diff --git a/pathspec/_backends/agg.py b/pathspec/_backends/agg.py
index 3e9c366..ee0c0a9 100644
--- a/pathspec/_backends/agg.py
+++ b/pathspec/_backends/agg.py
@@ -100,9 +100,12 @@ def make_pathspec_backend(
 	"""
 	if name == 'best':
 		name = _BEST_BACKEND
-		if name != 'simple' and has_unsupported_flags(patterns):
-			# Only the simple backend can keep the compile flags of the patterns it
-			# was given.
+		if name != 'simple' and (
+			any(not isinstance(pattern, RegexPattern) for pattern in patterns)
+			or has_unsupported_flags(patterns)
+		):
+			# Only the simple backend supports custom patterns and compile flags that
+			# cannot be translated.
 			name = 'simple'
 
 	if name == 'hyperscan':
diff --git a/pathspec/pathspec.py b/pathspec/pathspec.py
index fa11546..0f79e58 100644
--- a/pathspec/pathspec.py
+++ b/pathspec/pathspec.py
@@ -76,6 +76,8 @@ def __init__(
 		matching backend to use. Default is :data:`None` for "best" to use the best
 		available backend. Priority of backends is: "re2", "hyperscan", "simple".
 		The "simple" backend is always available.
+		Automatic selection uses "simple" when any pattern is not a
+		:class:`.RegexPattern`. Native backends only support regular expressions.
 		"""
 		if isinstance(patterns, Sequence):
 			use_patterns = patterns
diff --git a/tests/test_05_pathspec.py b/tests/test_05_pathspec.py
index 97771a0..3226724 100644
--- a/tests/test_05_pathspec.py
+++ b/tests/test_05_pathspec.py
@@ -1410,3 +1410,59 @@ def test_3_character_after_slash(self):
 					self.assertTrue(spec.match_file(name))
 					self.assertFalse(spec.match_file('foo/'))
 					self.assertFalse(spec.match_file('foo/other'))
+
+
+class SuffixPattern(Pattern):
+	def match_file(self, file: str) -> Optional[str]:
+		return file if file.endswith('.secret') else None
+
+
+class CustomPatternBackendTest(TestCase):
+	"""
+	Automatic backend selection must preserve custom matching implementations.
+	"""
+
+	def test_custom_pattern(self):
+		for preferred in ('simple', 're2', 'hyperscan'):
+			for backend in (None, 'best'):
+				with self.subTest(preferred=preferred, backend=backend):
+					with patch('pathspec._backends.agg._BEST_BACKEND', preferred):
+						spec = PathSpec([SuffixPattern(True)], backend=backend)
+					self.assertEqual(list(spec.match_files(['public.txt', 'key.secret'])), ['key.secret'])
+					self.assertEqual(list(spec.match_files(['public.txt', 'key.secret'], negate=True)), ['public.txt'])
+					self.assertEqual(spec.check_file('key.secret').index, 0)
+
+	def test_custom_pattern_factory(self):
+		for preferred in ('simple', 're2', 'hyperscan'):
+			with self.subTest(preferred=preferred):
+				with patch('pathspec._backends.agg._BEST_BACKEND', preferred):
+					spec = PathSpec.from_lines(lambda line: SuffixPattern(True), ['secrets'])
+				self.assertTrue(spec.match_file('key.secret'))
+				self.assertFalse(spec.match_file('public.txt'))
+
+	def test_mixed_patterns(self):
+		for preferred in ('simple', 're2', 'hyperscan'):
+			for include in (False, True):
+				patterns = [
+					RegexPattern(re.compile('.*'), include=not include),
+					Pattern(None),
+					SuffixPattern(include),
+				]
+				with self.subTest(preferred=preferred, include=include):
+					with patch('pathspec._backends.agg._BEST_BACKEND', preferred):
+						spec = PathSpec(patterns)
+					result = spec.check_file('key.secret')
+					self.assertEqual((result.include, result.index), (include, 2))
+					self.assertEqual(spec.match_file('public.txt'), not include)
+
+	def test_add_custom_pattern(self):
+		spec = PathSpec.from_lines('gitignore', ['*'])
+		other = PathSpec([SuffixPattern(False)], backend='simple')
+		combined = spec + other
+		self.assertTrue(spec.match_file('key.secret'))
+		self.assertFalse(combined.match_file('key.secret'))
+		original = spec
+		spec += other
+		self.assertIs(spec, original)
+		self.assertFalse(spec.match_file('key.secret'))
+		self.assertTrue(spec.match_file('public.txt'))