irb> (path, name) = File.split("bla/bla/bla/Prefix@invoice.csv")
=> ["bla/bla/bla", "Prefix@invoice.csv"]
irb> (prefix, postfix) = name.split("@")
=> ["Prefix", "invoice.csv"]

>> wpath="blah\blah\blah\Prefix@invoice.csv"
=> "blah\blah\blah\Prefix@invoice.csv"
>> upath="bla/bla/bla/Prefix@invoice.csv"
=> "bla/bla/bla/Prefix@invoice.csv"
>> r=Regexp.new(".+[\\/]([^@]+)@(.+)")
=> /.+[\/]([^@]+)@(.+)/
>> wpath.match(r)
=> #<MatchData "blah\blah\blah\Prefix@invoice.csv" 1:"Prefix" 2:"invoice.csv">
>> upath.match(r)
=> #<MatchData "bla/bla/bla/Prefix@invoice.csv" 1:"Prefix" 2:"invoice.csv">

.+[\/]([^@]+)@(.+)

".+[\\/]([^@]+)@(.+)"

>> evil_path="/foo/bar@baz/blorp/Prefix@invoice.csv"
=> "/foo/bar@baz/blorp/Prefix@invoice.csv"
>> evil_path.match(r)
=> #<MatchData "/foo/bar@baz/blorp/Prefix@invoice.csv" 1:"Prefix" 2:"invoice.csv">

var fileName = "file.txt"
var filePath = new RegExp('(.+)[\\/]' + fileName + '$')