Painless scripting language literal newline character

painless-newline.md

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

_source['message'].splitOnToken((String)(char)0x0a);

And yes that sounds retarted.