我可以使用什么正则表达式从未格式化的文本体中提取XML文本体?

假设我有以下正文:

Call me Ishmael. Some years ago- never mind how long precisely- having little 
or no money in my purse, and nothing particular to interest me on shore, I 
thought I would sail about a little and see the watery part of the world. It is  
<?xml version="1.0" encoding="utf-8"?>
<RootElement xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
     xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <ChildElement />
   <ChildElement />
</RootElement>
a way I have of driving off the spleen and regulating the circulation. Whenever  
I find myself growing grim about the mouth; whenever it is a damp, drizzly 
November in my soul; 

我可以使用什么正则表达式来返回嵌入在字符串中的XML?

注:我可以假设 <RootElement> 和 </RootElement> 总是有相同的名字。