Update regex for FnSub (#3878)

Author: kddejong

src/cfnlint/helpers.py

@@ -136,7 +136,7 @@
 REGEX_DYN_REF_SSM_SECURE = re.compile(
     r"^.*{{resolve:ssm-secure:[a-zA-Z0-9_\.\-/]+(:\d+)?}}.*$"
 )
-REGEX_SUB_PARAMETERS = re.compile(r"\${([^!].*?)}")
+REGEX_SUB_PARAMETERS = re.compile(r"\${\s*([^!\s].*?)\s*}")
 
 FUNCTIONS = frozenset(
     [

test/unit/rules/functions/test_sub.py

@@ -162,6 +162,18 @@ def context(cfn):
             {"type": "string"},
             [],
         ),
+        (
+            "Valid Fn::Sub with a space and escape char",
+            {"Fn::Sub": "${ !Foo }"},
+            {"type": "string"},
+            [],
+        ),
+        (
+            "Valid Fn::Sub with a space and no escape char",
+            {"Fn::Sub": "${ AWS::AccountId }"},
+            {"type": "string"},
+            [],
+        ),
         (
             "Invalid Fn::Sub with a too to many elements",
             {"Fn::Sub": ["${foo}", {"foo": "bar"}, {}]},