Bill Fisher: This patch fixes a less likely case where '\b' can back up into invalid memory, when driven by a regex_iterator (for case 1, see r185273 or http://llvm.org/bugs/show_bug.cgi?id=16240)

The attached test program also supplies a test for the case 1 fix in r185273.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@186089 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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);
+
+#include <regex>
+#include <string>
+#include <list>
+#include <cassert>
+
+int main()
+{
+    // This regex_iterator uses regex_search(__wrap_iter<_Iter> __first, ...)
+    // Test for http://llvm.org/bugs/show_bug.cgi?id=16240 fixed in r185273.
+    {
+        std::string s{"aaaa a"};
+        std::regex re{"\\ba"};
+        std::sregex_iterator it{s.begin(), s.end(), re};
+        std::sregex_iterator end{};
+
+        assert(it->position(0) == 0);
+        assert(it->length(0) == 1);
+
+        ++it;
+        assert(it->position(0) == 5);
+        assert(it->length(0) == 1);
+
+        ++it;
+        assert(it == end);
+    }
+
+    // This regex_iterator uses regex_search(_BidirectionalIterator __first, ...)
+    {
+        std::string s{"aaaa a"};
+        std::list<char> l{s.begin(), s.end()};
+        std::regex re{"\\ba"};
+        std::regex_iterator<std::list<char>::iterator> it{l.begin(), l.end(), re};
+        std::regex_iterator<std::list<char>::iterator> end{};
+
+        assert(it->position(0) == 0);
+        assert(it->length(0) == 1);
+
+        ++it;
+        assert(it->position(0) == 5);
+        assert(it->length(0) == 1);
+
+        ++it;
+        assert(it == end);
+    }
+}