Ah, I didn't even think of insert, that'd make it even easier!
PLC's are generally not geared toward string handling at all. They can do it, but it's never been a focus, because why would you do string handling in a PLC when you could do it in the HMI/OIT/software? You'd need a HMI/OIT/etc to allow for string manipulation in the first place, so it makes sense to do it at that level.
I'm curious now - I can't think of any likely reason to need a string search-and-replace function within a PLC - what's your application?