You could try it with Convert Tag -> Tag to see the immediate preview:
Field: TITLE $regexp(%TITLE%,(.*)\/(.*),$1)
would leave the part in front of the slash /
and $regexp(%TITLE%,(.*)\/(.*),$2)
would leave the part after the slash /
Please be careful with values that contains a slash itself, like AC/DC or This/or/that/or/something/else
It seems that you don't have to escape the slash in Mp3tag, so even $regexp(%TITLE%,(.*)/(.*),$1)
should work.
If you are not not too friendly with regular expression, you could also use an action of the type "Guess value"
Source string: %title%
Guessing format string: %artist%/%title%