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

Fairly easy, although it combines a few features not immediately obvious:

@echo off
setlocal enableextensions enabledelayedexpansion
rem iterate over all PNG files:
for %%f in (*.png) do (
    rem store file name without extension
    set FileName=%%~nf
    rem strip the "output_"
    set FileName=!FileName:output_=!
    rem Add leading zeroes:
    set FileName=000!FileName!
    rem Trim to only four digits, from the end
    set FileName=!FileName:~-4!
    rem Add "output_" and extension again
    set FileName=output_!FileName!%%~xf
    rem Rename the file
    rename "%%f" "!FileName!"
)

Edit: Misread that you're not after a batch file but any solution in any language. Sorry for that. To make up for it, a PowerShell one-liner:

gci *.png|%{rni $_ ('output_{0:0000}.png' -f +($_.basename-split'_')[1])}

Stick a ?{$_.basename-match'_\d+'} in there if you have other files that do not follow that pattern.