Need to parse the [HTTP::uri] and modify it for a redirect

could you please explain what exactly the code is doing especially the regular expression part and the string map ?

/~ is literal chars (i.e. it matches /~)

%[^/] matches every chars but not / and the matching string will be stored in variable specified later (e.g. it matches geverest and geverest will be stored in first variable)

[scan [HTTP::uri] {/~%[^/]} first] returns number of matching string. so, we can use it to determine whether it is uri we are interested (i.e. 1).

[string map "/~$first/ /" [HTTP::uri]] replace /~$first/ with / in uri (e.g. it replaces /~geverest/ with /).

[root@bip1a:Active:In Sync] config  tclsh
% set uri "/~geverest/home/data-modeling-course/course-flyer"
/~geverest/home/data-modeling-course/course-flyer

% scan $uri {/~%[^/]} first
1

% put $first
geverest

% string map "/~$first/ /" $uri
/home/data-modeling-course/course-flyer