I don't think arrays are built in to mIRC which is a shame. I believe I used a pre-existing character set builder class to specify which set to build from. So it can be modified to use only upper, lower, both, numerical, alpha numerical, or all characters (to a certain degree). Chosen at random, of course.
Yeah! There are a lot of random string snippet XD
I was bored, so I decided to make my own "random pass generator"
As you can see, there is 1/3 chance to return a number otherwise a letter (50% uppercase 50% lowercase).
XD