I think you were talking about this one:

$path =~ /^(\w):\\([^\\]+)\\?/;
[download]

In this one [\\] would match just backslashes, but with the ^, [^\\], matches all characters Except backslashes, and because of the +, it matches as much as possible up to the 1st backslash. $2 is populates, because the character class (ie. [^\\]) is in parenthesis, which happens to be the second set of them, so therefore is stored in $2