<TABLE>
<form name="form1" id="form1">
<TR>
    <TD>用户名：</TD>
    <TD><INPUT TYPE="text" NAME="user"></TD>
    <TD rowspan="3" id="msg" style="color:red" valign="top"></TD>
</TR>
<TR>
    <TD>密码：</TD>
    <TD><INPUT TYPE="password" NAME="psw"></TD>
</TR>
<TR>
    <TD colspan="2" align="center"><INPUT TYPE="button" value="登录" onclick="check()"></TD>
</TR>
</form>
</TABLE>
  <SCRIPT LANGUAGE="JavaScript">
  <!--
  function check(){
    var len = 6;
    var form1 = document.getElementById("form1");
    var msg = document.getElementById("msg");
        var reg = /^[0-9a-zA-Z_]+$/
    if (form1.user.value=="") msg.innerHTML = "请填写用户名";
    else if (form1.psw.value=="") msg.innerHTML = "请填写密码";
    else if (form1.psw.value.length< len) msg.innerHTML = "密码至少为6位";
    else if (!reg.test(form1.user.value)) msg.innerHTML = "用户名只能由数字、字母或下划线组成";
    else if (!reg.test(form1.psw.value)) msg.innerHTML = "密码只能由数字、字母或下划线组成";
    else alert("pass!")
  }
  //-->
  </SCRIPT>