pattern replacement

[Vulpes Velox]

If I wanted to replace every instance of a pattern in a file with
something, what would I use?

Sorta like doing a include in a html file and then something goes
threw and replaces that include with the file that include pointed it
at?

Sorta want something to make web admining easier and I am pretty sure
this all ready exists in some form...

Any ways, much thanks for any help offered.
-VV