Free virtual Uno simulation environment

Hello,
I've come across a possible bug.
In the code below, in block //4 the replace seems not to function.
John

/*
Sample Program to show some string functionsn
*/
//#include "string.h";

void setup() {
String my_str = "This is my string.";
Serial.begin(9600);

// (1) print the string
Serial.println(my_str);

// (2) change the string to upper-case
my_str.toUpperCase();
Serial.println(my_str);

// (3) overwrite the string
my_str = "My new string.";
Serial.println(my_str);

// (4) replace a word in the string
my_str.replace("string", "Arduino sketch");
Serial.println(my_str);

// (5) get the length of the string
Serial.print("String length is: ");
Serial.println(my_str.length());}

void loop()
{
}