Batch rename sequential files by padding with zeroes

Question

I have a bunch of files named like so:

output_1.png
output_2.png
...
output_10.png
...
output_120.png

What is the easiest way of renaming t

Answer 1

$rename output_ output_0 output_?   # adding 1 zero to names ended in 1 digit
$rename output_ output_0 output_??  # adding 1 zero to names ended in 2 digits
$rename output_ output_0 output_??? # adding 1 zero to names ended in 3 digits

That's it!