How do I replace only 1 match?

I'm trying to make a "Find in page" feature, like the one in Google but it's for a string. I tried to use string.gsub(), but it replaced all the matches.

How can I make it so that it only removes 1 match?

Code so far:

local s = "this is a text string"

local function find(s, matchingString)
   s = s:lower()
   matchingString = matchingString:lower()
   local _, matches = string.gsub(s, matchingString, matchingString)
   if matches == 1 then
      after = " match"
   elseif matchingString == "" then
      print("0 matches")
      return
   else
      after = " matches"
   end

   print(matches..after)
   
   if matches > 0 then
      for i = 1, matches do
         local a, b = s:find(matchingString)
         s = s:gsub(matchingString, "")--what can I change this to so that it only removes 1 match
         print(s)
         print("Location of match "..i..": "..a..", "..b)
      end
   end   
end

find(s, "a")