Here's an even faster version than the binvar one (more than twice as fast), using $regsubex():
alias rev var %a = $1 | return $regsubex(%a,/./g,$mid(%a,-\n,1))
The variable is there because of a $regsubex() bug, when it's fixed it could be as short/simple as
return $regsubex($1,/./g,$mid($1,-\n,1))
$regsubex() has a lot of potential, with a little imagination you can even do things unrelated to pattern matching
Edit: FiberOPtics pointed out that the alias kills spaces... as you know $mid() does retain them, it's whatever's being done internally in $regsubex() that kills them 
I want to believe that those $regsubex() problems will be ironed out eventually though.
