怎么从字符串中提取字母和数字

如何从字符串中提取字母和数字
比如string a="xzhke873PQJ0-d_D)us/@fjda'543214321#%^&*#";
怎么才能使a="xzhke873PQJ0dDusfjda543214321";

[解决办法]
string a="xzhke873PQJ0-d_D)us/@fjda'543214321#%^&*#";
a=Regex.Replace(a,"[^\da-zA-Z]+","");