diff --git a/ChangeLog b/ChangeLog
index 66b79d950a..2cd5cf182b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Tue Dec 10 23:58:30 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/date/date_strptime.c (date__strptime_internal): unset
+	  case-insensitive flag for [:alpha:], which already implies both
+	  cases, to get rid of backtrack explosion.  [ruby-core:58984]
+	  [Bug #9221]
+
 Tue Dec 10 23:44:42 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* array.c (rb_ary_hash): add salt to differentiate false and empty
diff --git a/ext/date/date_strptime.c b/ext/date/date_strptime.c
index c6a5969172..3e1b0f85ea 100644
--- a/ext/date/date_strptime.c
+++ b/ext/date/date_strptime.c
@@ -567,8 +567,8 @@ date__strptime_internal(const char *str, size_t slen,
 		    static const char pat_source[] =
 			"\\A("
 			"(?:gmt|utc?)?[-+]\\d+(?:[,.:]\\d+(?::\\d+)?)?"
-			"|[[:alpha:].\\s]+(?:standard|daylight)\\s+time\\b"
-			"|[[:alpha:]]+(?:\\s+dst)?\\b"
+			"|(?-i:[[:alpha:].\\s]+)(?:standard|daylight)\\s+time\\b"
+			"|(?-i:[[:alpha:]]+)(?:\\s+dst)?\\b"
 			")";
 		    static VALUE pat = Qnil;
 		    VALUE m, b;