private static String quoteIfNecessary(String s) {
        if (needsQuoting(s))
            return "\"" + s + "\"";
        return s;
    }
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