diff -ruN ruby-1.8.4.orig/re.c ruby-1.8.4/re.c
--- ruby-1.8.4.orig/re.c	2005-12-13 12:27:51.000000000 +0900
+++ ruby-1.8.4/re.c	2006-07-19 18:07:59.000000000 +0900
@@ -1332,6 +1332,8 @@
 {
     struct RRegexp *re = RREGEXP(obj);
 
+    if (!OBJ_TAINTED(obj) && rb_safe_level() >= 4)
+       rb_raise(rb_eSecurityError, "Insecure: can't modify regexp");
     if (re->ptr) re_free_pattern(re->ptr);
     if (re->str) free(re->str);
     re->ptr = 0;