diff --git a/conservancy_beancount/plugin/txn_date.py b/conservancy_beancount/plugin/txn_date.py
index 2aff09c9bbb04b88454dbd3d06058104b06b81ae..41f8de26305edea84203307ea562838a9f0d7575 100644
--- a/conservancy_beancount/plugin/txn_date.py
+++ b/conservancy_beancount/plugin/txn_date.py
@@ -5,6 +5,7 @@
 # Full copyright and licensing details can be found at toplevel file
 # LICENSE.txt in the repository.
 
+import os.path
 import re
 
 from ..beancount_types import (
@@ -24,11 +25,11 @@ class TransactionDate(core.TransactionHook):
                 "books dir setting is required to check transaction dates",
             )
         books_pat = re.escape(str(books_path))
-        self.filename_re = re.compile(rf'^{books_pat}/(\d{{4,}})\.beancount$')
+        self.filename_re = re.compile(rf'^{books_pat}/(\d{{4,}})[{os.path.sep}_.]')
         self.fy = config.fiscal_year_begin()
 
     def run(self, txn: Transaction) -> errormod.Iter:
-        match = self.filename_re.fullmatch(txn.meta.get('filename', ''))
+        match = self.filename_re.match(txn.meta.get('filename', ''))
         if match is None:
             return
         file_fy = int(match.group(1))
diff --git a/setup.py b/setup.py
index 1cf9c852890d1538103946ff740a50c387420821..940b34c916a96f1bdf1ef480037364fca0f6936c 100755
--- a/setup.py
+++ b/setup.py
@@ -5,7 +5,7 @@ from setuptools import setup
 setup(
     name='conservancy_beancount',
     description="Plugin, library, and reports for reading Conservancy's books",
-    version='1.18.3',
+    version='1.18.4',
     author='Software Freedom Conservancy',
     author_email='info@sfconservancy.org',
     license='GNU AGPLv3+',
diff --git a/tests/test_plugin_txn_date.py b/tests/test_plugin_txn_date.py
index 956bba4a2ab3563ccc670ce3e35e580085b4f8d5..7c08ce17659559d22a46a7e3243444a5c5c50752 100644
--- a/tests/test_plugin_txn_date.py
+++ b/tests/test_plugin_txn_date.py
@@ -5,6 +5,8 @@
 # Full copyright and licensing details can be found at toplevel file
 # LICENSE.txt in the repository.
 
+import itertools
+
 from datetime import date
 
 import pytest
@@ -18,30 +20,35 @@ from conservancy_beancount.plugin import txn_date as hookmod
 BOOKS_PATH = testutil.test_path('books')
 CONFIG = testutil.TestConfig(books_path=BOOKS_PATH)
 HOOK = hookmod.TransactionDate(CONFIG)
+SUFFIXES = itertools.cycle([
+    '',
+    '_sidefile',
+    '/subdirfile',
+])
 
-@pytest.mark.parametrize('txn_date,fyear', [
-    (date(2016, 1, 1), 2015),
-    (date(2016, 2, 29), 2015),
-    (date(2016, 3, 1), 2016),
-    (date(2016, 12, 31), 2016),
-    (date(2017, 2, 28), 2016),
-    (date(2017, 3, 1), 2017),
+@pytest.mark.parametrize('txn_date,fyear,suffix', [
+    (date(2016, 1, 1), 2015, next(SUFFIXES)),
+    (date(2016, 2, 29), 2015, next(SUFFIXES)),
+    (date(2016, 3, 1), 2016, next(SUFFIXES)),
+    (date(2016, 12, 31), 2016, next(SUFFIXES)),
+    (date(2017, 2, 28), 2016, next(SUFFIXES)),
+    (date(2017, 3, 1), 2017, next(SUFFIXES)),
 ])
-def test_good_txn(txn_date, fyear):
-    filename = str(BOOKS_PATH / f'{fyear}.beancount')
+def test_good_txn(txn_date, fyear, suffix):
+    filename = str(BOOKS_PATH / f'{fyear}{suffix}.beancount')
     txn = testutil.Transaction(date=txn_date, filename=filename, postings=[
         ('Assets:Cash', 5),
         ('Income:Donations', -5),
     ])
     assert not list(HOOK.run(txn))
 
-@pytest.mark.parametrize('txn_date,fyear', [
-    (date(2018, 1, 1), 2017),
-    (date(2018, 12, 31), 2018),
-    (date(2019, 3, 1), 2019),
+@pytest.mark.parametrize('txn_date,fyear,suffix', [
+    (date(2018, 1, 1), 2017, next(SUFFIXES)),
+    (date(2018, 12, 31), 2018, next(SUFFIXES)),
+    (date(2019, 3, 1), 2019, next(SUFFIXES)),
 ])
-def test_bad_txn(txn_date, fyear):
-    filename = str(BOOKS_PATH / '2020.beancount')
+def test_bad_txn(txn_date, fyear, suffix):
+    filename = str(BOOKS_PATH / f'2020{suffix}.beancount')
     txn = testutil.Transaction(date=txn_date, filename=filename, postings=[
         ('Assets:Cash', 5),
         ('Income:Donations', -5),