I believe this:
defp adjust_position(reversed_pre, post) do
case move_spaces(post, reversed_pre) do
# If we are between spaces and a dot, move past the dot
{[?. | post], reversed_pre} when hd(post) != ?. and hd(reversed_pre) != ?. ->
{post, reversed_pre} = move_spaces(post, [?. | reversed_pre])
{reversed_pre, post}
_ ->
case strip_spaces(reversed_pre, 0) do
# If there is a dot to our left, make sure to move to the first character
{[?. | rest], _} when rest == [] or hd(rest) not in ~c".:" ->
{post, reversed_pre} = move_spaces(post, reversed_pre)
{reversed_pre, post}
# If there is a % to our left, make sure to move to the first character
{[?% | _], _} ->
case move_spaces(post, reversed_pre) do
{[h | _] = post, reversed_pre} when h in ?A..?Z ->
{reversed_pre, post}
_ ->
{reversed_pre, post}
end
_ ->
{reversed_pre, post}
end
end
end
Can be simplified down quite a bit. It looks like everything below the first case is a no-op. But I’m not sure if I understood it. At least the tests pass. 
defp adjust_position(reversed_pre, post) do
case move_spaces(post, reversed_pre) do
# If we are between spaces and a dot, move past the dot
{[?. | post], reversed_pre} when hd(post) != ?. and hd(reversed_pre) != ?. ->
{post, reversed_pre} = move_spaces(post, [?. | reversed_pre])
{reversed_pre, post}
{post, reversed_pre} ->
{reversed_pre, post}
end
end
