Sorry, I focused on the Regex part of your post. I've been playing with too many MC functions and Regex lately.
The "Library Tools -> Find and Replace" function will do what you want, without the need for Regex. Just select the [Artist] field and set the Find to " &, " and Replace to ";" all without quotes, but note the spaces. You can apply that to just one file, and if it works, apply it to many. Then repeat for all variations you want to fix.
What I'm talking about is the ability of MC to update fields themselves by editing the field and entering an Expression, prefixed by an equals sign, into the field. What I would call "In Place Editing". MC reads the content of the field, applies the Expression to it, and write it back to the field. It's a neat trick, and the way I do most edits of that type. Hence I didn't think of the "Find and Replace" function when I read your thread.
One advantage of using the "Find and Replace" function is that you don't have to worry escaping certain characters like "&" and "," using the "/" Escape Character, which is why my expression included "/&/," which escaped the "&" and "," characters, preventing them from being treated as part of the expression function.
The attached image shows another example using your " feat., " variant, which escapes the spaces either side so they are also removed.