I am performing a grep find and replace in my text editor TextWrangler on a very large CSV data file.

I have a source string of

hello??world ??goodnight?? moon?? wake up

and would like to replace the ?? substring with -- ONLY when it is bounded by non-whitespace characters on both sides, yielding a target string of

hello--world ??goodnight?? moon?? wake up

 I have tried the following regex:

([\S]{1})?[\?]{2}([\S]{1})?

with the replace operation

\1\-\-\2

but the ?? within ??goodnight?? and  moon??  are still being matched even though they are both bordered by whitespace on one side.