Prepend leading zeros to each line of a file

I have a file that looks like this:

1:line1
14:line2
135:line3
15:line4

I need to prepend leading zeros to each line to make it look like this:

00001:line1
00014:line2
00135:line3
00015:line4

Is there an easy way to do this in Linux?

I have tried using

awk '{printf "%05d:%s\n", FNR, $0}' file

but this outputted:

00001:1:line1
00002:14:line2
00003:135:line3
00004:15:line4

I should note I did not write this command, I got it from Google and don't really understand how it works

Solution

There are many ways, one way is to use awk

awk -F":" '{OFS=FS; $1 = sprintf("%05d", $1); print}' "${filename}"

To break it down:

-F":" set the field seperator to ":", awk will split the lines into columns for each :.
OFS=FS set the output field separator to the field separator, this essentially puts ":" back into each line when we output it.
$1 = sprintf("%05d", $1) set the first column, $1, to be itself padded with 0's and of length 5.
print print the line.