Index: include/regex
===================================================================
--- include/regex	(revision 186019)
+++ include/regex	(working copy)
@@ -5811,9 +5811,10 @@
              const basic_regex<_CharT, _Traits>& __e,
              regex_constants::match_flag_type __flags = regex_constants::match_default)
 {
-    basic_string<_CharT> __s(__first, __last);
+    int __offset = (__flags & regex_constants::match_prev_avail) ? 1 : 0;
+    basic_string<_CharT> __s(_VSTD::prev(__first, __offset), __last);
     match_results<const _CharT*> __mc;
-    bool __r = __e.__search(__s.data(), __s.data() + __s.size(), __mc, __flags);
+    bool __r = __e.__search(__s.data() + __offset, __s.data() + __s.size(), __mc, __flags);
     __m.__assign(__first, __last, __mc, __flags & regex_constants::__no_update_pos);
     return __r;
 }