use warnings;
use strict;

while (<DATA>) {
    s/(
                (?:(?:[^\/][^\/]*?|)*?  # anything, except comment symbol
                ("|').*?\2              # quoted string (and take everything)
                (?:[^\/][^\/]*?|)*?     # anything, except comment symbol
              )*?
            )
            (?:
              \/\/                      # comment symbol
              [^\\]*                    # anything, except continuation
            )??$
          /$1/x;

    print;
}

__DATA__
/* //
*/
/*
*/
*str = "//\"//";