code: static boolean isStartOfIcuMessage(String part) {
      // ICU messages start with a '{' followed by an identifier, followed by a ',' and then 'plural'
      // or 'select' follows by another comma.
      // the 'startsWith' check is redundant but should allow us to skip using the matcher
      if (!part.startsWith("{")) {
        return false;
      }
      int commaIndex = part.indexOf(',', 1);
      // if commaIndex == 1 that means the identifier is empty, which isn't allowed.
      if (commaIndex <= 1) {
        return false;
      }
      int nextBracketIndex = part.indexOf('{', 1);
      return (nextBracketIndex == -1 || nextBracketIndex > commaIndex)
          && (part.startsWith("plural,", commaIndex + 1)
              || part.startsWith("select,", commaIndex + 1));
    }
docs: Detects an ICU-formatted plural or select message. Any placeholders occurring inside these
  messages must be rewritten in ICU format.