function userPwds(){
                var pass=document.getElementById("upass").value;
                var upassd=document.getElementById("upassnu");
                if(pass==""){
                        upassd.innerHTML="密码不能为空！";
                        document.getElementById("upass").focus();
                        return false;
                    }
                    else{
                    upassd.innerHTML="";
                    var regster= /^[A-Za-z0-9]+$/;　
                    if(regster.test(pass)==false){
                    upassd.innerHTML="密码格式输入不正确！";
                    document.getElementById("upass").focus();
                    return false;
                }                    
            }
        }
    function userName(){
            var name=document.getElementById("uname").value;
            var unamenull=document.getElementById("unamenull");
            if(name==""){
                    unamenull.innerHTML="用户名不能为空！";
                    document.getElementById("uname").focus();
                    return false;
                }
                else{
                    unamenull.innerHTML="";
                    var username= /^[A-Za-z0-9]+$/;　
                    if(username.test(name)==false){
                    unamenull.innerHTML="用户名格式输入不正确！只能字母和数字";
                    document.getElementById("uname").focus();
                    return false;
                }                    
            }
        }

        <form action="" method="post" name="myForm" onsubmit="checklogin()">
        <table>
            <tr>
                <td>用户名：<input type="text"  id="uname" name="username" class="size" onblur=                                     "userName()"/></td>
            </tr>
            
            <tr>
                <td id="unamenull"></td>
            </tr>            
            
            <tr>
            <td>密&nbsp;&nbsp;码：<input type="password" id="upass" name="pwd" class="size" onblur=                                                                                                                                     "userPwds()"  /></td>
            </tr>
            
            <tr>
                <td id="upassnu"></td>
            </tr>
        </table>        
                <input type="button" src="images/sms_btn_login.png" class="but" />
                <input type="button" src="images/sms_btn_reg.png" class="but2" />
        </form>