How to add zero-padding to a folder full of numbered files

Have you got a folder of files that look like this?

file_1.png
file_2.png
file_3.png
file_10.png
file_11.png
file_12.png
file_100.png

But what you really want this:

file_001.png
file_002.png
file_003.png
file_010.png
file_011.png
file_012.png
file_100.png

Then, my linux-loving friend, run this:

rename 's/\d+/sprintf("%03d",$&)/e' *.png