Can you please let us know the regex variant you are using - unfortunately they are not all the same and the pattern I have suggested below may or may not work for you.

I think I have come close with:

(\x20\x20|^)E(?!/)[\w/]*(\x20\x20|\r?$)

with the 'multiline' options set (and the 'ignore case' not set - it doesn't matter with the example you have provided but it might in some other situations).

This will pick up the following:

E9/C
E
E
Esus
E
E9/C
Emaj

As you will see, these are the same as your examples EXCEPT that it also picks up the "Esus". If this is not to be picked up, then you will need to tell how we can differentiate these cases: is it as simply as "always ignore 'sus'" or is it more complex than that.

The way I'm doing this is:

(\x20\x20|^) - match either 2 consecutive spaces or the start of a line
E               - match the key letter
(?!/)            - make sure that the next character is NOT "/" (to prevent the "E/B" matching)
[\w/]*          - any number (including zero) of digits, letters and the "/" character
(\x20\x20|\r?$) - match either 2 consecutive spaces or the end of a line (the '\r?' is there to cover some environments where the end of a line is marked by "\r\n")

Susan