diff --git a/lib/rack/request.rb b/lib/rack/request.rb
index be8f0b3bf..750a0dc44 100644
--- a/lib/rack/request.rb
+++ b/lib/rack/request.rb
@@ -598,7 +598,7 @@ def split_header(value)
         value ? value.strip.split(/[,\s]+/) : []
       end
 
-      AUTHORITY = /
+      AUTHORITY = /^
         # The host:
         (?<host>
           # An IPv6 address:
@@ -612,7 +612,7 @@ def split_header(value)
         )
         # The optional port:
         (:(?<port>\d+))?
-      /x
+      $/x
 
       private_constant :AUTHORITY
 
diff --git a/test/spec_request.rb b/test/spec_request.rb
index 4e6e52d63..f3eaae934 100644
--- a/test/spec_request.rb
+++ b/test/spec_request.rb
@@ -116,6 +116,11 @@ class RackRequestTest < Minitest::Spec
     req.host.must_equal "www2.example.org"
     req.hostname.must_equal "www2.example.org"
 
+    req = make_request \
+      Rack::MockRequest.env_for("/", "HTTP_HOST" => "123foo.example.com")
+    req.host.must_equal "123foo.example.com"
+    req.hostname.must_equal "123foo.example.com"
+      
     req = make_request \
       Rack::MockRequest.env_for("/", "SERVER_NAME" => "example.org", "SERVER_PORT" => "9292")
     req.host.must_equal "example.org"