def generate_AST(string):

  # my implementation is currently supporting negative numbers and decimals,
  # but not exponentiation

  number_symbols = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.', '-']

  ind = 1
  arr_to_return = []

  while ind < len(string):
    char = string[ind]

    if char == "(":
      open_cnt = 1
      closed_cnt = 0
      sub_str = "("
      for c in string[ind + 1:]:
        if c == "(": open_cnt += 1
        if c == ")": closed_cnt += 1
        sub_str += c
        if open_cnt == closed_cnt: break
      arr_to_return.append(generate_AST(sub_str))
      ind += len(sub_str)

    elif char == " " or char == ")":
      ind += 1

    else:
      stop_ind = string.find(" ", ind)
      if stop_ind == -1:
        stop_ind = string.find(")", ind)
      s = string[ind:stop_ind]
      if all(x in number_symbols for x in list(s)):
        if s.find('-', 1) == -1:
          num = float(s)
          arr_to_return.append(num)
      else:
        arr_to_return.append(s)
      ind = stop_ind + 1

  return arr_to_return

print(generate_AST("(first (list -1.5 (+ 2 3) 9))"))