diff --git a/todo.txt b/todo.txt
index f559dc9..4f59bf9 100644
--- a/todo.txt
+++ b/todo.txt
@@ -1,4 +1,5 @@
 1. Error-checking - invalid regex, Out-of-bounds stuff. Do more testing.
-2. More operators - ?, character classes
+2. More operators - numeric specifier, character classes
+    a. Make the zero-length checking more rigorous - a? does not match empty string, even though it should.
 3. Add support for zero-length matches - see https://regex101.com/r/8Ec4ue/1
 4. Fix adding concatenation operators in shunting-yard function (very janky, compares against operators individually)