Decently optimized function for escaping rich text in Roblox. 18% faster than a naive gsub implementation in a small pessimistic case, and 53% faster in the hot case of text with no bad characters. CC0

escapeRichText.lua

local ESCAPES: { [string]: string? } = {
	["&"] = "&",
	["<"] = "&lt;",
	[">"] = "&gt;",
	['"'] = "&quot;",
	["'"] = "&apos;",
}

local function escapeRichText(text: string): string
	local start, finish = 1, 1
	local pieces = {}
	
	while finish <= #text do
		local character = string.sub(text, finish, finish)
		
		local escaped = ESCAPES[character]
		if escaped == nil then
			finish += 1
			continue
		end
		
		table.insert(pieces, string.sub(text, start, finish - 1) .. escaped)
		
		finish += 1
		start = finish
	end
	
	if start == 1 then
		return text
	end
	
	return table.concat(pieces) .. string.sub(text, start)
end

return escapeRichText

zzz_benchmark.lua

--[[
This file is for use by Benchmarker (https://boatbomber.itch.io/benchmarker)

|WARNING| THIS RUNS IN YOUR REAL ENVIRONMENT. |WARNING|
--]]

local escapeRichTextNew = require(game.ReplicatedStorage.escapeRichTextNew)

local function escapeRichTextOld(text: string): string
	return (text:gsub("&", "&amp;"):gsub("<", "&lt;"):gsub(">", "&gt;"):gsub('"', "&quot;"):gsub("'", "&apos;"))
end

return {
	ParameterGenerator = function()
		return
	end,

	Functions = {
		["old - escape with lots of bad characters (1000x)"] = function()
			for _ = 1, 1000 do
				escapeRichTextOld('oh <b>my</b> "god"')
			end
		end,

		["new - escape with lots of bad characters (1000x)"] = function()
			for _ = 1, 1000 do
				escapeRichTextNew('oh <b>my</b> "god"')
			end
		end,
		
		["old - escape with no bad characters (1000x)"] = function()
			for _ = 1, 1000 do
				escapeRichTextOld("hello world")
			end
		end,
		
		["new - escape with no bad characters (1000x)"] = function()
			for _ = 1, 1000 do
				escapeRichTextNew("hello world")
			end
		end,
	},
}