Definition: In this task, you are given an input list A. You need to convert all the alphabets in the list with a number representing their position in the English alphabet. E.g., replace A by 1, B by 2, a by 1, b by 2, and so on.
Input: ['r', 'C', 'U', '2951', '3187', 'k', '5233', 'i', 'B', 'J', 'n', 'S', 'U']

18, 3, 21, 2951, 3187, 11, 5233, 9, 2, 10, 14, 19, 21