Note that this isn't a fix-all solution. It's right here as that code can't be easily improved, but we don't want to add @TruffleBoundary everywhere just to get things to compile. And you won't be able to add @TruffleBoundary if a method takes a VirtualFrame or if it executes child nodes.

Will Graal support recursive methods later (Regex uses this) ? or could Graal update that method to not recurse ?

Long term we will probably implement a Truffle interpreter specially for Regex. However no we're unlikely to support unbounded recursion. Note that @TruffleBoundary doesn't mean no performance - the method is still JIT compiled and you should get the same performance as in JRuby - it just isn't partially evaluated with the rest of your code.