var re = /^[1-9]{4,10}$/gi;
var str = "123456";
alert(re.test(str));   //返回true

alert(re.lastIndex); // 弹出6

<script type="text/javascript">
var re = /^[1-9]{4,10}$/gi;
var str = "123456";
alert(re.test(str));   //返回true
re.lastIndex=0;
alert(re.test(str));   //返回true
</script>

function check(){
var email=document.getElementById("email").value;
var isemail=/^[a-z]([a-z0-9]*[-_]?[a-z0-9]+)*@([a-z0-9]*[-_]?[a-z0-9]+)+[\.][a-z]{2,3}([\.][a-z]{2})?$/i;
if (email=="") {
   alert("请输入您的邮箱!");
   return false;
  }
if (email.length>25){
   alert("长度太长");
   return false
}
if (!isemail.test(email)){
   alert("不是邮箱");
  return false;
}
}