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

Question

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 cases where the difference could be sensitive to a string pattern, like this:

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

print(text2:match("pattern"))

In this example, I'd want to print "(" since at that point the new string is no longer consistent with the old one.

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.

Answer 1

local function get_inserted_text(old, new)
   local prv = {}
   for o = 0, #old do
      prv[o] = ""
   end
   for n = 1, #new do
      local nxt = {[0] = new:sub(1, n)}
      local nn = new:sub(n, n)
      for o = 1, #old do
         local result
         if nn == old:sub(o, o) then
            result = prv[o-1]
         else
            result = prv[o]..nn
            if #nxt[o-1] <= #result then
               result = nxt[o-1]
            end
         end
         nxt[o] = result
      end
      prv = nxt
   end
   return prv[#old]
end

Usage:

print(get_inserted_text("hello there", "hello.there"))    -->  .
print(get_inserted_text("hello there", "hello()there"))   -->  ()
print(get_inserted_text("hello there", "hello htere"))    -->  h
print(get_inserted_text("hello there", "heLlloU theAre")) -->  LUA

Answer 2

Just iterate over the strings and find when they don't match.

function StringDifference(str1,str2)
    for i = 1,#str1 do --Loop over strings
        if str1:sub(i,i) ~= str2:sub(i,i) then --If that character is not equal to it's counterpart
            return i --Return that index
        end
    end
    return #str1+1 --Return the index after where the shorter one ends as fallback.
end

print(StringDifference("hello there", "hello.there"))