D2: empty string in a conditional statement

Question

In the following code, why does 2 give output but not 3? The removechars statement returns a string with length 0

import std.stdio, std.string;

void main() {
    string str = null;
    if (str) writeln(1); // no

    str = "";
    if (str) writeln(2); // yes

    if (",&%$".removechars(r"^a-z"))  writeln(3); // no
}

Edit: Ok, it may return null, but I'm still a bit puzzled because all of these print true

writeln(",&%$".removechars(r"^a-z") == "");
writeln(",&%$".removechars(r"^a-z") == null);
writeln(",&%$".removechars(r"^a-z").length == 0);

Edit 2: This also prints true, but put either of them in a conditional and you get a different result

writeln("" == null);

Edit 3: Alright, I understand that I cannot test for an empty string the way I did. What led to this question is the following situation. I want to remove chars from a word, but don't want to store an empty string:

if (auto w = word.removechars(r"^a-z"))
    wordcount[w]++;

This works when I try it, but that must be because removechars is returning null rather than ""

Answer 1

Because removeChars will return null when no characters match.

(This happens because .dup of an empty string will always be null.)