The "backspace" escape character '\b': unexpected behavior?

Your result will vary depending on what kind of terminal or console program you're on, but yes, on most \b is a nondestructive backspace. It moves the cursor backward, but doesn't erase what's there.

So for the hello worl part, the code outputs

hello worl          ^

...(where ^ shows where the cursor is) Then it outputs two \b characters which moves the cursor backward two places without erasing (on your terminal):

hello worl        ^

Note the cursor is now on the r. Then it outputs d, which overwrites the r and gives us:

hello wodl         ^

Finally, it outputs \n, which is a non-destructive newline (again, on most terminals, including apparently yours), so the l is left unchanged and the cursor is moved to the beginning of the next line.

..........^ <= pointer to "print head"

            /* part1 */            printf("hello worl");

hello worl          ^ <= pointer to "print head"

            /* part2 */            printf("\b");

hello worl         ^ <= pointer to "print head"

            /* part3 */            printf("\b");

hello worl        ^ <= pointer to "print head"

            /* part4 */            printf("d\n");

hello wodl^ <= pointer to "print head" on the next line

If you want a destructive backspace, you'll need something like

"\b \b"

i.e. a backspace, a space, and another backspace.