Faster checkPlain()?

gistfile1.txt

function checkPlain(str) {
    var r = new RegExp('(&)|(\")|(<)|(>)|([^&\"<>]*)', 'g');
    var replacements = ["&amp;", "&quot;", "&lt;", "&gt;"];
    var retval = '';
    var match = r.exec(str);
    while (match[0] != "") {
      for (var i = 1; i < match.length; ++i) {
        if (match[i] != undefined) {
          retval += (i == 5 ? match[i] : replacements[i-1]);
        }
      }
      match = r.exec(str);
    }
    
    return retval;
  }