How to write a parser in LUA?

Question

Programming

The task is to write a function for fast thinning of a text file in Lua.
html or javascript file. At the input we have string. You need to thin out a large block, for example like this:

function lua_filter_bloсk ("<-- Great work begin -->","<--Great work done-->","Shit happens")

Essentially, you need a function that simply looks for start and stop and replaces all text between fragments with replace. In PHP, this function might look like this:

function subs_filter_block ($start, $stop, $replace) {
        global $result;
        $start_pos = stripos ($result, $start);
        if ($start_pos !== false) {
                $end_pos = stripos($result, $stop);
                if (($end_pos !== false) && ($start_pos < $end_pos)) {
                        $result =substr_replace ($result, $replace, $start_pos, ($end_pos - $start_pos + strlen($stop)));
                }
        }
        return;
}

Now we need to do it in Lua. For educational purposes, two extreme options are of interest - as simple as possible and as quickly as possible. Should the data be treated as a string? Is it worth converting to a table? How?
The ideal completion would be a ready reference to a library function. Unfortunately, the quality and relevance of the Lua documentation does not even allow you to quickly find out if there is a desired function or library. Interesting options I would be willing to test for performance.
UPDATE 1. The first two who responded did not give a working solution, but forced them to conduct tests and read the manual about patterns (not to be confused with regexp !)
As a result, we got the following version of the working code:

function subs_filter_block(startWord, endWord, replaceStr)
  res = res:gsub(startWord .. "(.-)" .. endWord, replaceStr);
end

It remains to understand how fast Lua patterns work.

Answer 1

As far as I know, there is no non-exciting search, but in theory it should work :)

testStr = "Hello Johny !";

function do_replace(inputStr, startWord, endWord, replaceStr)
  return string.gsub(inputStr, startWord .. " (%S+) " .. endWord, startWord .. " " .. replaceStr .. " " .. endWord);
end

result = do_replace(testStr, "Hello", "!", "habrahabr");
print(result);

Answer 2

Can be done using string.gsub. Something like string.gsub(givenString, start… '(.*)'… stop, replace)
This is in regex.
You can make the equivalent of the php code using the corresponding functions from the string module.