Avoid running the extradata parsing regex unnecessarily

src/ass_dialogue.cpp

@@ -123,6 +123,7 @@ void AssDialogue::Parse(std::string const& raw) {
 
 	std::string text{tkn.next_tok().begin(), str.end()};
 
+	if (text.size() > 1 && text[0] == '{' && text[1] == '=') {
 		static const boost::regex extradata_test("^\\{(=\\d+)+\\}");
 		boost::match_results<std::string::iterator> rematch;
 		if (boost::regex_search(text.begin(), text.end(), rematch, extradata_test)) {
@@ -140,6 +141,7 @@ void AssDialogue::Parse(std::string const& raw) {
 			}
 			ExtradataIds = ids;
 		}
+	}
 
 	Text = text;
 }