diff --git a/ChangeLog b/ChangeLog
index 15f164e096c5eb..dd05994d5f905b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Thu Dec 15 13:15:51 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* regcomp.c (onig_region_memsize): implemented for memsize_of().
+
+	* ext/objspace/objspace.c (memsize_of): use it.
+
 Thu Dec 15 10:44:54 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* array.c (rb_ary_reject_bang, rb_ary_delete_if): update rdoc.
diff --git a/ext/objspace/objspace.c b/ext/objspace/objspace.c
index 66e33a38c47077..f37499fc27c922 100644
--- a/ext/objspace/objspace.c
+++ b/ext/objspace/objspace.c
@@ -97,7 +97,7 @@ memsize_of(VALUE obj)
       case T_MATCH:
 	if (RMATCH(obj)->rmatch) {
             struct rmatch *rm = RMATCH(obj)->rmatch;
-	    size += sizeof(struct re_registers); /* TODO: onig_region_memsize(&rm->regs); */
+	    size += onig_region_memsize(&rm->regs);
 	    size += sizeof(struct rmatch_offset) * rm->char_offset_num_allocated;
 	    size += sizeof(struct rmatch);
 	}
diff --git a/regcomp.c b/regcomp.c
index 98ecbe8180c60f..d81334a0024b9b 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -5244,6 +5244,7 @@ size_t
 onig_memsize(const regex_t *reg)
 {
     size_t size = sizeof(regex_t);
+    if (!reg) return 0;
     if (IS_NOT_NULL(reg->p))                size += reg->alloc;
     if (IS_NOT_NULL(reg->exact))            size += reg->exact_end - reg->exact;
     if (IS_NOT_NULL(reg->int_map))          size += sizeof(int) * ONIG_CHAR_TABLE_SIZE;
@@ -5254,6 +5255,15 @@ onig_memsize(const regex_t *reg)
     return size;
 }
 
+size_t
+onig_region_memsize(const OnigRegion *regs)
+{
+    size_t size = sizeof(*regs);
+    if (!regs) return 0;
+    size += regs->allocated * (sizeof(*regs->beg) + sizeof(*regs->end));
+    return size;
+}
+
 #define REGEX_TRANSFER(to,from) do {\
   (to)->state = ONIG_STATE_MODIFY;\
   onig_free_body(to);\
diff --git a/regint.h b/regint.h
index cd3c2a103571de..ca293edcc52957 100644
--- a/regint.h
+++ b/regint.h
@@ -842,6 +842,7 @@ typedef int (*ONIGENC_INIT_PROPERTY_LIST_FUNC_TYPE)(void);
 extern int onigenc_property_list_init P_((ONIGENC_INIT_PROPERTY_LIST_FUNC_TYPE));
 
 extern size_t onig_memsize P_((const regex_t *reg));
+extern size_t onig_region_memsize P_((const struct re_registers *regs));
 
 #if defined __GNUC__ && __GNUC__ >= 4
 #pragma GCC visibility pop
diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index 04a33817de7f71..3c22dae551e410 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -17,6 +17,10 @@ def test_memsize_of
     f.close
     assert_kind_of(Integer, ObjectSpace.memsize_of(/a/.match("a")))
     assert_kind_of(Integer, ObjectSpace.memsize_of(Struct.new(:a)))
+
+    assert_operator(ObjectSpace.memsize_of(Regexp.new("(a)"*1000).match("a"*1000)),
+                    :>,
+                    ObjectSpace.memsize_of(//.match("")))
   end
 
   def test_memsize_of_all