Replace variable parsing in guide with pattern matching

guide.lua

@@ -1193,9 +1193,10 @@ local function preprocessHypertext(content,vars)
 		return S(sourceString)
 	end)
 
-	for key,value in pairs(vars) do
-		content=string.gsub(content,string.format("{{%s}}",key),value)
-	end
+	-- Replace all sequences which should be treated as variables with values provided in vars
+	content=string.gsub(content,"{{([^{}]+)}}",function(var)
+		return vars[var] or "nil"
+	end)
 
 	return content
 end