private static boolean isValidAny(char c) {
        // [A-Za-z0-9_-]*
        return c == '-' || isValidFirst(c);
    }
Returns the single argument of the given feature name, without any quotes,
or empty if it is not a valid query, attribute or constant feature name
searchlib/src/main/java/com/yahoo/searchlib/ranking/features/FeatureNames.java