这段正则怎么写？

[wudichaoren2010]

这么写能达到这个要求的效果

①用户名的正则怎么写？

②这个样式怎么表现出来？

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>


<style type="text/css"> 

#titel_img{ 
width:417px; 
} 
#log_image { 
z-index: 0; 
position: absolute; 
left: 50%;top:50%; 
height: 151px;width:400; 
margin-left: -200px;margin-top:-100px; 
} 
#text_box{ 
position: absolute; 
top:65px; 
left:40px; 
z-index: 1; 
/* background-color: #FF0000;*/ 
} 
#text_box div{ 

} 
#titel_text{ 
position: absolute; 
} 
</style> 



</head>

<body>

<script type="text/javascript">
/*
function checkusername(){
	var regex=/^[0-9A-Za-z_]{6,15}$/;
	var s=document.getElementById("username").value;

alert(regex.exec(s));
}
*/

function checkusername()
{
var patrn=/^[0-9A-Za-z_]{6,15}$/
	var s=document.getElementById("username").value;
	if(patrn.exec(s)){ alert('正确'); }else{ alert('错误'); }

}



   function validate() {
              var pw1 = document.getElementById("pw1").value;
              var pw2 = document.getElementById("pw2").value;
              if(pw1 == pw2) {
                  document.getElementById("tishi").innerHTML="<font color='green'>两次密码相同</font>";
                  document.getElementById("submit").disabled = false;
              }
              else {
                  document.getElementById("tishi").innerHTML="<font color='red'>两次密码不相同</font>";
                document.getElementById("submit").disabled = true;
              }
          }
		  
		  
	function checkpass() {	  
		 var  password=document.getElementById("pw1").value;
		 if(password.length<8){
			 alert("密码至少大于等于8位");
			 return;
	}
	var reg = /^[0-9]+$/;
	if(!reg.test(str)){ 
	alert("密码只能由数字组成");
	}
	}

</script>






<form>

<div id="text_box">


<table width="800">
  <tr>
   <td><div>用户名*：<input id="username" type="text" onkeydown="checkusername()"></div> </td>
    <td></td>


  </tr>
    <tr>
    <td width="600px" >九位有效数字，前三个只能输入字母（不区分大小写），后六个只能输入数字</td>
    <td></td>

  </tr>

  
    <tr> <td>输入密码： <input type="password" name="pw" id="pw1" onkeyup="checkpass()"/></td>
    
    <td><span style="color:#F00">八位有效数字</span></td>
    </tr>
      <tr>  <td>确认密码： <input type="password" name="pw" id="pw2" onkeyup="validate()"/><span id="tishi"></span> </td>
      
       <td><span style="color:#F00">密码必须一致</span></td>
      </tr>

  
 
</table>





</div> 
</form>





</body>
</html>

[天际的海浪]

/^[A-Za-z]{3}\d{6}$/

[娃都会打酱油了]

^[a-zA-Z]{3}\d{6}$