RegEx to find and replace a word

Question

Hi all
I need a RegEx that allow me to search and replace
words chosen by me (eg. "some" instead of "same").
However, replacement should not happen if the word to replace
represents a resource (a file);
i.e. If some.jpg appears in the text, I don't have to replace anything.
How can I write such a thing?
Thanks in advance.

Luigi