my ($np);
 # The initial balance parentheses expression with an embedded set is:
 # $np=qr/\( ([^()] | (??{$np})) *\)/x;
 # And quoted strings are "(?:[^"]|\")*" and '(?:[^']|\')*' or ('|")(?:[^\1]|\\1)*\1 so
 $np=qr/
          \(                           # The opening "("
              ((                       # We'll want this hence the capturing ()
                  '(?:[^']|\')*?'      #' a single quote string
                  | "(?:[^"]|\")*?"    #" a double quote string
                  | [^()]              # not a parentheses
                  | (??{$np})
              )*)
          \)                           # and the closing ")"
      /x;