Add warning for regular expression with [\/] (pre-commit#2043)

pre_commit/clientlib.py

@@ -156,6 +156,30 @@ def check(self, dct: Dict[str, Any]) -> None:
             )
 
 
+class OptionalSensibleSlashesAtHook(cfgv.OptionalNoDefault):
+    def check(self, dct: Dict[str, Any]) -> None:
+        super().check(dct)
+
+        if r'[\/]' in dct.get(self.key, ''):
+            logger.warning(
+                f'Pre-commit normalizes the slashes in {self.key!r} field in '
+                f"hook {dct.get('id')!r} to forward slashes, so you can use "
+                f'/ instead of [\\/]',
+            )
+
+
+class OptionalSensibleSlashesAtTop(cfgv.OptionalNoDefault):
+    def check(self, dct: Dict[str, Any]) -> None:
+        super().check(dct)
+
+        if r'[\/]' in dct.get(self.key, ''):
+            logger.warning(
+                f'Pre-commit normalizes the slashes in the top-level '
+                f'{self.key!r} field to forward slashes, so you can use / '
+                f'instead of [\\/]',
+            )
+
+
 class MigrateShaToRev:
     key = 'rev'
 
@@ -273,6 +297,8 @@ def warn_unknown_keys_repo(
     ),
     OptionalSensibleRegexAtHook('files', cfgv.check_string),
     OptionalSensibleRegexAtHook('exclude', cfgv.check_string),
+    OptionalSensibleSlashesAtHook('files', cfgv.check_string),
+    OptionalSensibleSlashesAtHook('exclude', cfgv.check_string),
 )
 CONFIG_REPO_DICT = cfgv.Map(
     'Repository', 'repo',
@@ -343,6 +369,8 @@ def warn_unknown_keys_repo(
     ),
     OptionalSensibleRegexAtTop('files', cfgv.check_string),
     OptionalSensibleRegexAtTop('exclude', cfgv.check_string),
+    OptionalSensibleSlashesAtTop('files', cfgv.check_string),
+    OptionalSensibleSlashesAtTop('exclude', cfgv.check_string),
 
     # do not warn about configuration for pre-commit.ci
     cfgv.OptionalNoDefault('ci', cfgv.check_type(dict)),

tests/clientlib_test.py

@@ -281,6 +281,40 @@ def test_validate_optional_sensible_regex_at_top_level(caplog):
     ]
 
 
+def test_validate_optional_sensible_slashes_at_hook_level(caplog):
+    config_obj = {
+        'id': 'flake8',
+        'files': r'dir[\/].*\.py',
+    }
+    cfgv.validate(config_obj, CONFIG_HOOK_DICT)
+
+    assert caplog.record_tuples == [
+        (
+            'pre_commit',
+            logging.WARNING,
+            "Pre-commit normalizes the slashes in 'files' field in hook "
+            "'flake8' to forward slashes, so you can use / instead of [\\/]",
+        ),
+    ]
+
+
+def test_validate_optional_sensible_slashes_at_top_level(caplog):
+    config_obj = {
+        'files': r'dir[\/].*\.py',
+        'repos': [],
+    }
+    cfgv.validate(config_obj, CONFIG_SCHEMA)
+
+    assert caplog.record_tuples == [
+        (
+            'pre_commit',
+            logging.WARNING,
+            "Pre-commit normalizes the slashes in the top-level 'files' field "
+            'to forward slashes, so you can use / instead of [\\/]',
+        ),
+    ]
+
+
 @pytest.mark.parametrize('fn', (validate_config_main, validate_manifest_main))
 def test_mains_not_ok(tmpdir, fn):
     not_yaml = tmpdir.join('f.notyaml')