With some help from one of my friends, I was able to get it to work using this line!

sed 's/(MIMG109AL1ARP..*)($)/\1\twhatIwant\2/g'

The solution was to break up the expression into two sub expressions. Instead of changing what I already had, I was able to flank what I wanted between the two sub-expressions.

Thank you both for posting and trying to help me out.

With some help from one of my friends, I was able to get it to work using this line!

sed 's/\(MIMG109AL1ARP..*\)\($\)/\1\twhatIwant\2/g'

The solution was to break up the expression into two sub expressions. Instead of changing what I already had, I was able to flank what I wanted between the two sub-expressions.

Thank you both for posting and trying to help me out.