question: --- Day 4: Ceres Search ---
  
  "Looks like the Chief's not here. Next!" One of The Historians pulls out a device and pushes the only button on it. After a brief flash, you recognize the interior of the Ceres monitoring station!
  
  As the search for the Chief continues, a small Elf who lives on the station tugs on your shirt; she'd like to know if you could help her with her word search (your puzzle input). She only has to find one word: XMAS.
  
  This word search allows words to be horizontal, vertical, diagonal, written backwards, or even overlapping other words. It's a little unusual, though, as you don't merely need to find one instance of XMAS - you need to find all of them. Here are a few ways XMAS might appear, where irrelevant characters have been replaced with .:
  
  ..X...
  .SAMX.
  .A..A.
  XMAS.S
  .X....
  The actual word search will be full of letters instead. For example:
  
  MMMSXXMASM
  MSAMXMSMSA
  AMXSXMAAMM
  MSAMASMSMX
  XMASAMXAMM
  XXAMMXXAMA
  SMSMSASXSS
  SAXAMASAAA
  MAMMMXMMMM
  MXMXAXMASX
  In this word search, XMAS occurs a total of 18 times; here's the same word search again, but where letters not involved in any XMAS have been replaced with .:
  
  ....XXMAS.
  .SAMXMS...
  ...S..A...
  ..A.A.MS.X
  XMASAMX.MM
  X.....XA.A
  S.S.S.S.SS
  .A.A.A.A.A
  ..M.M.M.MM
  .X.X.XMASX
  Take a look at the little Elf's word search. How many times does XMAS appear?
  
  Your puzzle answer was 2434.
  
  --- Part Two ---
  
  The Elf looks quizzically at you. Did you misunderstand the assignment?
  
  Looking for the instructions, you flip over the word search to find that this isn't actually an XMAS puzzle; it's an X-MAS puzzle in which you're supposed to find two MAS in the shape of an X. One way to achieve that is like this:
  
  M.S
  .A.
  M.S
  Irrelevant characters have again been replaced with . in the above diagram. Within the X, each MAS can be written forwards or backwards.
  
  Here's the same example from before, but this time all of the X-MASes have been kept instead:
  
  .M.S......
  ..A..MSMS.
  .M.S.MAA..
  ..A.ASMSM.
  .M.S.M....
  ..........
  S.S.S.S.S.
  .A.A.A.A..
  M.M.M.M.M.
  ..........
  In this example, an X-MAS appears 9 times.
  
  Flip the word search from the instructions back over to the word search side and try again. How many times does an X-MAS appear?
solution: import re
  
  def get_grids_3x3(grid, grid_len):
      subgrids = []
  
      for row_start in range(grid_len - 2):
          for col_start in range(grid_len - 2):
              subgrid = [row[col_start:col_start + 3] for row in grid[row_start:row_start + 3]]
              subgrids.append(subgrid)
      return subgrids
  
  input = [line.strip() for line in open("input.txt", "r")]
  count = 0
  grids_3x3 = get_grids_3x3(input, len(input))
  
  for grid in grids_3x3:
      if re.match(r".A.", grid[1]):
          if (re.match(r"M.M", grid[0]) and re.match(r"S.S", grid[2])) or \
              (re.match(r"S.S", grid[0]) and re.match(r"M.M", grid[2])) or \
              (re.match(r"S.M", grid[0]) and re.match(r"S.M", grid[2])) or \
              (re.match(r"M.S", grid[0]) and re.match(r"M.S", grid[2])):
              count+=1
  
  print(count)
  
  