How to sed and put back the part of the match?

Question

So these are the original texts:

$ printf 'bbb\nb2b\n'
bbb
b2b

This is what I'm doing

$ printf 'bbb\nb2b\n' | sed 's/^b[0-9]/XXX/g'
bbb
XXXb

And this is the output that I want :D

$ printf 'bbb\nb2b\n' | SOME_SED_MAGIC
bbb
XXX2b

My question: How can I swap the "b2b" to "XXX2b", so I put back the "2" from the sed match? "2" could vary.

Answer 1

You can use backreference:

$ printf 'bbb\nb2b\n' | sed 's/^b\([0-9]\)/XXX\1/'
bbb
XXX2b

^{(BTW, you don't need the g flag since that regex can match only once because of the ^).}