diff --git a/javatests/com/google/gerrit/plugins/codeowners/acceptance/testsuite/TestPathExpressionsTest.java b/javatests/com/google/gerrit/plugins/codeowners/acceptance/testsuite/TestPathExpressionsTest.java
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+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.plugins.codeowners.acceptance.testsuite;
+
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+
+import com.google.gerrit.acceptance.config.GerritConfig;
+import com.google.gerrit.entities.Project.NameKey;
+import com.google.gerrit.extensions.registration.DynamicMap;
+import com.google.gerrit.extensions.registration.PrivateInternals_DynamicMapImpl;
+import com.google.gerrit.extensions.registration.RegistrationHandle;
+import com.google.gerrit.plugins.codeowners.acceptance.AbstractCodeOwnersTest;
+import com.google.gerrit.plugins.codeowners.backend.CodeOwnerBackend;
+import com.google.gerrit.plugins.codeowners.backend.CodeOwnerConfig;
+import com.google.gerrit.plugins.codeowners.backend.CodeOwnerConfigUpdate;
+import com.google.gerrit.plugins.codeowners.backend.GlobMatcher;
+import com.google.gerrit.plugins.codeowners.backend.PathExpressionMatcher;
+import com.google.gerrit.server.IdentifiedUser;
+import com.google.inject.Key;
+import com.google.inject.util.Providers;
+import java.nio.file.Path;
+import java.util.Optional;
+import javax.annotation.Nullable;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.junit.Before;
+import org.junit.Test;
+
+/** Tests for {@link TestPathExpressions}. */
+public class TestPathExpressionsTest extends AbstractCodeOwnersTest {
+  private TestPathExpressions testPathExpressions;
+  private DynamicMap<CodeOwnerBackend> codeOwnerBackends;
+
+  @Before
+  public void setUp() {
+    testPathExpressions = plugin.getSysInjector().getInstance(TestPathExpressions.class);
+    codeOwnerBackends =
+        plugin.getSysInjector().getInstance(new Key<DynamicMap<CodeOwnerBackend>>() {});
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = "find-owners")
+  public void matchFileType_findOwnersBackend() throws Exception {
+    assertThat(testPathExpressions.matchFileType("md")).isEqualTo("*.md");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = "proto")
+  public void matchFileType_protoBackend() throws Exception {
+    assertThat(testPathExpressions.matchFileType("md")).isEqualTo("....md");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = TestCodeOwnerBackend.ID)
+  public void matchFileType_backendWithGlobMatcher() throws Exception {
+    try (AutoCloseable registration = registerTestBackend(GlobMatcher.INSTANCE)) {
+      assertThat(testPathExpressions.matchFileType("md")).isEqualTo("**.md");
+    }
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = TestCodeOwnerBackend.ID)
+  public void matchFileType_backendWithUnknownMatcher() throws Exception {
+    PathExpressionMatcher pathExpressionMatcher =
+        new PathExpressionMatcher() {
+          @Override
+          public boolean matches(String pathExpression, Path relativePath) {
+            return false;
+          }
+        };
+    try (AutoCloseable registration = registerTestBackend(pathExpressionMatcher)) {
+      IllegalStateException exception =
+          assertThrows(IllegalStateException.class, () -> testPathExpressions.matchFileType("md"));
+      assertThat(exception)
+          .hasMessageThat()
+          .isEqualTo(
+              String.format(
+                  "path expression matcher %s not supported",
+                  pathExpressionMatcher.getClass().getName()));
+    }
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = TestCodeOwnerBackend.ID)
+  public void matchFileType_backendWithoutMatcher() throws Exception {
+    TestCodeOwnerBackend testCodeOwnerBackend =
+        new TestCodeOwnerBackend(/* pathExpressionMatcher= */ null);
+    try (AutoCloseable registration = registerTestBackend(testCodeOwnerBackend)) {
+      IllegalStateException exception =
+          assertThrows(IllegalStateException.class, () -> testPathExpressions.matchFileType("md"));
+      assertThat(exception)
+          .hasMessageThat()
+          .isEqualTo(
+              String.format(
+                  "code owner backend %s doesn't support path expressions",
+                  testCodeOwnerBackend.getClass().getName()));
