Skip rewriting of External URLs

As Bhattman said, I think the last example is closest for doing validation of the links. If you provide more exact (but anonymized) examples of the response content you do and don't want to rewrite, we can give you more specific ideas on how to implement it. You could potentially have a whitelist of domains in a string datagroup and then use the class command to check the domains in the STREAM_MATCHED event:

 From: http://devcentral.f5.com/Wiki/default.aspx/iRules/STREAM__expression
when HTTP_REQUEST {
    Disable the stream filter for all requests
   STREAM::disable
}
when HTTP_RESPONSE {  

    Check if response type is text  
   if {[HTTP::header value Content-Type] contains "text"}{  

       Match an http://*example.com string and replace it with nothing yet
      STREAM::expression {&http://.*?example\.com&&}

       Enable the stream filter for this response only  
      STREAM::enable  
   }  
}   
when STREAM_MATCHED {  

    Check if the matched string meets some condition that can't easily be checked for using a single regex in STREAM::expression
   if {[STREAM::match] starts_with "host1"}{

       Replace http:// with https:// and do the replacement
      STREAM::replace "[string map {http:// https://} [STREAM::match]]"
      log local0. "[IP::client_addr]:[TCP::local_port]: matched: [STREAM::match], replaced with: [string map {http:// https://} [STREAM::match]]"  
   }
}

http://devcentral.f5.com/Wiki/default.aspx/iRules/class

Aaron