replacing a space with an _ is a bad idea. What if I want to reverse "this_is_a_test" you've just changed all those _ to spaces giving a result of "tset a si siht" which is incorrect. Instead, you should take advantage of the fact that mIRC does allow you to have a space at the beginning of a variable, just not at the end and therefore you can use:
alias reverse {
var %x,%i = 1
while ($mid($1-,%i,1)) {
%x = $ifmatch $+ %x
inc %i
}
return $strip(%x)
}
That is obviously going to be much faster since there are no $replace's involved.