Thanks Yemi Bedu,
Except that the Dog value is replacing my whole search string in my string. What I got is the equivalent of <get_eco>dog<get_eco>boydog
I did however solve the problem using another variable -replace1- for example containing the -dog- value, Then I used
<get_eco>%replace1%<get_eco>
as my replaceString value and checked the Perform variable substitution box. and I got:
<get_eco>dog<get_eco>boy<get_eco>dog<get_eco>
Thanks a lot for your help and the references.