Add a helper to check whether a user ID is compliant without parsing it

synapse/types/__init__.py

@@ -593,6 +593,24 @@ def is_compliant_user_id_localpart(localpart: str) -> bool:
     return all(0x21 <= ord(c) <= 0x7E for c in localpart)
 
 
+def is_compliant_user_id(user_id: str) -> bool:
+    """
+    Validates that the localpart of the given user ID is within the "compliant"
+    range. See `is_compliant_user_id_localpart`.
+
+    Unlike parsing the user ID with `UserID.from_string`, this never raises on a
+    malformed user ID.
+
+    Args:
+        user_id: the user ID to be checked
+
+    Returns:
+        True if the localpart of the user ID is compliant, False otherwise
+    """
+    localpart, _, _ = user_id[1:].partition(":")
+    return is_compliant_user_id_localpart(localpart)
+
+
 UPPER_CASE_PATTERN = re.compile(b"[A-Z_]")
 
 # the following is a pattern which matches '=', and bytes which are not allowed in a mxid

tests/test_types.py

@@ -37,6 +37,7 @@ from synapse.types import (
     UserID,
     get_domain_from_id,
     get_localpart_from_id,
+    is_compliant_user_id,
     map_username_to_mxid_localpart,
 )
 
@@ -142,6 +143,21 @@ class MapUsernameTestCase(unittest.TestCase):
         self.assertEqual(map_username_to_mxid_localpart("têst".encode()), "t=c3=aast")
 
 
+class IsCompliantUserIdTestCase(unittest.TestCase):
+    def test_compliant(self) -> None:
+        self.assertTrue(is_compliant_user_id("@test_1234:test"))
+        self.assertTrue(is_compliant_user_id("@TEST!~:test"))
+
+    def test_non_compliant(self) -> None:
+        self.assertFalse(is_compliant_user_id("@têst:test"))
+        self.assertFalse(is_compliant_user_id("@te st:test"))
+        self.assertFalse(is_compliant_user_id("@:test"))
+
+    def test_malformed_does_not_raise(self) -> None:
+        self.assertFalse(is_compliant_user_id(""))
+        self.assertFalse(is_compliant_user_id("@"))
+
+
 @parameterized_class(
     ("token_type",),
     [