ASCII newline `0x0a`, cast to char and then cast to string https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-casting.html#character-string-casting

So to split your string in Painless on newline you have to

```painless
_source['message'].splitOnToken((String)(char)0x0a);
```
And yes that sounds retarted.