RegexReplace is looking for a regex expression to use for matching. Your task could have been done using a Replace as Thomas indicated, but for more complex matching you may need a regex expression. Entering \t will match the first tab, but you want to match all the tabs so you need to enter \t+ as your regex expression which will match all of the tabs.
@jeffbadger Thanks Jeff. I did some research and created a sample solution and it looks like \t should work (even without the + sign). I am not sure why Jonathon's example isn't working though as it certainly does in the attached example. I believe I misspoke earlier in suggesting a script as this functionality should work out-of-the-box with the StringUtils.