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, 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."
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command