diff --git a/tests/lean/run/crlfToLf.lean b/tests/lean/run/crlfToLf.lean
new file mode 100644
index 000000000000..278c621dce3e
--- /dev/null
+++ b/tests/lean/run/crlfToLf.lean
@@ -0,0 +1,39 @@
+/-!
+# Test `String.crlfToLf`
+-/
+
+/-!
+Leaves single `\n`'s alone.
+-/
+/-- info: "hello\nworld" -/
+#guard_msgs in #eval String.crlfToLf "hello\nworld"
+
+/-!
+Turns `\r\n` into `\n`.
+-/
+/-- info: "hello\nworld" -/
+#guard_msgs in #eval String.crlfToLf "hello\r\nworld"
+
+/-!
+In a string of `\r...\r\n`, only normalizes the last `\r\n`.
+-/
+/-- info: "hello\x0d\nworld" -/
+#guard_msgs in #eval String.crlfToLf "hello\r\r\nworld"
+
+/-!
+Two in a row.
+-/
+/-- info: "hello\n\nworld" -/
+#guard_msgs in #eval String.crlfToLf "hello\r\n\r\nworld"
+
+/-!
+Normalizes at the end.
+-/
+/-- info: "hello\nworld\n" -/
+#guard_msgs in #eval String.crlfToLf "hello\r\nworld\r\n"
+
+/-!
+Can handle a loose `\r` as the last character.
+-/
+/-- info: "hello\nworld\x0d" -/
+#guard_msgs in #eval String.crlfToLf "hello\r\nworld\r"