Question

    String[] parts = msg.split(" +\n?");

    String room = parts[0];
    System.out.println(msg);
    System.out.println("Laczenie do" + room + ", " + parts.length);
    for (String s : parts)
        System.out.println("-" + s + "-");

output:

  main                                                               

Laczenie do, 2
 --
 -main-

What is this first char? How to cut it using regexp?

Answer 1

That is the empty string at the start of the string. Your regex splits on one or more spaces, optionally followed by a newline character. Since your string starts with spaces, the first split occurs around those spaces – the left-hand part is empty, the right-hand part is main etc.

Visualizing the spaces using _, your string looks like:

__main

So when you split it, it gets divided into the part before __ ("") and the part after __ ("main").

What did you expect to happen?