fix(core): cannot override properties with . in the name (#10441)

Closes #10109 

While researching this, I saw some comments multiple years old saying that JavaScript doesn't support RegExp lookbehind assertions, but from the looks of https://node.green/ it is supported all the way back to 10.3.0, so that aligns with current CDK supported node versions.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/core/lib/cfn-resource.ts

@@ -146,6 +146,10 @@ export class CfnResource extends CfnRefElement {
    * If the override is nested, separate each nested level using a dot (.) in the path parameter.
    * If there is an array as part of the nesting, specify the index in the path.
    *
+   * To include a literal `.` in the property name, prefix with a `\`. In most
+   * programming languages you will need to write this as `"\\."` because the
+   * `\` itself will need to be escaped.
+   *
    * For example,
    * ```typescript
    * addOverride('Properties.GlobalSecondaryIndexes.0.Projection.NonKeyAttributes', ['myattribute'])
@@ -177,7 +181,7 @@ export class CfnResource extends CfnRefElement {
    * @param value - The value. Could be primitive or complex.
    */
   public addOverride(path: string, value: any) {
-    const parts = path.split('.');
+    const parts = splitOnPeriods(path);
     let curr: any = this.rawOverrides;
 
     while (parts.length > 1) {
@@ -495,3 +499,25 @@ function deepMerge(target: any, ...sources: any[]) {
 
   return target;
 }
+
+/**
+ * Split on periods while processing escape characters \
+ */
+function splitOnPeriods(x: string): string[] {
+  // Build this list in reverse because it's more convenient to get the "current"
+  // item by doing ret[0] than by ret[ret.length - 1].
+  const ret = [''];
+  for (let i = 0; i < x.length; i++) {
+    if (x[i] === '\\' && i + 1 < x.length) {
+      ret[0] += x[i + 1];
+      i++;
+    } else if (x[i] === '.') {
+      ret.unshift('');
+    } else {
+      ret[0] += x[i];
+    }
+  }
+
+  ret.reverse();
+  return ret;
+}

packages/@aws-cdk/core/test/resource.test.ts

@@ -586,6 +586,39 @@ nodeunitShim({
       test.done();
     },
 
+    'addOverride(p, v) will not split on escaped dots'(test: Test) {
+      // GIVEN
+      const stack = new Stack();
+      const r = new CfnResource(stack, 'MyResource', { type: 'AWS::Resource::Type' });
+
+      // WHEN
+      r.addOverride(String.raw`Properties.Hello\.World.Foo\.Bar\.Baz`, 42);
+      r.addOverride(String.raw`Properties.Single\Back\Slashes`, 42);
+      r.addOverride(String.raw`Properties.Escaped\\.Back\\.Slashes`, 42);
+      r.addOverride(String.raw`Properties.DoublyEscaped\\\\Back\\\\Slashes`, 42);
+      r.addOverride('Properties.EndWith\\', 42); // Raw string cannot end with a backslash
+
+      // THEN
+      test.deepEqual(toCloudFormation(stack), {
+        Resources:
+        {
+          MyResource:
+          {
+            Type: 'AWS::Resource::Type',
+            Properties:
+            {
+              'Hello.World': { 'Foo.Bar.Baz': 42 },
+              'SingleBackSlashes': 42,
+              'Escaped\\': { 'Back\\': { Slashes: 42 } },
+              'DoublyEscaped\\\\Back\\\\Slashes': 42,
+              'EndWith\\': 42,
+            },
+          },
+        },
+      });
+      test.done();
+    },
+
     'addPropertyOverride(pp, v) is a sugar for overriding properties'(test: Test) {
       // GIVEN
       const stack = new Stack();