str.replace(/display=\w*/g,"display=localhost");

Regex reg=new Regex(@"display=\w*");
str=reg.Replace(str,"display=localhost");

var reg = /(display=)(\w*)/g;
var result;
while ((result= reg.exec(str))!=null) {
  str= str.replace(result[0], result[1] + "localhost" + result[2]);
}

/// <summary>
/// 定义处理方法
/// </summary>
/// <param name="match">符合的字符串</param>
/// <returns></returns>
private string Evaluator(Match match)
{
  //(display=)(\w*) Groups按查找到的字符串再根据分组进行分组
  //第0组为整个符合的字符串，后面的组按括号顺序排
  string str =match.Groups[1].Value+"localhost"+ match.Groups[2].Value;
  return str;
}
Regex regex = new Regex(@"(display=)(\w*)");
string result = regex.Replace(str, Evaluator);