Rudder

regex

 sed "s/\([^&]\)RudderUniqueID\([^&]\)/\1\&RudderUniqueID\&\2/g" 

4 seds work but seems a bit overkill

sed "s/\([^&]\)RudderUniqueID\([^&]\)/\1\&RudderUniqueID\&\2/g" test.st | sed "s/^RudderUniqueID\([^&]\)/\&RudderUniqueID\&\1/g" | sed "s/\([^&]\)RudderUniqueID$/\1\&RudderUniqueID\&/g" | sed "s/^RudderUniqueID$/\&RudderUniqueID\&/g" 

sed "s/\([^&]\|^\)RudderUniqueID\([^&]\|$\)/\1\&RudderUniqueID\&\2/g"