Need Help with iRule

regex should be avoided at all cost. A simple string map would be appropriate here. Just in the tcl shell, this is accomplished with an lindex/split (you can use getfield in an iRule which is just an lindex/split shortcut):

% set x "/t/BA/CkA/E2I/ABqYWA/AA/MTUxMzV8aHR0cHM6Ly9wcm8ubW9uZXltYXBwcmVzcy5jb20vbS83ODM4MzM-ZW1haWw9Sk9ITiU0MDE0d2VzdGl0Lm9ubWljcm9zb2Z0LmNvbSZhPTQmbz0yNjI0JnM9NDk2MiZ1PTE3NDI5MzYmbD0xNTEzNSZyPU1DMiZ2aWQ9NDJjTGZKJmc9MA/AQ/fWN2"

/t/BA/CkA/E2I/ABqYWA/AA/MTUxMzV8aHR0cHM6Ly9wcm8ubW9uZXltYXBwcmVzcy5jb20vbS83ODM4MzM-ZW1haWw9Sk9ITiU0MDE0d2VzdGl0Lm9ubWljcm9zb2Z0LmNvbSZhPTQmbz0yNjI0JnM9NDk2MiZ1PTE3NDI5MzYmbD0xNTEzNSZyPU1DMiZ2aWQ9NDJjTGZKJmc9MA/AQ/fWN2

% set y [lindex [split $x /] 7]

MTUxMzV8aHR0cHM6Ly9wcm8ubW9uZXltYXBwcmVzcy5jb20vbS83ODM4MzM-ZW1haWw9Sk9ITiU0MDE0d2VzdGl0Lm9ubWljcm9zb2Z0LmNvbSZhPTQmbz0yNjI0JnM9NDk2MiZ1PTE3NDI5MzYmbD0xNTEzNSZyPU1DMiZ2aWQ9NDJjTGZKJmc9MA

% string map [list ${y} ${y}.] $x

/t/BA/CkA/E2I/ABqYWA/AA/MTUxMzV8aHR0cHM6Ly9wcm8ubW9uZXltYXBwcmVzcy5jb20vbS83ODM4MzM-ZW1haWw9Sk9ITiU0MDE0d2VzdGl0Lm9ubWljcm9zb2Z0LmNvbSZhPTQmbz0yNjI0JnM9NDk2MiZ1PTE3NDI5MzYmbD0xNTEzNSZyPU1DMiZ2aWQ9NDJjTGZKJmc9MA./AQ/fWN2

In the iRule, it will consolidate to something like this:

set uri_chunk [getfield [HTTP::uri] / 8]
string map [list ${uri_chunk} ${uri_chunk}.] [HTTP::uri]