For the following HTML document:
<html>
<body>
<a href="http://www.example.com">Example</a>
<a href="http://www.stackoverflow.com">SO</a>
</body>
</html>
The xpath query /html/body//a/@href (or simply //a/@href) will return:
http://www.example.com
http://www.stackoverflow.com
To select a specific instance use /html/body//a[N]/@href,
$ /html/body//a[2]/@href
http://www.stackoverflow.com
To test for strings contained in the attribute and return the attribute itself place the check on the tag not on the attribute:
$ /html/body//a[contains(@href,'example')]/@href
http://www.example.com
Mixing the two:
$ /html/body//a[contains(@href,'com')][2]/@href
http://www.stackoverflow.com