Found a couple things

sexpr.lua

@@ -0,0 +1,70 @@
+--- simple s expressions parser
+
+local parse, parse_exp, parse_atom
+
+-- expression: starts with ( and ends with ), contains a whietspace separated list of expressions and tokens
+-- s has no leading whitespace, starts with (
+-- returns exp, r (r has no leading whitespace)
+-- returns nil, err
+parse_exp = function(s)
+	local r = s:match("^%(%s*(.*)$")
+	if not r then return nil, "no expression found" end
+	local exp = {}
+	repeat
+		local item, r_item = parse(r)
+		if item then
+			table.insert(exp, item)
+			r = r_item
+		end
+	until not item
+	if not r:match("^%)") then
+		return nil, "expected closing )"
+	end
+	return exp, r:match("^%)%s*(.-)$")
+end
+
+-- atom: litteral delimited by whitespace, ), or (; and with escaping using \
+-- s has no leading whitespace
+-- returns exp, r (r has no leading whitespace)
+-- returns nil, err
+parse_atom = function(s)
+	local atom = {}
+	local n, r = s:match("^([^%s%(%)\\]*)(.-)$")
+	if #n > 0 then table.insert(atom, n) end
+	while r:match("^\\") do
+		table.insert(atom, r:match("^\\(.)"))
+		n, r = r:match("^\\.([^%s%(%)\\]*)(.-)$")
+		if #n > 0 then table.insert(atom, n) end
+	end
+	if #atom == 0 then return nil, "no atom found" end
+	return table.concat(atom), r:match("^%s*(.-)$")
+end
+
+-- s has no leading whitespace
+-- returns exp, r (r has no leading whitespace)
+-- returns nil, err
+parse = function(s)
+	local i, r = parse_exp(s)
+	if i then return i, r end
+	i, r = parse_atom(s)
+	if i then return i, r end
+	return nil, "no expression found"
+end
+
+local function test(s)
+	local trimmed = s:match("^%s*(.-)$")
+	local parsed, r = parse(s)
+	if not parsed then
+		print(r)
+	elseif r:match(".") then
+		print(("unexpected %q at end of expression"):format(r))
+	else
+		print(require("inspect")(parsed))
+	end
+end
+
+test("((str (Hel\\)lo world) sa mère) (lol))")
+
+test("\\lol\\ wut")
+
+test("()")