+    }
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = "find-owners")
+  public void matchAllFilesInSubfolder_findOwnersBackend() throws Exception {
+    assertThat(testPathExpressions.matchAllFilesInSubfolder("foo")).isEqualTo("foo/**");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = "proto")
+  public void matchAllFilesInSubfolder_protoBackend() throws Exception {
+    assertThat(testPathExpressions.matchAllFilesInSubfolder("foo")).isEqualTo("foo/...");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = TestCodeOwnerBackend.ID)
+  public void matchAllFilesInSubfolder_backendWithGlobMatcher() throws Exception {
+    try (AutoCloseable registration = registerTestBackend(GlobMatcher.INSTANCE)) {
+      assertThat(testPathExpressions.matchAllFilesInSubfolder("foo")).isEqualTo("foo/**");
+    }
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = TestCodeOwnerBackend.ID)
+  public void matchAllFilesInSubfolder_backendWithUnknownMatcher() throws Exception {
+    PathExpressionMatcher pathExpressionMatcher =
+        new PathExpressionMatcher() {
+          @Override
+          public boolean matches(String pathExpression, Path relativePath) {
+            return false;
+          }
+        };
+    try (AutoCloseable registration = registerTestBackend(pathExpressionMatcher)) {
+      IllegalStateException exception =
+          assertThrows(
+              IllegalStateException.class,
+              () -> testPathExpressions.matchAllFilesInSubfolder("foo"));
+      assertThat(exception)
+          .hasMessageThat()
+          .isEqualTo(
+              String.format(
+                  "path expression matcher %s not supported",
+                  pathExpressionMatcher.getClass().getName()));
+    }
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.backend", value = TestCodeOwnerBackend.ID)
+  public void matchAllFilesInSubfolder_backendWithoutMatcher() throws Exception {
+    TestCodeOwnerBackend testCodeOwnerBackend =
+        new TestCodeOwnerBackend(/* pathExpressionMatcher= */ null);
+    try (AutoCloseable registration = registerTestBackend(testCodeOwnerBackend)) {
+      IllegalStateException exception =
+          assertThrows(
+              IllegalStateException.class,
+              () -> testPathExpressions.matchAllFilesInSubfolder("foo"));
+      assertThat(exception)
+          .hasMessageThat()
+          .isEqualTo(
+              String.format(
+                  "code owner backend %s doesn't support path expressions",
+                  testCodeOwnerBackend.getClass().getName()));
+    }
+  }
+
+  private AutoCloseable registerTestBackend(PathExpressionMatcher pathExpressionMatcher) {
+    return registerTestBackend(new TestCodeOwnerBackend(pathExpressionMatcher));
+  }
+
+  private AutoCloseable registerTestBackend(CodeOwnerBackend codeOwnerBackend) {
+    RegistrationHandle registrationHandle =
+        ((PrivateInternals_DynamicMapImpl<CodeOwnerBackend>) codeOwnerBackends)
+            .put("gerrit", TestCodeOwnerBackend.ID, Providers.of(codeOwnerBackend));
+    return registrationHandle::remove;
+  }
+
+  private static class TestCodeOwnerBackend implements CodeOwnerBackend {
+    static final String ID = "test-backend";
+
+    @Nullable private final PathExpressionMatcher pathExpressionMatcher;
+
+    public TestCodeOwnerBackend(@Nullable PathExpressionMatcher pathExpressionMatcher) {
+      this.pathExpressionMatcher = pathExpressionMatcher;
+    }
+
+    @Override
+    public boolean isCodeOwnerConfigFile(NameKey project, String fileName) {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public Optional<CodeOwnerConfig> getCodeOwnerConfig(
+        CodeOwnerConfig.Key codeOwnerConfigKey, RevWalk revWalk, ObjectId revision) {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public Path getFilePath(CodeOwnerConfig.Key codeOwnerConfigKey) {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public Optional<CodeOwnerConfig> upsertCodeOwnerConfig(
+        CodeOwnerConfig.Key codeOwnerConfigKey,
+        CodeOwnerConfigUpdate codeOwnerConfigUpdate,
+        IdentifiedUser currentUser) {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public Optional<PathExpressionMatcher> getPathExpressionMatcher() {
+      return Optional.ofNullable(pathExpressionMatcher);
+    }
+  }
+}