Development of a "find and replace" function in external file

The developer shall develop a stand-alone function that reads from an external file and replaces specific parts of code therein with given variables before including it.

For example:

(input) string code_old="OLD";
(input) string code_new="NEW";

// (if NEW file exists, skip to #include)

// open OLD file?
// 
find and replace in OLD file?
// write NEW file?

#include "NEW file"

and in external OLD file (i.e. before the function):

OLD_1
OLD_2
OLD_3
OLD_4

and in external NEW file (i.e. after the function):

NEW_1
NEW_2
NEW_3
NEW_4

In order to qualify for the selection, the developer shall submit a simplified solution to demonstrate the feasibility of his/her code.