[regexp] do not assume short external strings have a minimum size.

test/cctest/test-regexp.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1955 matching lines @@
   // .toString() throws on non-RegExps that aren't RegExp.prototype
   v8::Local<v8::Value> resultToStringError = CompileRun(
       "var exception;"
       "try { RegExp.prototype.toString.call(null) }"
       "catch (e) { exception = e; }"
       "exception");
   CHECK_EQ(2, use_counts[v8::Isolate::kRegExpPrototypeStickyGetter]);
   CHECK_EQ(1, use_counts[v8::Isolate::kRegExpPrototypeToString]);
   CHECK(resultToStringError->IsObject());
 }
+
+class UncachedExternalString
+    : public v8::String::ExternalOneByteStringResource {
+ public:
+  const char* data() const override { return "abcdefghijklmnopqrstuvwxyz"; }
+  size_t length() const override { return 26; }
+  bool IsCompressible() const override { return true; }
+};
+
+TEST(UncachedExternalString) {
+  v8::Isolate* isolate = CcTest::isolate();
+  v8::HandleScope scope(isolate);
+  LocalContext env;
+  v8::Local<v8::String> external =
+      v8::String::NewExternalOneByte(isolate, new UncachedExternalString())
+          .ToLocalChecked();
+  CHECK(v8::Utils::OpenHandle(*external)->map() ==
+        CcTest::i_isolate()->heap()->short_external_one_byte_string_map());
+  v8::Local<v8::Object> global = env->Global();
+  global->Set(env.local(), v8_str("external"), external).FromJust();
+  CompileRun("var re = /y(.)/; re.test('ab');");
+  ExpectString("external.substring(1).match(re)[1]", "z");
+}