How to mask a String to show only the last 3 characters?

I just tried to mask String as below, but I didn't find what I want after do some search and research.

string a = "0123456789" masked = "xxxxxxx789"

I modified solutions in this questions http://stackoverflow.com/questions/41224637/masking-first-and-last-name-string-with but it just change the String that doesn't match with the pattern. I have no idea how to change the pattern to match with what I mean.