超简单的JAVASCRIPT显示问题

Question

function RegUser() 
{           
     var usernamelength=document.getElementById("username").value.length; 
     if(usernamelength==0) 
     { 
        document.getElementById("Regularexpressionvalidator1").style.display=block; 
     }  
} 
<INPUT class=kuan id="username"  runat="server"   onblur=RegUser(); maxLength=30 /> 
<SPAN class=style5>*** <SPAN id=RequiredFieldValidator1 style="DISPLAY: none; COLOR: red">请输入用户名 </SPAN> 这样试试！

Answer 1

<script>
function RegUser() 
        { 
          x= document.form1; 
          var usernamelength=x.username.value.length; 
          if(usernamelength==0) 
          { 
        document.getElementById("RequiredFieldValidator1").style.display="block"; 
          }  
        } 
</script>
<form name="form1">
<INPUT class=kuan id="username"  runat="server"      onblur=RegUser(this.value); maxLength=30 name="username"> 
                  *** <SPAN id="RequiredFieldValidator1" 
                  style="DISPLAY: none; COLOR: red">请输入用户名 </SPAN> 
  </form>

Answer 2

<script>
function RegUser() { 
x= document.form1; 
var usernamelength=x.username.value.length; 
if(usernamelength==0) { 
document.getElementById("RequiredFieldValidator1").style.display="block"; 
} 
} 
function RegUser2() { 
document.getElementById("RequiredFieldValidator1").style.display="none"; 
} 
</script>
<form name="form1">
<INPUT class=kuan id="username"  runat="server"  onblur="RegUser(this.value);" onfocus="RegUser2();" maxLength="30" name="username"> 
<SPAN class="style5">*** <SPAN id="RequiredFieldValidator1" style="DISPLAY: none; COLOR: red">请输入用户名 </SPAN> 
</form>

Answer 3

<SCRIPT LANGUAGE="JavaScript">
<!--
function RegUser(obj) 
        { 
 
          if(obj.value.length < 1 ) 
          { 
document.getElementById("RequiredFieldValidator1").style.display="block"; 
          }  
        } 
//-->
</SCRIPT>
<INPUT class=kuan id="username"  runat="server"    onblur='RegUser(this);' maxLength=30 name=username> 
<SPAN class=style5>*** <SPAN id='RequiredFieldValidator1' style="DISPLAY: none; COLOR: red">请输入用户名 </SPAN> 

Answer 4

function RegUser() 
{           
     var usernamelength=document.getElementById("username").value.length; 
     if(usernamelength==0) 
     { 
        document.getElementById("Regularexpressionvalidator1").style.display=block; 
     }  
} 
<INPUT class=kuan id="username"  runat="server"   onblur=RegUser(); maxLength=30 /> 
<SPAN class=style5>*** <SPAN id=RequiredFieldValidator1 style="DISPLAY: none; COLOR: red">请输入用户名 </SPAN>