About linux : Bash-sed-command-insert-between-special-characters-duplicate

I am quite new to Bash and sed.

I have a file named test.xml. This looks like this:

variable1=”” variable2=”” variable3=””

This is only one line of the file there is more above and under of this line.
My goal is that at the end with sed the line looks like this:

variable1=”” variable2=”123″ variable3=””

I tried with this command: sed -i 's/\bvariable2="\b/& 123/' test.xml
But this didnt work. Any ideas?

may be :

sed -i '' 's/variable2=""/variable2="123"/g' test.xml