#Haskell, 298

main=interact g
g s|all(`elem`"HQ9+")s=s>>=(%s)|0<1="Source code contains invalid characters"
'H'%_="hello World!"
'Q'%s=s
'9'%_=drop 32$d 99
_%_=""
k=" bottles of beer on the wall"
d 0="No more"++k++"."
d n|a<-shows n k=[a,".\n\n",a,",\n",take 18a,".\ntake one down, pass it around\n",d$n-1]>>=id

this is quite straightforward. % returns a command's output (given the source cod for use on Q). d returns the 99 bottles song with a junk line at the start for golfing reasons. everything is wrapped in an interact (you might want to use g instead of main when testing)

Haskell, 298

main=interact g
g s|all(`elem`"HQ9+")s=s>>=(%s)|0<1="Source code contains invalid characters"
'H'%_="hello World!"
'Q'%s=s
'9'%_=drop 32$d 99
_%_=""
k=" bottles of beer on the wall"
d 0="No more"++k++"."
d n|a<-shows n k=[a,".\n\n",a,",\n",take 18a,".\ntake one down, pass it around\n",d$n-1]>>=id

this is quite straightforward. % returns a command's output (given the source cod for use on Q). d returns the 99 bottles song with a junk line at the start for golfing reasons. everything is wrapped in an interact (you might want to use g instead of main when testing)