1. You don't need multiple text events per trigger.
2. You don't need to use the global variables; the local ones will do.

on $*:TEXT:$(/((h)i|ello|what's up|nothing|sad| $+ $me $+ )$/iS):#: {
  var %hi = Hi $nick $+ , how are you today?|What's up $nick $+ ?|Wazzup $nick $+ ?
  var %whatsup = What's up with you?|Nothing but the sky, $nick $+ .|You say what's up. I say what's down.
  if ($regml(1) == hi) || ($regml(1) == ello) {
    msg # $gettok(%hi,$r(1,$numtok(%hi,124)),124) 
  }
  if ($regml(1) == what's up) {
    msg # $gettok(%whatsup,$r(1,$numtok(%whatsup,124)),124) 
  }
  if ($regml(1) == nothing) {
    var %nothing = $r(1,2)
    if (%nothing == 1) msg # If nothing's up then what are you talking about?
    if (%nothing == 2) describe # slaps $nick "You're boring" :/
  }
  if ($regml(1) == sad) { 
    var %sad = $r(1,4)
    if (%sad == 1) msg # Awwwww don't be sad $nick $+ .
    if (%sad == 2) describe # pats $nick on the back "Being sad makes you miss out on all of the good things in life."
    if (%sad == 3) msg # Be the opposite of sad and be glad! =D
    if (%sad == 4) msg # You're sad too much $nick $+ .
  }
  if ($regml(1) == $me) {
    var %me = $r(1,5)
    if (%me == 1) msg # That's my name. Don't stick a knife through it.
    if (%me == 2) msg # WHO SAID MY NAME?!?!?!
    if (%me == 3) msg # Yes, $nick $+ .
    if (%me == 4) describe # shoots $nick $+ , "DID I SAY YOU COULD SAY MY NAME?!"
    if (%me == 5) msg # What can I help you with, $nick $+ ?
  }
}

$r() is the same as $rand()