Creating a HQ9+ interpreter

APL (Dyalog) (326 - 8 = 318)

Paste it in an editor window and call H. Input is taken from the keyboard.

H
⎕←'Source code contains invalid characters'/⍨~∧/'HhQq9+'∊⍨V←⍞
B←' of beer'
W←' on the wall'
L←⎕TC[2]
Z←{k←' bottle','s'/⍨⍵≠1⋄⍵<0:'99',k⋄⍵=0:'No',k⋄k,⍨⍕⍵}
{⍵∊'Hh':⎕←'Hello, world!'
⍵∊'Qq':⎕←V
⍵∊'9':{⎕←G,K,(G←Z⍵),B,L,(⊃'Go to the store and buy some more' 'Take one down and pass it around'[1+×⍵]),L,(Z⍵-1),K←B,W,L}¨1-⍨⌽⍳100}¨V

C, 599 587 487 481 467 characters

I'm sure this can be beaten. I'm using C after all. There's a reason you don't see a lot of winning golf entries written in C. This is now 467 characters thanks to criminal abuse of #define.

Specify the HQ9+ source file as a command line argument.

EDIT: Now accepts the source from stdin, not a file. Start the program, start typing the code, CTRL‑C when done to run it.

It compiles at least in MinGW/GCC with: gcc -o hq9+.exe hq9+.c

Should work in MSVC, but I didn't feel like creating a whole solution just for this. :)

#define B "bottles of beer"
#define C case
#define P printf
#define R break
a,i,j,s;
main(){
char p[9999],c;
for(;;) {
    switch(c=getch()){
    C 'H':C 'Q':C '9':C '+': C 3: R;
    default:P("Source code contains invalid characters\n");
    }
    if (c==3) R;
    p[s++]=c;
}
for(i=0;i<s;i++){
    c = p[i];
    switch(c){
    C 'H':
        P("Hello world!");
        R;
    C 'Q':
        for(j=0;j<s;j++)putchar(p[j]);
        R;
    C '9':
        j=99;
        while(j){
            P("%d "B" on the wall,\n%d "B".\nTake one down, pass it around,\n%d "B".\n",j,j,j-1);
            j--;
        }
        R;
    C '+':
        a++;
    }
}
}

Or:

#define B "bottles of beer"
#define C case
#define P printf
#define R break
a,i,j,s;main(){char p[9999],c;for(;;){switch(c=getch()){C 'H':C 'Q':C '9':C '+': C 3: R;default:P("Source code contains invalid characters\n");}if (c==3) R;p[s++]=c;}for(i=0;i<s;i++){c = p[i];switch(c){C 'H':P("Hello world!");R;C 'Q':for(j=0;j<s;j++)putchar(p[j]);R;C '9':j=99;while(j){P("%d "B" on the wall,\n%d "B".\nTake one down, pass it around,\n%d "B".\n",j,j,j-1);j--;}R;C '+':a++;}}}

Mathematica, 349 346 341 chars

h = If[StringMatchQ[#, ("H" | "Q" | "9" | "+") ...], 
   b = If[# > 0, ToString@#, "No"] <> " bottle" <> 
      If[# == 1, "", "s"] <> " of beer" &; 
   w = b@# <> " on the wall" &; 
   Print /@ 
    StringCases[#, {"H" -> "Hello, world!", "Q" -> #, 
      "9" -> "" <> 
        Table[w@n <> ",\n" <> b@n <> 
          ".\ntake one down, pass it around,\n" <> w[n - 1] <> 
          If[n == 1, ".", ".\n\n"], {n, 99, 1, -1}]}];, 
   "Source code contains invalid characters"] &