4 diagonals
20. And the fact that it is regular is irrelevant. 5 from each vertex. So 8 vertices would (you'd think) give 8*5 = 40. Except that you would then be counting each diagonal twice - once from each...
19* * * * *The correct answer is 8*(8-3)/2 = 20
27 diagonals
35 diagonals