William Fisher: A bug in __lookahead::exec causes /(?=^)b/ to match ab. When makes a recursive call to , it passes true for the value of . This causes a beginning-of-line anchor (^) inside a lookahead assertion to match anywhere in the text. This fixes http://llvm.org/bugs/show_bug.cgi?id=11118

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185196 91177308-0d34-0410-b5e6-96231b3b80d8

include/regex

@@ -2921,7 +2921,7 @@ __lookahead<_CharT, _Traits>::__exec(__state& __s) const
     bool __matched = __exp_.__match_at_start_ecma(__s.__current_, __s.__last_,
                                                   __m,
                                                   __s.__flags_ | regex_constants::match_continuous,
-                                                  true);
+                                                  __s.__at_first_ && __s.__current_ == __s.__first_);
     if (__matched != __invert_)
     {
         __s.__do_ = __state::__accept_but_not_consume;

test/re/re.alg/re.alg.search/lookahead.pass.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <regex>
+
+// template <class BidirectionalIterator, class Allocator, class charT, class traits>
+//     bool
+//     regex_search(BidirectionalIterator first, BidirectionalIterator last,
+//                  match_results<BidirectionalIterator, Allocator>& m,
+//                  const basic_regex<charT, traits>& e,
+//                  regex_constants::match_flag_type flags = regex_constants::match_default);
+
+// http://llvm.org/bugs/show_bug.cgi?id=11118
+
+#include <regex>
+#include <cassert>
+
+int main() 
+{
+    assert(!std::regex_search("ab", std::regex("(?=^)b")));
+    assert(!std::regex_search("ab", std::regex("a(?=^)b")));
+}