Try Notepad++ or EditPlus. They work independent of notepad but they have additional features including the one you asked for. If want any open source alternative, try vim for windows.
If you want to develop an exe yourself, here is what you need to do (assuming c++)
1) open the file to modify (no notepad needed use IOstreams)
2) use a string matching algorithm( first try using Brute Force, for efficient algo use KMP string matching algo)
3) move file pointer to beginning of the word and replace it
4) save file to disk
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