code: function(input) {
        var dollarCurly = input.indexOf('${');
        if (dollarCurly >= 0) {
          var endCurly = input.indexOf('}', dollarCurly);
          if (endCurly >= 0) {
            return { start: dollarCurly, stop: endCurly, expr: input.substring(dollarCurly + 2, endCurly) };
          }
        }
        return null;
      }
docs: gets expression coordinates