diff --git a/requirements.txt b/requirements.txt
index 11825e9..59432f5 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,6 +3,9 @@
 # there is one source for the package names.
 pyyaml>=6,<7
 jsonschema>=4,<5
+# lce_schema.py needs Unicode property escapes (\p{Ll} etc.) in the slug
+# pattern; Python's built-in `re` cannot compile those, only `regex` can.
+regex>=2023.1.1
 # export_quiz_pdf.py (graded-quiz -> test + answer-sheet PDF). Pure Python,
 # no system libraries. Only the PDF emission needs it; the page logic does not.
 fpdf2>=2.7,<3