I want to match all strings ending in .htm unless it ends in foo.htm. I'm generally decent with regular expressions, but negative lookaheads have me stumped. Why doesn't this work?

/(?!foo).htm$/i.test(/foo.htm);  // returns true. I want false.

What should I be using instead? I think I need a negative lookbehind expression (if JavaScript supported such a thing, which I know it doesn't).