diff --git a/core/src/main/java/google/registry/reporting/billing/CopyDetailReportsAction.java b/core/src/main/java/google/registry/reporting/billing/CopyDetailReportsAction.java
index e5a152ca1..ac9f26309 100644
--- a/core/src/main/java/google/registry/reporting/billing/CopyDetailReportsAction.java
+++ b/core/src/main/java/google/registry/reporting/billing/CopyDetailReportsAction.java
@@ -21,10 +21,8 @@ import static jakarta.servlet.http.HttpServletResponse.SC_OK;
 import static java.util.stream.Collectors.joining;
 
 import com.google.cloud.storage.BlobId;
-import com.google.common.base.Splitter;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMultimap;
-import com.google.common.collect.Iterables;
 import com.google.common.flogger.FluentLogger;
 import com.google.common.io.ByteStreams;
 import com.google.common.net.MediaType;
@@ -41,6 +39,8 @@ import jakarta.inject.Inject;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.Optional;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 /** Copy all registrar detail reports in a given bucket's subdirectory from GCS to Drive. */
 @Action(
@@ -54,6 +54,9 @@ public final class CopyDetailReportsAction implements Runnable {
 
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();
 
+  private static final Pattern FILENAME_PATTERN =
+      Pattern.compile("^invoice_details_[0-9]{4}-[0-9]{2}_(.+)_.+\\.csv$");
+
   private final String billingBucket;
   private final String invoiceDirectoryPrefix;
   private final DriveConnection driveConnection;
@@ -101,8 +104,13 @@ public final class CopyDetailReportsAction implements Runnable {
         new ImmutableMultimap.Builder<>();
     for (String detailReportName : detailReportObjectNames) {
       // The standard report format is "invoice_details_yyyy-MM_registrarId_tld.csv
-      // TODO(larryruili): Determine a safer way of enforcing this.
-      String registrarId = Iterables.get(Splitter.on('_').split(detailReportName), 3);
+      Matcher matcher = FILENAME_PATTERN.matcher(detailReportName);
+      if (!matcher.matches()) {
+        logger.atWarning().log(
+            "Detail report filename '%s' does not match the expected pattern.", detailReportName);
+        continue;
+      }
+      String registrarId = matcher.group(1);
       Optional<Registrar> registrar = Registrar.loadByRegistrarId(registrarId);
       if (registrar.isEmpty()) {
         logger.atWarning().log(
diff --git a/core/src/test/java/google/registry/reporting/billing/CopyDetailReportsActionTest.java b/core/src/test/java/google/registry/reporting/billing/CopyDetailReportsActionTest.java
index b0ca522d4..f7c040613 100644
--- a/core/src/test/java/google/registry/reporting/billing/CopyDetailReportsActionTest.java
+++ b/core/src/test/java/google/registry/reporting/billing/CopyDetailReportsActionTest.java
@@ -248,4 +248,38 @@ class CopyDetailReportsActionTest {
     action.run();
     verifyNoInteractions(driveConnection);
   }
+
+  @Test
+  void testSuccess_registrarIdWithUnderscores() throws IOException {
+    persistResource(
+        loadRegistrar("TheRegistrar")
+            .asBuilder()
+            .setRegistrarId("The_Registrar")
+            .setDriveFolderId("0B-99999")
+            .build());
+
+    gcsUtils.createFromBytes(
+        BlobId.of("test-bucket", "results/invoice_details_2017-10_The_Registrar_test.csv"),
+        "hello,world\n1,2".getBytes(UTF_8));
+
+    action.run();
+    verify(driveConnection)
+        .createOrUpdateFile(
+            "invoice_details_2017-10_The_Registrar_test.csv",
+            MediaType.CSV_UTF_8,
+            "0B-99999",
+            "hello,world\n1,2".getBytes(UTF_8));
+    assertThat(response.getStatus()).isEqualTo(SC_OK);
+  }
+
+  @Test
+  void testSuccess_invalidFilenamePattern_skipped() throws IOException {
+    gcsUtils.createFromBytes(
+        BlobId.of("test-bucket", "results/invoice_details_2017-10.csv"),
+        "hello,world\n1,2".getBytes(UTF_8));
+
+    action.run();
+    verifyNoInteractions(driveConnection);
+    assertThat(response.getStatus()).isEqualTo(SC_OK);
+  }
 }