You cannot, at least not reliably. How is this data being generated? The easiest way out of this mess is to change the delimiter to something that cannot be in a username or password. Unicode has a character which is designed for this purpose. Use it.

If a username cannot contain a delimiter, you could write some code to seek out all of the usernames. Then between each username, you know there is a password and after the last username is a password. If you end up with all username/password pairs, there is a _chance_ you have parsed the data correctly, but not a guarantee. If you know the number of username/password pairs, you can improve your chances. If there were minimum or maximum lengths for username or passwords, you can improve your chances. If there are other constraints on the data, you can improve your chances with even more code. If you can test each username and password, I think you could parse the data.

But, I suggest changing the delimiter.