substitution

[superpollo]

it looked simpler when i posted, but i realize that the problem is non 
trivial.

thanks to everybody.

i guess that the algorithm would be easier if it was known in advance 
that the string to substitute must have some specific property, say:

1) they all must start with "XYZ"
2) they all have the same length N (e.g. 5)

like this:

qweXYZ12asdXYZ1345XYZ ---> qweIWAS12asdIWAS1345XYZ

bye