Rather than simply testing for a question mark followed by a character that is not a quote (\?[^"]), I should have tested for a question mark with a negative look-ahead (\?(?!")) for a quote. This appears to work:
Unfortunately, Benchmark shows that it's not quite as fast as merlyn's version.$myvar =~ /"((?:[^?"]|\?(?!"))*)\?"/';
For those unfamiliar with lookaheads, they allow you to test for text without "bumping along" the regex. In other words, \?[^"] will check for a question mark followed by a non-quote character, but further matching of the regex continues after the non-quote character. \?(?!") allows you to check for a question mark not followed by a quote, but continues matching after the question mark.
Note: There is a subtle difference between the negated character class and the negative lookahead. The negated character class generally requires a character after the question mark (in the above example), while the negative lookahead just makes sure that a quote doesn't follow the question mark and doesn't require a character.
<code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>