i want to remove the google ad html in the text,such as

```
xxxxxxx<div class="gg200x300" style="padding: 19px; margin: 0px 22px 0px 0px; overflow: hidden; text-align: center; font-size: 0px; line-height: 0; float: left; border: 1px solid rgb(229, 229, 229); color: rgb(37, 37, 37); font-family: 宋体, sans-serif;"><iframe src="http://g.163.com/r?site=netease&amp;affiliate=news&amp;cat=article&amp;type=logo300x250&amp;location=13" width="300" height="250" frameborder="no" border="0" marginwidth="0" marginheight="0" scrolling="no"></iframe></div>yyyyyy

```

i want to remove the html between xxxxxxx and yyyyyy, and return 

```
xxxxxxxyyyyyy 

```

how to set regular expression using c# and could you please describe why use the regular expression ?
thanks.