diff --git a/ChangeLog b/ChangeLog
index 497aa89f67..1f1f5dbd91 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Sep 10 15:59:05 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* regparse.c (UNKNOWN_ESC_WARN): added.
+
+	* regparse.c (conv_backslash_value): Warn unknown
+	  escaped chars in regexp. [ruby-dev:39104]
+
 Wed Sep  9 22:02:02 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/fileutils.rb (FileUtils::Entry_#copy_file): open the source
diff --git a/regparse.c b/regparse.c
index 9295e1d5ab..b9667c6f67 100644
--- a/regparse.c
+++ b/regparse.c
@@ -2115,6 +2115,8 @@ or_cclass(CClassNode* dest, CClassNode* cc, ScanEnv* env)
     return 0;
 }
 
+static void UNKNOWN_ESC_WARN(ScanEnv *env, int c);
+
 static int
 conv_backslash_value(int c, ScanEnv* env)
 {
@@ -2133,6 +2135,8 @@ conv_backslash_value(int c, ScanEnv* env)
       break;
 
     default:
+      if (('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z'))
+	  UNKNOWN_ESC_WARN(env, c);
       break;
     }
   }
@@ -2885,6 +2889,22 @@ CC_DUP_WARN(ScanEnv *env)
   }
 }
 
+static void
+UNKNOWN_ESC_WARN(ScanEnv *env, int c)
+{
+  UChar buf[WARN_BUFSIZE];
+  if (onig_warn == onig_null_warn || !RTEST(ruby_verbose)) return ;
+
+  onig_snprintf_with_pattern(buf, WARN_BUFSIZE, env->enc,
+	  env->pattern, env->pattern_end,
+	  (UChar* )"Unknown escape \\%c is ignored", c);
+
+  if (env->sourcefile == NULL)
+      (*onig_warn)((char* )buf);
+  else
+      rb_compile_warn(env->sourcefile, env->sourceline, (char* )buf);
+}
+
 static UChar*
 find_str_position(OnigCodePoint s[], int n, UChar* from, UChar* to,
 		  UChar **next, OnigEncoding enc)