I don't really even know where to start on this...

A new SoPW. And a much clearer description of

1. The contents of the file and its formatting.

   With preferably an example of the data that contains typical contents and at least 3 records that match. If that means it would be too big to post here, then making an example available off-site shouldn't be too hard.

2. What you are actually searching for.

   Ie. less "(Say, all blocks of character 0x20 that are at least 12 bytes long)" & "say, they shouldn't overlap between groups of 1000 byte" & "(say, the ASCII characters "U S E D" followed by the block length)".

3. And what you want to do once you've found them.