Copy and Edit Data in File

Question

I've just started C# and have little experience with programming. I've browsed trhough many topics in this forum but none seem to be exactly what I'm looking for...

I want to create a tool which compares to files, not in bytesize but in by content. I've managed to create a tool who does so, but he compares complete files. I'm unable to create a way the tool stops when a certain phrase comes up. So I had the idea to create two temporary files which I compare, but here I have the same problem. I'm unable to copy a certain block of data.

Is there anybody who can help me with my problem?