http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/subst.htm
What's the use of subst? As Joe English, who has deep experience in the area, explains, "[subst] is massively handy in text-processing applications, especially SGML and XML down-translators. [subst] and [ string map] make Tcl particularly well-suited for this type of task." Many jobs for which Perl uses "right-hand-side" variables with regular expression substitution, Tcl does as REs whose results are subst-ituted.
Very simple example of using [subst] with XML/HTML.
set html {<html><head>$title</head></html>} set title "Hello, World!" set output [subst -nocommands $html] set output <html><head>Hello, World!</head></html>
Another alternative would be to use XPath I use something like this in an application of mine. Of course the "text" of the 'html' variable above would probably come from a file.
I would much rather use XPath to hunt down the place to change.
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command