Something

if i >= len then
	failwith "Unexpected end of string"
else if s.[i] = '[' then
	failwith "Invalid \"[\" inside character class"
else
	if s.[i] = ']' then
		(i + 1, cs)
	else
		let (i1, c1) = parse_char i in
		if i1 < len && s.[i1] = '-' then
			let (i2, c2) = parse_char (i1 + 1) in
			cs#add_range c1 c2;
		else
			()
	parse_inner_charclass i2 cs
	else (
		cs#add c1;
		parse_inner_charclass i1 cs
	)