SED substitutions: use the original pattern in the substitution part

Question

How can I use sed to match the search 'pattern' and reuse it again in the 'substitution' part?

Example: I want the command:

echo "12a34" | sed 's/a/#bcd/g'

to return

12abcd34

What should be the # parameter to get the right substitution?

Answer 1

As already commented, the solution is to take the & character in the substitution part:

echo "12a34" | sed 's/a/&bcd/g'

Like this the & is just replaced by the a in the search pattern.