diff --git a/ChangeLog b/ChangeLog
index 53622a677d..cac116f62c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Mar 17 14:25:16 2009  Tanaka Akira  <akr@fsij.org>
+
+	* lib/pathname.rb (Pathname#sub): set $~ in block.binding.
+	  [ruby-dev:38173]
+
 Tue Mar 17 13:48:08 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* win32/Makefile.sub (config.h): added RUBY_COREDLL.
diff --git a/lib/pathname.rb b/lib/pathname.rb
index 8a07d455fd..75faaef8b2 100644
--- a/lib/pathname.rb
+++ b/lib/pathname.rb
@@ -254,7 +254,21 @@ class Pathname
 
   # Return a pathname which is substituted by String#sub.
   def sub(pattern, *rest, &block)
-    self.class.new(@path.sub(pattern, *rest, &block))
+    if block
+      path = @path.sub(pattern, *rest) {|*args|
+        begin
+          old = Thread.current[:pathname_sub_matchdata]
+          Thread.current[:pathname_sub_matchdata] = $~
+          eval("$~ = Thread.current[:pathname_sub_matchdata]", block.binding)
+        ensure
+          Thread.current[:pathname_sub_matchdata] = old
+        end
+        yield *args
+      }
+    else
+      path = @path.sub(pattern, *rest)
+    end
+    self.class.new(path)
   end
 
   if File::ALT_SEPARATOR
diff --git a/test/pathname/test_pathname.rb b/test/pathname/test_pathname.rb
index d979ff7023..a9f7149633 100644
--- a/test/pathname/test_pathname.rb
+++ b/test/pathname/test_pathname.rb
@@ -471,6 +471,15 @@ class TestPathname < Test::Unit::TestCase
   defassert(:pathsubext, 'fooaa.o', 'fooaa', '.o')
   defassert(:pathsubext, 'd.e/aa.o', 'd.e/aa', '.o')
 
+  def test_sub_matchdata
+    result = Pathname("abc.gif").sub(/\..*/) {
+      assert_not_nil($~)
+      assert_equal(".gif", $~[0])
+      ".png"
+    }
+    assert_equal("abc.png", result.to_s)
+  end
+
   def root?(path)
     Pathname.new(path).root?
   end