Since you refused to answer most of my questions, I will make one final attempt at trying to handle a general case. Since you didn't give enough information about the desired output format, I'll make some code that reads input strings and produces output strings with the following characteristics:Since you refused to specify the length of the desired ending zero filled decimal digit output string, I'll use awk for this example to avoid reading the input file twice:

Code:

awk '
BEGIN {	m = 1
}
{	tail[NR] = substr($0, match($0, /[[:digit:]]*$/))
	head[NR] = substr($0, 1, RSTART - 1)
	if(RLENGTH > m)
		m = RLENGTH
}
END {	for(i = 1; i <= NR; i++)
		printf("%s%0*d\n", head[i], m, tail[i])
}' file

If file contains:

Code:

S1
S2
S12
S21
sk1
sk12
sk321
sk1344
strange1prefix2long99
123

it produces the output:

Code:

S0001
S0002
S0012
S0021
sk0001
sk0012
sk0321
sk1344
strange1prefix2long0099
0123

and if file just contains the 1st four input lines shown above, the output would be:

Code:

S01
S02
S12
S21