stats: find regex prefixes more aggressively, taking into account ignored dot token

STYLE.md

@@ -67,6 +67,10 @@
   annotations](https://github.com/abseil/abseil-cpp/blob/master/absl/base/thread_annotations.h),
   such as `GUARDED_BY`, should be used for shared state guarded by
   locks/mutexes.
+* Functions intended to be local to a cc file should be declared in an anonymonus namespace,
+  rather than using the 'static' keyword. Note that the
+  [Google C++ style guide](https://google.github.io/styleguide/cppguide.html#Unnamed_Namespaces_and_Static_Variables)
+   allows either, but in Envoy we prefer annonymous namespaces.
 
 # Error handling
 

source/common/stats/stats_impl.cc

@@ -28,6 +28,10 @@ size_t roundUpMultipleNaturalAlignment(size_t val) {
   return (val + multiple - 1) & ~(multiple - 1);
 }
 
+bool regexStartsWithDot(absl::string_view regex) {
+  return absl::StartsWith(regex, "\\.") || absl::StartsWith(regex, "(?=\\.)");
+}
+
 } // namespace
 
 size_t RawStatData::size() {
@@ -75,7 +79,7 @@ std::string TagExtractorImpl::extractRegexPrefix(absl::string_view regex) {
       if (!absl::ascii_isalnum(regex[i]) && (regex[i] != '_')) {
         if (i > 1) {
           const bool last_char = i == regex.size() - 1;
-          if ((!last_char && (regex[i] == '\\') && (regex[i + 1] == '.')) ||
+          if ((!last_char && regexStartsWithDot(regex.substr(i))) ||
               (last_char && (regex[i] == '$'))) {
             prefix.append(regex.data() + 1, i - 1);
           }

test/common/stats/stats_impl_test.cc

@@ -396,6 +396,7 @@ TEST(TagExtractorTest, ExtractRegexPrefix) {
 
   EXPECT_EQ("", extractRegexPrefix("^prefix(foo)."));
   EXPECT_EQ("prefix", extractRegexPrefix("^prefix\\.foo"));
+  EXPECT_EQ("prefix_optional", extractRegexPrefix("^prefix_optional(?=\\.)"));
   EXPECT_EQ("", extractRegexPrefix("^notACompleteToken"));   //
   EXPECT_EQ("onlyToken", extractRegexPrefix("^onlyToken$")); //
   EXPECT_EQ("", extractRegexPrefix("(prefix)"));