tweak to format_string_for_each_named_arg

Summary:
* Skip past the seen names initially rather than scanning the seen names for the next `{`.
* Avoid `std::isalpha` which is locale-dependent.

Reviewed By: vitaut

Differential Revision: D67282453

fbshipit-source-id: 6675baf70d5e367741f58d2f706cf25596c06564

folly/String.h

@@ -771,7 +771,7 @@ struct format_string_for_each_named_arg_fn {
       auto const pos = str.find('{');
       auto const beg = pos == view::npos ? str.size() : pos + 1;
       if (beg == str.size()) {
-        return; // malformed
+        return; // completed
       }
       if (str[beg] == '{') {
         str = str.substr(beg + 1);
@@ -782,10 +782,11 @@ struct format_string_for_each_named_arg_fn {
         return; // malformed
       }
       auto const arg = str.substr(beg, end - beg);
-      if (!arg.empty() && (arg[0] == '_' || std::isalpha(arg[0]))) {
+      auto const c = arg.empty() ? 0 : arg[0];
+      if (c == '_' || (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z')) {
         fn(arg);
       }
-      str = str.substr(beg);
+      str = str.substr(end);
     }
   }
 };

folly/test/StringTest.cpp

@@ -1487,4 +1487,5 @@ TEST(String, format_string_for_each_named_arg) {
   EXPECT_THAT(fn("hello{3}world{bob}go"), testing::ElementsAre("bob"));
   EXPECT_THAT(fn("hello{bob}world{3}go"), testing::ElementsAre("bob"));
   EXPECT_THAT(fn("hello{bob}world{sam}go"), testing::ElementsAre("bob", "sam"));
+  EXPECT_THAT(fn("{bob}world{sam}"), testing::ElementsAre("bob", "sam"));
 }