I wrote a script to try it. It's boggling that anyone could do this task in milliseconds.

  import os
  import random

  cols,rows = os.get_terminal_size()
  numchars = cols*rows
  randletter = lambda: chr(random.randrange(ord('A'), ord('Z')+1))
  randnumber = lambda: chr(random.randrange(ord('0'), ord('9')+1))

  letters = []
  placement = random.randrange(numchars)
  for i in range(numchars): letters.append(randnumber()) if i == placement else letters.append(randletter())

  for l in letters: print(l,end='')
  input()

  CEND      = '\33[0m'
  CSELECTED = '\33[7m'

  for i,l in enumerate(letters): print(CSELECTED + l + CEND,end='') if i == placement else print(l,end='')
  input()