Add a test with a more complicated substitution

test/mechanicaldiff.bats

@@ -120,3 +120,21 @@ setup() {
   assert_failure
   assert_output "${output_one}${output_two}"
 }
+
+@test "supports regex group substitutions" {
+  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
+	foo(123)
+	bar
+	EOF
+  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
+	foo-123
+	bar
+	EOF
+
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff" || true
+
+  run mechanicaldiff.py 'foo\((\d+)\)' 'foo-\1' <"$BATS_TEST_TMPDIR/diff"
+  assert_success
+  assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
+}