How to write a javascript funcion to check that my text box is containg special character or not?

I want to check that my text box is containing special character or not, using java script.
for this i am using following code

function CheckUserId() {
     var txt = document.getElementById('<%=TextBox1.ClientID%>').value;
     var regexp = "/^[a-zA-Z0-9]+$/";
     if (regexp.match(txt)) {
         alert("No special character ");
         return true;
     }
     else {
         alert("Special character");
         return false;
     }
     }
 

I am calling this java script function on textbox onblur. TextBox code is following

  <asp:TextBox ID="TextBox1" runat="server" onblur="CheckUserId()" ></asp:TextBox>
 

but it is showing always alert message "special character" .Whats wrong with my code?
Please help me

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