New feature in #Tcl9 – part 33:

regsub -command ... — Generate the replacement for a regular expression by calling a command

Sometimes, a simple `regsub` is not good enough. You may need to do more than just substituting matches into placeholders. That is what the new `-command` option is good for. You can provide a command that takes the matches (substrings) and does with them want you want to happen!