code: public static boolean isValid(String s) {
  		if (s == null) {
  			return false;
  		}
  		s = StrUtil.removeAll(s, "-");
  		final int len = s.length();
  		if (len != 24) {
  			return false;
  		}
  
  		char c;
  		for (int i = 0; i < len; i++) {
  			c = s.charAt(i);
  			if (c >= '0' && c <= '9') {
  				continue;
  			}
  			if (c >= 'a' && c <= 'f') {
  				continue;
  			}
  			if (c >= 'A' && c <= 'F') {
  				continue;
  			}
  			return false;
  		}
  		return true;
  	}
docs: 给定的字符串是否为有效的ObjectId
  
  @param s 字符串
  @return 是否为有效的ObjectId