How to not expand in strings

Hi-

I have a newbie question.  I'm using m4 to preprocess files in
the Scheme language.  I've changed the quotes to
U+007B LEFT CURLY BRACKET and U+007D RIGHT CURLY BRACKET which
works great with Scheme.  But now I need to avoid processing
a curly bracket that appears inside a string, as in the 
following

(set! str 
      m4_ifdef({USE_REGEX},
               {" } "},
               {" > "}))


I guess I could just temporarily change the quote characters
again, but, is there some more compact way to not expand just that
one string that contains the curly bracket?

Thanks,

Mike Gran