Pad one-digit, two-digit and three-digit numbers with zeros with sed

Question

I'm programming a bash script that pads one-digit, two-digit and three-digit numbers with zeros. For example:

file.txt:

2
22
222
2222
a2a2
2a2a
a22a
2aa2
22a2

output.txt:

The purpose of this script is to fill the digits until they reach a maximum of 4 digits.

I have to do it with the sed command. I thought about doing it like this, but it doesn't work like I want.

sed -r -i 's/[0-9]{1}/0&/g' file.txt

`I have to do it with the sed command` - why? sed is completely the wrong tool for the job. Its like saying you have to plant a tree using just a screwdriver. — Ed Morton, Oct 05 '17 at 16:34
Maybe they're on an embedded system with `sed` but no AWK (I know, I know, busybox has both). Or a Windows system with a `sed.exe` binary, but no AWK. Maybe it's homework for a Unix programming class. Maybe they just want to stretch their brain, or get really good with `sed`. I mean, why on _earth_ did some guy write [an RPN calculator in `sed`](http://sed.sourceforge.net/local/scripts/dc.sed.html) when nearly every Unix already has `dc`? — TheDudeAbides, May 04 '19 at 21:58

score 4 · Answer 1 · answered Oct 05 '17 at 16:43

$ sed -E 's/([[:digit:]]+)/000&/g;s/0+([[:digit:]]{4})/\1/g' file.txt
0002
0022
0222
2222
a0002a0002
0002a0002a
a0022a
0002aa0002
0022a0002

The first substitution prepends 000 to every group of digits in the line. The second substitution removes zeroes from the beginning of every group of digits until there are only four digits left.

The intermediate output after the first substitution is thus

$ sed -E 's/([[:digit:]]+)/000&/g' file.txt
0002
00022
000222
0002222
a0002a0002
0002a0002a
a00022a
0002aa0002
00022a0002

and the second substitution removes the extra zeroes.

RomanPerekhrest · Answer 2 · 2017-10-05T20:59:56.133

3

GNU awk solution:

awk -v FPAT='[0-9]+|[^0-9]+' '{ 
           for (i=1;i<=NF;i++) printf ($i~/[0-9]/? "%04d":"%s"),$i; 
           print "" 
       }' file.txt

The output:

edited Oct 05 '17 at 20:59

answered Oct 05 '17 at 16:22

RomanPerekhrest

88,541
4
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OP asked for a solution using `sed`. – TheDudeAbides May 03 '19 at 08:05
@TheDudeAbides, you could first establish if the "OP" notion is still actual/relevant here. – RomanPerekhrest May 03 '19 at 08:14

Pad one-digit, two-digit and three-digit numbers with zeros with sed

2 Answers2