WIP router: implement path rewrite with regex

api/envoy/api/v2/route/route.proto

@@ -734,6 +734,33 @@ message RouteAction {
   //   requests to */prefix/etc* will be stripped to */etc*.
   string prefix_rewrite = 5;
 
+  // Indicates that during forwarding, paths that match the pattern should be
+  // rewritten, interpolating capture groups from the path pattern into the new path as specified
+  // by the rewrite pattern. This option allows application URLs to be rewritten at the reverse
+  // proxy layer in a way that is aware of segments with variable contents like IDs. The router
+  // filter will place the original path before rewrite into the :ref:`x-envoy-original-path //
+  // <config_http_filters_router_x-envoy-original-path>` header. Examples:
+  //
+  // * The path pattern *^/foo/(bar)/(.*)$ paired with rewrite pattern */$1/$2* would transform
+  // */foo/bar/baz/bang* into */bar/baz/bang*.
+  //
+  //  [#next-major-version: RegexRewrite should be merged with prefix_rewrite and converted to a
+  //  oneof.]
+  RegexRewrite regex_rewrite = 30;
+
+  message RegexRewrite {
+    // Represents a regular expression that the path must match in order for the substitution
+    // to be applied. The regular expression may include capture groups to be interpolated into the
+    // substitution.
+    string pattern = 1;
+
+    // Represents the substitution used to build the final path of the request if the initial path
+    // matches
+    // the pattern. The substitution may include references to capture groups defined in the
+    // pattern which will be interpolated into the final path.
+    string substitution = 2;
+  }
+
   oneof host_rewrite_specifier {
     // Indicates that during forwarding, the host header will be swapped with
     // this value.

docs/root/configuration/operations/tools/router_check.rst

@@ -63,6 +63,7 @@ expects a cluster name match of "instant-server".::
       virtual_host_name: ...,
       host_rewrite: ...,
       path_rewrite: ...,
+      regex_rewrite: ...,
       path_redirect: ...,
       - header_fields:
         key: ...,