:-D

Ok, let's take it a a little at a time:

s! you know, although it's possible you didn't know you can use pretty much any character for the expression delimiters.

(?:^|/) matches (without capturing) either the start of the string or a /.

[^\s,@;]* matches as many characters that aren't in the set of terminal characters as can be found.

(?<=/) looks back and asserts the last character matched was /.

([^\s,@;]+?) matches and captures as few non-terminal characters as it can and still find a match. That's the filename that you want.

(?=[\s,@;]|$) looks ahead and asserts that the next character is a terminal character or the end of the string.

!$1!g you are probably completely familiar with - replace all the matched stuff with the captured string and do it for every match that can be found.

So with a little head scratching the introductory line of my initial reply might make a more sense along with the regex. For further study consult perlretut, perlre and perlreref.