sed to insert a character

Hi all,

I have a file like this

Q8N302
21-84
Q8N157
15-45
Q99996
167-201
202-251
269-318

I want to insert a character or space if the line starts with a number and I used the command

sed 's/^[0-9]/#/'

But in the output file, when it inserts this character, first digit in the number is deleted. How can I change the code so that I dont lose any digits. The output I am getting is given below

Q8N302
#1-84
Q8N157
#5-45
Q99996
#67-201
#02-251
#69-318
#20-321
#22-323

Use '&' to insert matched string:

sed 's/^[0-9]/#&/'

try also:

sed '/^[0-9]/s/^/#/'