The mIRC's $rand() identifier doesn't work with letters as it does with numbers.

//echo -a $rand(111,999) - Returns 346
//echo -a $rand(aaa,zzz) - Returns a

Now using $rand2()

//echo -a $rand2(111,999) - Returns 301
//echo -a $rand2(aaa,ccc) - Returns bca

alias rand2 {  
  if (($1 && $2) isnum) { return $rand($1,$2)  }   
  else { return $replace($($str($!rand($1,$2) $+ $chr(32),$len($1)),3),$chr(32),$chr(3)) } 
}