Here is one example, please share others:
the URI is the stuff after the URL:
Example:
www.cisco.com/index.cgi?name=billy
The URI is: /index.cgi?name=bily
The host field in http header is:
www.cisco.com
So look for [Ww][Ww][Ww][.][Cc][Ii][Ss][Cc][Oo][.][Cc][Oo][Mm] in the header section and if you know the rest of the URL you can append that section in the URI:
[\x2f\x5c][Ii][Nn][Dd][Ee][Xx][.][Cc][Gg][Ii]