feat(core): add parseDomainName to Fn class (aws#10465)

Add function to Fn class to parse the domain name given an URL.

Fixes aws#5433

----

*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-fn.ts

@@ -134,6 +134,15 @@ export class Fn {
     return Token.asList(new FnCidr(ipBlock, count, sizeMask));
   }
 
+  /**
+   * Given an url, parse the domain name
+   * @param url the url to parse
+   */
+  public static parseDomainName(url: string): string {
+    const noHttps = Fn.select(1, Fn.split('//', url));
+    return Fn.select(0, Fn.split('/', noHttps));
+  }
+
   /**
    * The intrinsic function ``Fn::GetAZs`` returns an array that lists
    * Availability Zones for a specified region. Because customers have access to

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

@@ -35,6 +35,24 @@ export = nodeunit.testCase({
       test.done();
     },
   },
+  'FnParseDomainName': {
+    'parse domain name from resolved url'(test: nodeunit.Test) {
+      test.deepEqual(Fn.parseDomainName('https://test.com/'), 'test.com');
+      test.done();
+    },
+    'parse domain name on token'(test: nodeunit.Test) {
+      const stack = new Stack();
+      const url = Fn.join('//', [
+        'https:',
+        Fn.join('/', [
+          'test.com',
+          'graphql',
+        ]),
+      ]);
+      test.deepEqual(Fn.parseDomainName(stack.resolve(url)), 'test.com');
+      test.done();
+    },
+  },
   'FnJoin': {
     'rejects empty list of arguments to join'(test: nodeunit.Test) {
       test.throws(() => Fn.join('.', []));