diff --git a/05-counting-mining.md b/05-counting-mining.md
index 59a900a7..06433a97 100644
--- a/05-counting-mining.md
+++ b/05-counting-mining.md
@@ -649,6 +649,16 @@ $ grep -iw hero *a.tsv > results/hero-i.tsv
 
 ::::::::::::::::::::::::::::::::::::::::::::::::::
 
+:::::::::::::::::::::::::::::::::::::::::::::::::: instructor
+
+### Watch out for different behaviour of the -E option across systems
+
+Learners may encounter some inconsistent behaviour between operating systems when solving the challenge below.
+ `grep -E` on macOS acts like `grep -P` on other platforms. 
+ On Windows and Linux, `grep -E` is halfway between `grep -P` and `grep`: it only does what `grep` can do, but uses Perl-compatible syntax to do it.
+
+:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+
 :::::::::::::::::::::::::::::::::::::::  challenge
 
 ## Searching with regular expressions
diff --git a/md5sum.txt b/md5sum.txt
index c5e72f2e..377eed1a 100644
--- a/md5sum.txt
+++ b/md5sum.txt
@@ -8,7 +8,7 @@
 "episodes/02-navigating-the-filesystem.md" "a55e2785748efba647329295a246802c" "site/built/02-navigating-the-filesystem.md" "2024-05-02"
 "episodes/03-working-with-files-and-folders.md" "f28760ce8c1c6e3b96c8b6ccacc55772" "site/built/03-working-with-files-and-folders.md" "2024-02-23"
 "episodes/04-loops.md" "ef86d9f8b71733dea97b44a886391bdd" "site/built/04-loops.md" "2024-02-23"
-"episodes/05-counting-mining.md" "d287fe1d39afedc4a4448a9b4427aafc" "site/built/05-counting-mining.md" "2024-05-07"
+"episodes/05-counting-mining.md" "8a1409e608d737dc37bb8057e9d6260b" "site/built/05-counting-mining.md" "2025-02-12"
 "episodes/06-free-text.md" "0189b30bfbfaaa9ab8e754834500b2f0" "site/built/06-free-text.md" "2024-06-05"
 "instructors/instructor-notes.md" "c317e03b34390725b50f49df1bf943b1" "site/built/instructor-notes.md" "2024-02-23"
 "learners/discuss.md" "498cf8840b7e5bb0897f7c15af83c052" "site/built/discuss.md" "2023-08-29"