Sed append to end of line directory - bash

Question

I'm very new to SED and I'm having a hard time trying to append to an end of a directory. What I'm doing involves 2 basic things with sed but for some reason, no changes are made after the script runs. I will show segments of my script
I have a bash script that pulls my home directory from the host and I define the ID variable.
USERNAME="test"
#pull the home directory
dir=$(ssh -n -t $SERVERNAME "echo \$HOME";)
the above example will store /export/home/ID in the dir variable
echo $dir | sed 's/\([/export/home]*\).*/\1/' > olddir
the sed command above stores /export/home/ in the file olddir (takes off the ending)
sed -i 's_/home/$ _\$USERNAME_' olddir
i am now trying to change /export/home/ to /export/home/test using the defined variable with the escaped $.
after the script runs, it still has /export/home/ as the entry in the olddir file.
I'm using the -i to modify the file and I think I'm using the deliminators correctly? what could I be doing wrong? i even took off the $ from the USERNAME variable which didn't do anything. I know I'm missing something small but i just can't figure it out. I really appreciate your time to answer my question.

Answer 1

I think your command line can be modified then redirect output to olddir as follows:
echo $dir | sed 's#\(/export/home\).*#\1#' > olddir
to add the USERNAME VARIABLE
sed -i "s#/export/home#&/$USERNAME#" olddir

Answer 2

After using the provided information and playing around with the code, I found a solution that will work for both /export/home/ID and /home/ID situations.
I used both Xorg and also gniourf_gniourf's suggestions and below is my result.
echo $dir | sed 's_\([/export/home\]*\).*_\1_' > olddir
The above code is the first part of my solution.
I used what Xorg provided for the above code but it looks like i need those [] so i can use the *\ if i have it as echo $dir | sed 's_\(/export/home\).*_\1_' > olddir then the olddir file will contain /export/home/ID/test after the following sed command is used.
Here is the following sed command that I used:
sed -i 's_/home_&/'"$USERNAME"'_' olddir
The above code is the second part of my solution. I figured that I really only need to put focus on /home/ since i'm appending after it. I looked at gniourf_gniourf's comment and used the example where the quotes to isolate $USERNAME with "" that seems to be the way you can tell the script to use $USERNAME as a variable and not just put in the characters $USERNAME.
so after the script is run, depending on the host, I either have export/home/test/ or /home/test/ I can now put either of these into a new variable on the script to use to specify the home directory when creating ID's on remote hosts!
newdir="$(cat olddir)"
Thank you all so much for your help. I wouldn't have been able to figure this out with out your help.
Update: it turns out that the traling / at the end of directory is problem so i found a much easier way to replace the ID in the home directory
newdir="${dir/ID/${USERNAME}}"
i used these if statments for both home directory situations
`if [[ $dir == "/home/dhabinsk" ]]; then
newdir="${dir/dhabinsk/${USERNAME}}"
fi
if [[ $dir == "/export/home/dhabinsk" ]]; then
newdir="${dir/dhabinsk/${USERNAME}}"
fi`
cheers