diff --git a/test/string_position_expr_test.exs b/test/string_position_expr_test.exs
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+++ b/test/string_position_expr_test.exs
@@ -0,0 +1,54 @@
+# SPDX-FileCopyrightText: 2019 ash_postgres contributors <https://github.com/ash-project/ash_postgres/graphs/contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule AshPostgres.StringPositionExprTest do
+  @moduledoc false
+  use AshPostgres.RepoCase, async: false
+
+  alias AshPostgres.Test.Post
+
+  require Ash.Query
+  import Ash.Expr
+
+  defp position(title, substring) do
+    post = Post |> Ash.Changeset.for_create(:create, %{title: title}) |> Ash.create!()
+
+    Post
+    |> Ash.Query.filter(id == ^post.id)
+    |> Ash.Query.calculate(:position, :integer, expr(string_position(title, ^substring)))
+    |> Ash.read_one!()
+    |> Map.get(:calculations)
+    |> Map.get(:position)
+  end
+
+  test "a match at the start of the string is zero" do
+    assert position("alpha", "a") == 0
+  end
+
+  test "a match inside the string is zero based" do
+    assert position("alpha", "pha") == 2
+  end
+
+  test "a substring that is absent is nil" do
+    assert position("alpha", "zzz") == nil
+  end
+
+  test "the whole string matches at zero" do
+    assert position("alpha", "alpha") == 0
+  end
+
+  test "a ci_string is matched case insensitively, and still zero based" do
+    post =
+      Post
+      |> Ash.Changeset.for_create(:create, %{title: "t", category: "Alpha"})
+      |> Ash.create!()
+
+    assert Post
+           |> Ash.Query.filter(id == ^post.id)
+           |> Ash.Query.calculate(:position, :integer, expr(string_position(category, "PHA")))
+           |> Ash.read_one!()
+           |> Map.get(:calculations)
+           |> Map.get(:position) == 2
+  end
+end