diff --git a/lib/uri/common.rb b/lib/uri/common.rb
index 4e8210d..cf93fb1 100644
--- a/lib/uri/common.rb
+++ b/lib/uri/common.rb
@@ -45,6 +45,15 @@ def self.parser=(parser = RFC3986_PARSER)
   end
   self.parser = RFC3986_PARSER
 
+  def self.const_missing(const)
+    if value = RFC2396_PARSER.regexp[const]
+      warn "URI::#{const} is obsolete. Use RFC2396_PARSER.regexp[#{const.inspect}] explicitly.", uplevel: 1 if $VERBOSE
+      value
+    else
+      super
+    end
+  end
+
   module Util # :nodoc:
     def make_components_hash(klass, array_hash)
       tmp = {}
diff --git a/test/uri/test_common.rb b/test/uri/test_common.rb
index aa9c689..bccdeaf 100644
--- a/test/uri/test_common.rb
+++ b/test/uri/test_common.rb
@@ -10,6 +10,15 @@ def setup
   def teardown
   end
 
+  def test_fallback_constants
+    orig_verbose = $VERBOSE
+    $VERBOSE = nil
+    assert URI::ABS_URI
+    assert_raise(NameError) { URI::FOO }
+  ensure
+    $VERBOSE = orig_verbose
+  end
+
   def test_parser_switch
     assert_equal(URI::Parser, URI::RFC3986_Parser)
     refute defined?(URI::REGEXP)