Let me give an example with some pictures:

The bytes are continuous in the file. To make it clear in the picture, I broke the stream at 0D0A.

The data fields are fixed length in bytes. The red Solid lines are where I want to insert the delimiter bytes.

It would be best if there is a way to insert the delimiter bytes into the byte stream directly. But I don’t know such a way (which is what I exactly want). So currently I have to open the files with a text editor and handle the text by characters.

The data fields are fixed length in bytes. But the number of characters could be different. (I highlighted some corresponding bytes and characters with different colors.)

UltraEdit’s Convert to Character Delimited command can only handle the text by characters. (And I don’t know if regular expressions can handle by bytes.) So I insert ! next to the multi-byte characters according to the numbers of the bytes.
Now the number of characters is equal to the number of the original bytes, which lets UltraEdit’s Convert to Character Delimited command insert the delimiters to the right positions.

Any better solutions/tools are welcome.

The example text and its Hex(UTF-8):
(Column Width: 8,30,15,10,13 bytes)

Field 1 Field 2 ăĕĭŏŭ âêîôû Field3(15bytes)Field 4   Field 5, etc.
123     Any unicode string without tab诸如此类             Field 5, etc.
12345678[---This field is 30 bytes---]エトセトラ[10 bytes]Field 5, etc.


By the way, about the picture above, if someone would be interested:
All the colorful highlightings and lines in the text are done within EmEditor, not by an image editor.

Thank you, Yutaka, for the new features.
The User-Defined Guides are useful.