code: function () {
                  var value;
  
                  // http://jsperf.com/case-insensitive-regex-vs-strtolower-then-regex/18
                  if (!parserInput.$re(/^opacity=/i)) { return; }
                  value = parserInput.$re(/^\d+/);
                  if (!value) {
                      value = expect(parsers.entities.variable, 'Could not parse alpha');
                      value = '@{' + value.name.slice(1) + '}';
                  }
                  expectChar(')');
                  return new tree.Quoted('', 'alpha(opacity=' + value + ')');
              }
docs: IE's alpha function  alpha(opacity=88)