What is the slice expression that gives every third character of string s, starting with the last character and proceeding backward to the first?

The slice expression that gives every third character of a string s, starting with the last character and proceeding backward to the first in Python is s[::-3].

Here's how it works:

• The first colon : is used to indicate the start and end indices. Leaving it blank means we want to include all elements.
• The second colon : is used to indicate the step, which is the number of indices between elements. In this case, we want every third element, so we use 3.
• The negative sign - before the 3 means we want to step in the reverse direction, starting from the end and going towards the start.

So, s[::-3] will give us every third character of the string s, starting from the end and going towards the start.