C#-方法-替换指定位置字符串

        /// <summary>
        /// 替换指定位置字符串
        /// </summary>
        /// <param name="str">字符串</param>
        /// <param name="position">替换位置</param>
        /// <param name="oldstr">旧字符串-正则，可以为""</param>
        /// <param name="newstr">新字符串</param>
        /// <returns>替换后的字符串</returns>
        public static string StrPositionReplace(this string str, int position, string oldstr, string newstr)
        {
            return Regex.Replace(str, "(?<=^.{" + position + "})" + oldstr, newstr);
        }