Thanks for your comments,

Something I failed to place in my original query was that I was aware of certain things about the files I'd be looking at:

A) They are all files for a webserver, each subdirectory of the root directory are separate domains.

B) I could and probably should have done a quick test on the file name to make sure I only changed .html files however this is hindsight.

C) Due to A) I knew none of the files would be bigger than a few hundred kilobytes at most and the server has plenty more than that in physical RAM.

Thanks again,
James