diff --git a/todo.txt b/todo.txt
index b0f1279..d42b2b7 100644
--- a/todo.txt
+++ b/todo.txt
@@ -1,3 +1,4 @@
 1. Error-checking - invalid regex, Out-of-bounds stuff. Do more testing.
 2. More opreators - +, ?, character classes
 3. '*' doesn't match trailing whitespace
+4. Fix adding concatenation operators in shunting-yard function (very janky, compares against operators individually)