question about find and replace

[Peter Alcibiades]

How do you pad out a series of numbers with leading zeros?  Like for instance 
the series is

.1.
.2.
.3.
.11.
.42.
.98.

and you want them to be

.001.
.002.
.003.
.011.
.042.
.098.

I know how to find them, using the fact that they appear as shown between 
two . characters,  but then I don't know how to use regular expressions to 
put part of what has been found back along with the padding zeros.  Or should 
you not be trying to use regex at all?  I seem to recall something about look 
ahead and greediness but never really understood it, is this relevant?

Peter