private static boolean isValidFirst(char c) {
        // [A-Za-z0-9_]
        return (c == '_') || ((c >= 'a') && (c <= 'z')) || ((c >= 'A') && (c <= 'Z')) || ((c >= '0') && (c <= '9'));
    }
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