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."
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. However 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