.Net在线编辑工具.NET Fiddle

using System;
public class Program{  public static void Main()  {    string img = "ijk";    Console.WriteLine(ReplacehtmlBody(img));    Console.WriteLine(ReplacehtmlBody2(img));  }
  public static string ReplaceRegex(string content, string regParen, string replyContent)  {    string strOutput = System.Text.RegularExpressions.Regex.Replace(content, regParen, replyContent);    return strOutput;  }
  public static string ReplacehtmlBody(string htmlBody)  {    if (string.IsNullOrWhiteSpace(htmlBody))    {      return htmlBody;    }
    return ReplaceRegex(htmlBody, "(i?)(\\]+\\>)", "$2max-width: 80% !important;$3");  }
  public static string ReplacehtmlBody2(string htmlBody)  {    if (string.IsNullOrWhiteSpace(htmlBody))    {      return htmlBody;    }
    var reg = new System.Text.RegularExpressions.Regex("(\\]+\\>)", System.Text.RegularExpressions.RegexOptions.IgnoreCase);    return reg.Replace(htmlBody, "$1max-width: 80% !important;$2");  }}

<img style='max-width: 80% !important;aaa' />j<Img style='bbb' />k<img style='max-width: 80% !important;ccc' >img>i<img style='max-width: 80% !important;aaa' />j<Img style='max-width: 80% !important;bbb' />k<img style='max-width: 80% !important;ccc' >img>