function trim(str) {
  while (str.charAt(0) == ' ') {
    str = str.substring(1, str.length);
  }
  while (str.charAt(str.length - 1) == ' ') {
    str = str.substring(0, str.length - 1);
  }
  return str;
}


function checkemailformat(em) {
  var len = em.length;
  if (len == 0) {
    return "請輸入電子郵件";
  }
  for(var i = 0; i < len; i++) {
    var c = em.charAt(i);
    if(!((c >= "A" && c <= "Z")||(c >= "a" && c <= "z")||(c >= "0" && c <= "9")||(c == "-")||(c == "_")||(c == ".")||(c == "@")))
    {
      return "請輸入限用英文，數字，點，'@'，橫線及底線的電子郵件";
    }
  }
  if((em.indexOf("@")==-1)||(em.indexOf("@")==0)||(em.indexOf("@")==(len-1)))   {
    return "請輸入正確的電子郵件";
  }
  if((em.indexOf("@")!=-1)&&(em.substring(em.indexOf("@")+1,len).indexOf("@")!=-1))   {
    return "請輸入正確的電子郵件";
  }
  if((em.indexOf(".")==-1)||(em.indexOf(".")==0)||(em.lastIndexOf(".")==(len-1)))   {
    return "您的電子郵件地址不完整";
  }
  return "";
}