Shell Programming and Scripting

I need to change R3.1.5 as 03015
similarly R4.1.7 as 04017

i need a single command
in tcl pls help

---------- Post updated at 05:19 PM ---------- Previous update was at 04:48 PM ----------

i had to do like this without using regexp
set old_release "R3.1.5"
set old_release [string replace $old_release 0 0 0]
set old_release [string replace $old_release 2 2 0]
set old_release [string replace $old_release 4 4]

---------- Post updated at 05:19 PM ---------- Previous update was at 05:19 PM ----------

i need in regexp as a single command

---------- Post updated at 05:53 PM ---------- Previous update was at 05:19 PM ----------

can anyone help???