I have had a look into Kotlin grammar (https://kotlinlang.org/docs/reference/grammar.html). The relevant parts are:

multiLineStringLiteral
  : '"""' (multiLineStringContent | multiLineStringExpression | '"')*
    TRIPLE_QUOTE_CLOSE
  ;

TRIPLE_QUOTE_CLOSE
  : ('"'? '"""') 
  ;

It seems that end: '"""(?=[^"])' is basically the same as end: '"?"""', but the latter is a bit simpler and closer to the grammar. Thoughts?

I don' think those are the same... Is this not a problem? (just look at the end matches) A negative look behind would also work but since we don't have those yet I went with look ahead.

Yeah, I played around a bit and looks like you're right. We need the rightmost match.