Port Translation

You can replace anything in the Header if you know specifically what your looking for and want to replace it with.

The first example that I gave searches for a string "server1" and replaces it with a string "test1".

when HTTP_RESPONSE {
    if { [HTTP::is_redirect] } {
        HTTP::header replace Location [ string map  {"server1" "test1"} [HTTP::header Location] ]
    }
}

Or you could do something like what hoolio did and replace the URI instead of the Host:

Replace Host (URI::host) = http://devcentral.f5.com/wiki/default.aspx/iRules/URI__host.html

Replace Path (URI::path) = http://devcentral.f5.com/wiki/default.aspx/iRules/URI__path.html

 
when HTTP_RESPONSE {

    Check if response is a redirect
   if {[HTTP::is_redirect]}{
      HTTP::header replace Location [string map -nocase "[URI::path [HTTP::header Location]] /some/value/index.html" [HTTP::header Location]]
   }
}

If you are looking to do replacements in the body, you can do that too. The Stream Profile or Stream Expression is a good place to look for ideas:

http://devcentral.f5.com/wiki/default.aspx/iRules/stream__expression

http://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/241/iControl-101--18--Stream-Profile.aspx

http://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/101/LTM-stream-profile-Multiple-replacements-regular-expressions.aspx