Using Perl's 'rename' to pad filenames with leading zeros

Here is what I have currently:

rename -n 's/(\d+)\.txt/sprintf "%03d", $1/e' ./*.txt

This adds leading zeros but unfortunately drops an extension:

rename(./1.txt, ./001)
rename(./2.txt, ./002)
rename(./3.txt, ./003)

I tried the following, but this didn't help:

• rename -n 's/(\d+)(\.txt)/sprintf "%03d", $1/e$2' ./*.txt
• rename -n 's/(\d+)(\.txt)/(sprintf "%03d", $1/e)$2' ./*.txt

How to make it work?

I believe your rename drops the extension because you tell it to. That is, your 's/(\d+)\.txt/sprintf "%03d", $1/e' replaces \d+\.txt with the results of sprintf "%03d", $1, which do not include the extension.

All you really need to do is replace the integer part of the file name with the zero-padded version. Something like

rename 's/(\d+)/sprintf "%03d", $1/e' *.txt

should do the trick.