diff --git a/pyanalyze/__init__.py b/pyanalyze/__init__.py
index f1621943..edce733c 100644
--- a/pyanalyze/__init__.py
+++ b/pyanalyze/__init__.py
@@ -58,6 +58,7 @@
 used(extensions.show_error)
 used(extensions.has_extra_keys)
 used(extensions.EnumName)
+used(extensions.ValidRegex)
 used(value.UNRESOLVED_VALUE)  # keeping it around for now just in case
 used(reexport)
 used(patma)
diff --git a/pyanalyze/test_implementation.py b/pyanalyze/test_implementation.py
index ad180af0..dea128e2 100644
--- a/pyanalyze/test_implementation.py
+++ b/pyanalyze/test_implementation.py
@@ -1494,3 +1494,16 @@ def capybara():
 
             re.match(r"a", "b")
             re.match(rb"(", b"b")  # E: incompatible_call
+
+    @assert_passes()
+    def test_extension(self):
+        from typing_extensions import Annotated
+
+        from pyanalyze.extensions import ValidRegex
+
+        def f(x: Annotated[str, ValidRegex()]):
+            pass
+
+        def capybara():
+            f("x")
+            f(r"[")  # E: invalid_regex