Enable format validation by default in check_schema.

This catches some additional schema problems (ones caught
by format validation in the metaschema, which was previously treated
as annotation-only in check_schema).

Closes: #904

CHANGELOG.rst

@@ -1,6 +1,11 @@
 v4.17.0
 =======
 
+* The ``check_schema`` method on ``jsonschema.protocols.Validator`` instances
+  now *enables* format validation by default when run. This can catch some
+  additional invalid schemas (e.g. containing invalid regular expressions)
+  where the issue is indeed uncovered by validating against the metaschema
+  with format validation enabled as an assertion.
 * The ``jsonschema`` CLI (along with ``jsonschema.cli`` the module) are now
   deprecated. Use ``check-jsonschema`` instead, which can be installed via
   ``pip install check-jsonschema`` and found

jsonschema/tests/test_validators.py

@@ -1483,6 +1483,16 @@ def test_enum_allows_non_unique_items(self):
         else:
             self.Validator.check_schema({"enum": [12, 12]})
 
+    def test_schema_with_invalid_regex(self):
+        with self.assertRaises(exceptions.SchemaError):
+            self.Validator.check_schema({"pattern": "*notaregex"})
+
+    def test_schema_with_invalid_regex_with_disabled_format_validation(self):
+        self.Validator.check_schema(
+            {"pattern": "*notaregex"},
+            format_checker=None,
+        )
+
 
 class ValidatorTestMixin(MetaSchemaTestsMixin, object):
     def test_it_implements_the_validator_protocol(self):

jsonschema/validators.py

@@ -215,9 +215,15 @@ def __attrs_post_init__(self):
                 )
 
         @classmethod
-        def check_schema(cls, schema):
+        def check_schema(cls, schema, format_checker=_UNSET):
             Validator = validator_for(cls.META_SCHEMA, default=cls)
-            for error in Validator(cls.META_SCHEMA).iter_errors(schema):
+            if format_checker is _UNSET:
+                format_checker = Validator.FORMAT_CHECKER
+            validator = Validator(
+                schema=cls.META_SCHEMA,
+                format_checker=format_checker,
+            )
+            for error in validator.iter_errors(schema):
                 raise exceptions.SchemaError.create_from(error)
 
         def evolve(self, **changes):