diff --git a/lib/uri/common.rb b/lib/uri/common.rb
index 6378541cbe..00051a88fa 100644
--- a/lib/uri/common.rb
+++ b/lib/uri/common.rb
@@ -379,7 +379,7 @@ module URI
   #
   # See URI.encode_www_form_component, URI.decode_www_form
   def self.decode_www_form_component(str, enc=Encoding::UTF_8)
-    raise ArgumentError, "invalid %-encoding (#{str})" unless /\A[^%]*(?:%\h\h[^%]*)*\z/ =~ str
+    raise ArgumentError, "invalid %-encoding (#{str})" if /%(?!\h\h)/ =~ str
     str.b.gsub(/\+|%\h\h/, TBLDECWWWCOMP_).force_encoding(enc)
   end
 
diff --git a/test/uri/test_common.rb b/test/uri/test_common.rb
index 7808bcdc3c..24a5325522 100644
--- a/test/uri/test_common.rb
+++ b/test/uri/test_common.rb
@@ -100,6 +100,9 @@ class TestCommon < Test::Unit::TestCase
                  URI.decode_www_form_component("\xE3\x81\x82%E3%81%82".force_encoding("UTF-8")))
 
     assert_raise(ArgumentError){URI.decode_www_form_component("%")}
+    assert_raise(ArgumentError){URI.decode_www_form_component("%a")}
+    assert_raise(ArgumentError){URI.decode_www_form_component("x%a_")}
+    assert_nothing_raised(ArgumentError){URI.decode_www_form_component("x"*(1024*1024))}
   end
 
   def test_encode_www_form