Add new testcase for --regex=ecmascript

The default for --regex is llvm, so it's covered by any existing tests
with mappings.

tests/cxx/mapping_regex.cc

@@ -0,0 +1,42 @@
+//===--- mapping_regex.cc - test input file for iwyu ----------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// IWYU_ARGS: -I . \
+//            -Xiwyu --regex=ecmascript \
+//            -Xiwyu --mapping_file=tests/cxx/mapping_regex.imp
+
+// This test is a little forced, but here's the idea:
+// * The include of tests/cxx/direct.h should nominally be replaced by
+//   tests/cxx/indirect.h, where IndirectClass is defined
+// * But we provide a mapping file with a negative lookahead assertion mapping
+//   any indirect.h **except in not-tests/not-cxx** to foobar.h
+// * Since tests/cxx/indirect.h does not match not-tests/not-cxx/indirect.h,
+//   the mapping will apply...
+// * ... but only since the --regex=ecmascript, which is the only dialect
+//   supporting negative lookahead.
+
+#include "tests/cxx/direct.h"
+
+void f() {
+  // IWYU: IndirectClass is defined in...*foobar.h
+  IndirectClass i;
+}
+
+/**** IWYU_SUMMARY
+
+tests/cxx/mapping_regex.cc should add these lines:
+#include "foobar.h"
+
+tests/cxx/mapping_regex.cc should remove these lines:
+- #include "tests/cxx/direct.h"  // lines XX-XX
+
+The full include-list for tests/cxx/mapping_regex.cc:
+#include "foobar.h"  // for IndirectClass
+
+***** IWYU_SUMMARY */

tests/cxx/mapping_regex.imp

@@ -0,0 +1,5 @@
+# Bogus header mapping using a negative lookahead regex.
+# The foobar.h header does not exist; we just want IWYU to suggest using it.
+[
+  { include: ["@\"(?!.*not-tests/not-cxx/).*indirect.h\"", private, "\"foobar.h\"", public ] },
+]