fix too restrictive regex on pathpart

packages/@aws-cdk/aws-apigateway/lib/resource.ts

@@ -138,9 +138,15 @@ function validateResourcePathPart(part: string) {
   // strip {} which indicate this is a parameter
   if (part.startsWith('{') && part.endsWith('}')) {
     part = part.substr(1, part.length - 2);
+
+    // proxy resources are allowed to end with a '+'
+    if (part.endsWith('+')) {
+      part = part.substr(0, part.length - 1);
+    }
   }
 
   if (!/^[a-zA-Z0-9\.\_\-]+$/.test(part)) {
-    throw new Error(`Resource's path part only allow a-zA-Z0-9._- and curly braces at the beginning and the end: ${part}`);
+    throw new Error(`Resource's path part only allow [a-zA-Z0-9._-], an optional trailing '+'
+      and curly braces at the beginning and the end: ${part}`);
   }
 }

packages/@aws-cdk/aws-apigateway/test/test.restapi.ts

@@ -223,6 +223,27 @@ export = {
     test.done();
   },
 
+  '"addResource" allows configuration of proxy paths'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const api = new apigateway.RestApi(stack, 'restapi', {
+      deploy: false,
+      cloudWatchRole: false,
+      restApiName: 'my-rest-api'
+    });
+
+    // WHEN
+    const proxy = api.root.addResource('{proxy+}');
+    proxy.addMethod('ANY');
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ApiGateway::Resource', {
+      PathPart: "{proxy+}",
+      ParentId: { "Fn::GetAtt": ["restapiC5611D27", "RootResourceId"] }
+    }));
+    test.done();
+  },
+
   '"addMethod" can be used to add methods to resources'(test: Test) {
     // GIVEN
     const stack = new cdk.Stack();