Finding the "difference" between two string texts (Lua example)

I'm trying to find the difference in text between two string values in Lua, and I'm just not quite sure how to do this effectively. I'm not very experienced in working with string patterns, and I'm sure that's my downfall on this one. Here's an example:

-- Original text
local text1 = "hello there"
-- Changed text
local text2 = "hello.there"

-- Finding the alteration of original text with some "pattern"
print(text2:match("pattern"))

In the example above, I'd want to output the text ".", since that's the difference between the two texts. Same goes for more complex changes, like this for example:

local text1 = "hello there"
local text2 = "hello()there"

print(text2:match("pattern"))

Where in that example, I'd want to print "()" since again, that is the difference between the two texts.

If anyone has any insight on this, I'd really appreciate it. Sorry I couldn't give more to work with code-wise, I'm just not sure where to begin.