Add rule E1154 to validate Subnet IDs

src/cfnlint/rules/formats/SubnetId.py

@@ -0,0 +1,33 @@
+"""
+Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+SPDX-License-Identifier: MIT-0
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+import regex as re
+
+from cfnlint.jsonschema import Validator
+from cfnlint.rules.formats.FormatKeyword import FormatKeyword
+
+
+class SubnetId(FormatKeyword):
+    id = "E1154"
+    shortdesc = "Validate VPC subnet id format"
+    description = "Check that a VPC subnet id matches a pattern"
+    tags = []
+    source_url = "https://github.com/aws-cloudformation/cfn-lint/blob/main/docs/format_keyword.md#AWS::EC2::Subnet.Id"
+
+    def __init__(self):
+        super().__init__(format="AWS::EC2::Subnet.Id")
+
+    def format(self, validator: Validator, instance: Any) -> bool:
+        if not isinstance(instance, str):
+            return True
+
+        if re.match(r"^subnet-(([0-9A-Fa-f]{8})|([0-9A-Fa-f]{17}))$", instance):
+            return True
+
+        return False

test/fixtures/templates/integration/aws-ec2-instance.yaml

@@ -4,7 +4,7 @@ Resources:
     Properties:
       Description: foobar
       SourceDestCheck: false
-      SubnetId: subnet-abcdefgh
+      SubnetId: subnet-0abc1def2345a678b
   Instance:
     Type: AWS::EC2::Instance
     Properties:

test/unit/module/cfn_json/test_cfn_json.py

@@ -35,7 +35,7 @@ def setUp(self):
             },
             "nat_instance": {
                 "filename": "test/fixtures/templates/quickstart/nat-instance.json",
-                "failures": 3,
+                "failures": 4,
             },
             "vpc_management": {
                 "filename": "test/fixtures/templates/quickstart/vpc-management.json",

test/unit/rules/formats/test_subnet_id.py

@@ -0,0 +1,40 @@
+"""
+Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+SPDX-License-Identifier: MIT-0
+"""
+
+import pytest
+
+from cfnlint.rules.formats.SubnetId import SubnetId
+
+
+@pytest.fixture(scope="module")
+def rule():
+    rule = SubnetId()
+    yield rule
+
+
+@pytest.mark.parametrize(
+    "name,instance,expected",
+    [
+        (
+            "Valid subnet id",
+            "subnet-abcd1234",
+            True,
+        ),
+        (
+            "Valid subnet id long",
+            "subnet-abcdefa1234567890",
+            True,
+        ),
+        (
+            "Valid but wrong type",
+            [],
+            True,
+        ),
+        ("Invalid subnet ID", "subnet-abc", False),
+    ],
+)
+def test_validate(name, instance, expected, rule, validator):
+    result = rule.format(validator, instance)
+    assert result == expected, f"Test {name!r} got {result!r}"