public String getKey(String expression) {
        if (expression == null || expression.length() == 0) {
            return null;
        }
        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);
            if (c == NESTED || c == INDEXED_START) {
                return null;
            } else if (c == MAPPED_START) {
                int end = expression.indexOf(MAPPED_END, i);
                if (end < 0) {
                    throw new IllegalArgumentException("Missing End Delimiter");
                }
                return expression.substring(i + 1, end);
            }
        }
        return null;
    }