printf("remember, /* this isn't a comment */");

printf("/* This isn't a comment %s */", /* but this is */ "/* this isn
+'t, though */);

// /* The code between these lines
int x = y + z;
// is not commented out. The line level comments take precedent */
[download]

In short, this is a potentially nasty problem. I would strongly recommend you read the very excellent Mastering Regular Expressions from O'Reilly by Jeffrey Friedl.

Regardless, any solution you come up with using regexes will probably only operate on a carefully crafted subset of the data, so proceed with caution. There are also variations depending upon which implementation of C you're using.