regsub is a command that matches the regular expression exp against string in TCL.
This command matches the regular expression exp against string, and either copies string to the variable whose name is given by varName or returns string if varName is not present. (Regular expression matching is described in the re_syntax reference page.) If there is a match, then while copying string to varName (or to the result of this command if varName is not present) the portion of string that matched exp is replaced with subSpec. If subSpec contains a “&” or “\0”, then it is replaced in the substitution with the portion of string that matched exp. If subSpec contains a “\n”, where n is a digit between 1 and 9, then it is replaced in the substitution with the portion of string that matched the n'th parenthesized subexpression of exp. Additional backslashes may be used in subSpec to prevent special interpretation of “&”, “\0”, “\n” and backslashes. The use of backslashes in subSpec tends to interact badly with the Tcl parser's use of backslashes, so it is generally safest to enclose subSpec in braces if it includes backslashes. If the initial arguments to regsub start with - then they are treated as switches.
Read more: http://www.tcl.tk/man/tcl/TclCmd/regsub.htm
