<![if !vml]> 
<span style='mso-ignore:vglayout;position: 
absolute;z-index:7;left:117px;top:103px;width:28px;height:12px'> 
<img width=28 height=12 src="民生银行电子商户（B2C___）证书下载步骤2.files/image002.gif" v:shapes="_x0000_s1032"> 
</span> 
<![endif]> 


<![if !vml]> 
<img width=28 height=12 src="民生银行电子商户（B2C___）证书下载步骤2.files/image002.gif" v:shapes="_x0000_s1032"> 
<![endif]> 

<!--变化成-->

<![if !vml]> 
<span style='mso-ignore:vglayout;position: 
absolute;z-index:7;left:117px;top:103px;width:28px;height:12px'> 
[img width=28 height=12 src="民生银行电子商户（B2C___）证书下载步骤2.files/image002.gif"/]
</span> 
<![endif]> 

<![if !vml]> 
[img width=28 height=12 src="民生银行电子商户（B2C___）证书下载步骤2.files/image002.gif"/]
<![endif]>

------解决方案--------------------

C# code

string message = "<![if !vml]><span style='mso-ignore:vglayout;position:absolute;z-index:7;left:117px;top:103px;width:28px;height:12px'><img width=28 height=12 src=\"民生银行电子商户（B2C___）证书下载步骤2.files/image002.gif\" v:shapes=\"_x0000_s1032\"></span><![endif]>";
Regex reg = new Regex("<img border=\"0\" width=(\"?[0-9]*\"?) height=(\"?[0-9]*\"?) src=\"(.*?)\".*/?>");
message = reg.Replace(message, "[img width=$1 height=$2 src=\"$3\" /]");
Console.WriteLine(message);