xsl code to find and replace a text in all the anchor tags

xsl code to find and replace a text in all the anchor tags

I am a newbie to xsl. I am trying to do the following thing.
I want to convert all the 'a href="http://a.com//aaaa/index.html">AAAABBBB
Scenario: 1. I am reading the content from an source xml file into my xslt
file. There could be multiple entries in the content for links. 2. After
reading the xml file, I want to change all the above links matching only
'aaaa' to 'bbbb'. 3. Hence, the output is the user when clicks the link
from my page, he should be going to the 'bbbb' site.
Hope its clear!