preprocessor: don't substitute 'dates' when preceded by an underscore

preprocessor/Statement.cc

@@ -83,7 +83,7 @@ NativeStatement::writeOutput(ostream &output, const string &basename) const
 {
   using namespace boost::xpressive;
   string date_regex = "(-?\\d+([YyAa]|[Mm]([1-9]|1[0-2])|[Qq][1-4]|[Ww]([1-9]{1}|[1-4]\\d|5[0-2])))";
-  sregex regex_lookbehind = sregex::compile("(?<!\\$|\\d|[a-zA-Z]|\\')" + date_regex);
+  sregex regex_lookbehind = sregex::compile("(?<!\\$|\\d|[a-zA-Z_]|\\')" + date_regex);
   sregex regex_dollar = sregex::compile("(\\$)"+date_regex);
 
   string ns = regex_replace(native_statement, regex_lookbehind, "dates('$&')");